theory and design of automotive engine

THEORY AND DESIGNOF

AUTOMOTIVE ENGINES

I Introduction1 General - Historical development of automobiles, Types of power plant, Principle of engine operation,

Classification of engines.2. Two stroke & four stroke engines; Principles of engine operation (SI & CI), Scavenging - systems,

theoretical processes, parameters, relative merits & demerits; Port timing diagrams, port design. Relative merits & demerits compared to petrol & diesel engines, scavenging pumps.

II Engine components –Classification/types, function, materials, construction details, design and manufacturing processes of the following engine components

3. Cylinders and liners - design, cylinder wear and corrosion, details of water jacket, dry and wet liners, Cylinder head - design;

4. Piston, piston rings, piston pin - design - stress analysis, methods of manufacture, compensation of thermal expansion in pistons, heat treatment, piston ring selection, limits of fit for pins

5. Connecting rod - design, effects of whipping, bearing materials, lubrication6. Crank shaft - design, firing order, balancing and torsional vibration analysis, vibration dampers,

bearings,. Lubrication7. Flywheel - design; Camshaft - drives of cams, materials, Types (only descriptive)8. Valve and valve mechanism - design, types of valve operating mechanisms, valve springs, guides,

push rods, rocker arms, tappets, valve timing diagrams 9. Crank Case- Design of crank case, oil sumps and cooling features10. Manifolds-construction and design of inlet and exhaust manifolds.

TEXT BOOKS:I. High Speed Engines - P .M.Heldt, Oxford & IBH , 19652. Auto Design - R.B Gupta, Satya Prakashan, New Delhi 1999

REFERENCE BOOKS:I.A course in I.c. Engine - Mathur & Sharma, Dhanput Rai & Sons, Delhi, 19942.Automobile Engineering VoU & II - Kirpal Singh, Standard publications, New Delhi, 19723. Modem Petrol Engine ~ A.W.Judge, B.I. Publications. 19834. I.c. Engine - Maleev &Litchy, McGrawHill5. I.C.Engines - H.B.Keshwani, Standard Pub New Delhi., 19826. Fundamentals of I.C.Engines - J.B.Heywood7. Machine design exercises - S.N.Trikha, Khanna publications, Delhi8. Automotive mechanics - N.K.Giri, Khanna publications,Delhi9. Automotive mechanics - William H. Crouse, Tata Mc,Graw Hill Publications Co. New Delhi10. I.C.Engines and Air Pollution - B.P.Obel'rlntext harper & Roni Pub, New york )

THEORY AND DESIGN OF AUTOMOTIVE ENGINES

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesCHAPTER - 1

HISTORY Automobiles through the Years - Since they originated in the late 1800s, automobiles have

changed and developed in response to consumer wishes, economic conditions, and advancing technology. The first gas-powered vehicles looked like horse buggies with engines mounted underneath because this was the style to which people were accustomed. By 1910, however, features like the front-mounted engine were already established, giving the automobile a look that was all its own. As public demand for cars increased, the vehicles became more stylized. The classic cars of the 1920s and 1930s epitomize the sleek, individually designed luxury cars called the “classic cars.” During the 1940s and 1950s, automobiles generally became larger until the advent of the “compact” car, which immediately became a popular alternative. The gasoline crisis is reflected in the fuel efficient cars made in the 1970s and 1980s. Current designs continue to reflect economy awareness, although many different markets exist.

The history of the automobile actually began about 4,000 years ago when the first wheel was used for transportation in India.

In the early 15th century the Portuguese arrived in China and the interaction of the two cultures led to a variety of new technologies, including the creation of a wheel that turned under its own power.By the 1600s small steam-powered engine models had been developed, but it was another century before a full-sized engine-powered vehicle was created.

In 1769 French Army officer Captain Nicolas-Joseph Cugnot built what has been called the first automobile. Cugnot’s three-wheeled, steam-powered vehicle carried four persons. Designed to move artillery pieces, it had a top speed of a little more than 3.2 km/h (2 mph) and had to stop every 20 minutes to build up a fresh head of steam.

Cugnot Steam Tractor -the first self-propelled road vehicle, thus, the earliest automobile. Powered by steam, the three-wheeled tractor- invented in 1769 by Nicolas-Joseph Cugnot. designed to carry artillery, but similar vehicles soon found many other uses in industry.

As early as 1801, successful but very heavy steam automobiles were introduced in England. Laws barred them from public roads and forced their owners to run them like

trains on private tracks.In 1802 a steam-powered coach designed by British engineer Richard Trevithick journeyed more than 160 km (100 mi) from Cornwall to London. Steam power caught the attention of other vehicle builders.In 1804 American inventor Oliver Evans built a steam-powered vehicle in Chicago, Illinois. French engineer Onésiphore Pecqueur built one in 1828.

British inventor Walter Handcock built a series of steam carriages in the mid-1830s that were used for the first omnibus service in London.

By the mid-1800s England had an extensive network of steam coach lines. Horse-drawn stagecoach companies and the new railroad companies pressured the British Parliament to approve heavy tolls on steam-powered road vehicles. The tolls quickly drove the steam coach operators out of business.

During the early 20th century steam cars were popular in the United States. Most famous was the Stanley Steamer, built by American twin brothers Freelan and Francis Stanley. A Stanley Steamer established a world land speed record in 1906 of 205.44 km/h (121.573 mph). Manufacturers produced about 125 models of steam-powered automobiles, including the Stanley, until 1932.

2

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesInternal-Combustion Engine

Development of lighter steam cars during the 19th century coincided with major developments in engines that ran on gasoline or other fuels. Because the newer engines burned fuel in cylinders inside the engine, they were called internal-combustion engines.

In 1860 French inventor Jean-Joseph-Étienne Lenoir patented a one-cylinder engine that used kerosene for fuel. Two years later, a vehicle powered by Lenoir’s engine reached a top speed of about 6.4 km/h (about 4 mph).

In 1864 Austrian inventor Siegfried Marcus built and drove a carriage propelled by a two-cylinder gasoline engine.

American George Brayton patented an internal-combustion engine that was displayed at the 1876 Centennial Exhibition in Philadelphia, Pennsylvania.In 1876 German engineer Nikolaus August Otto built a four-stroke gas engine, the most direct ancestor to today’s automobile engines. In a four-stroke engine the pistons move down to draw fuel vapor into the cylinder during stroke one; in stroke two, the pistons move up to compress the vapor; in stroke three the vapor explodes and the hot gases push the pistons down the cylinders; and in stroke four the pistons move up to push exhaust gases out of the cylinders. Engines with two or more cylinders are designed so combustion occurs in one cylinder after the other instead of in all at once. Two-stroke engines accomplish the same steps, but less efficiently and with more exhaust emissions.

Automobile manufacturing began in earnest in Europe by the late 1880s.German engineer Gottlieb Daimler and German inventor Wilhelm Maybach mounted a gasoline-

powered engine onto a bicycle, creating a motorcycle, in 1885.In 1887 they manufactured their first car, which included a steering tiller and a four-speed

gearbox. Another German engineer, Karl Benz, produced his first gasoline car in 1886.

Early CarThe first practical car, built by German engineer Karl Benz in 1885, initiated the era of automobile manufacturing. Benz made improvements to the internal combustion engine and invented the differential drive and other automotive components. The company Benz founded grew into one of the largest automobile manufacturers in Germany.

In 1890 Daimler and Maybach started a successful car manufacturing company, The Daimler Motor Company, which eventually merged with Benz’s manufacturing firm in 1926 to create Daimler-Benz. The joint company makes cars today under the Mercedes-Benz nameplate.

3

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesIn France, a company called Panhard-Levassor began making cars in 1894 using Daimler’s

patents. Instead of installing the engine under the seats, as other car designers had done, the company introduced the design of a front-mounted engine under the hood. Panhard-Levassor also introduced, a clutch and gears, and separate construction of the chassis, or underlying structure of the car, and the car body. The company’s first model was a gasoline-powered buggy steered by a tiller.

French bicycle manufacturer Armand Peugeot saw the Panhard-Levassor car and designed an automobile using a similar Daimler engine. In 1891 this first Peugeot automobile paced a 1,046-km (650-mi) professional bicycle race between Paris and Brest.

Other French automobile manufacturers opened shop in the late 1800s, including Renault.In Italy, Fiat (Fabbrica Italiana Automobili di Torino) began building cars in 1899.

4

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesAmerican automobile builders were not far behind. Brothers Charles Edgar Duryea and James

Frank Duryea built several gas-powered vehicles between 1893 and 1895. The first Duryea, a one-cylinder, four-horsepower model, looked much like a Panhard-Levassor model.

Horseless CarriageThe original horseless carriage was introduced in 1893 by brothers Charles and Frank Duryea. It was America’s first internal-combustion motor car, and it was

followed by Henry Ford’s first experimental car that same year.

In 1893 American industrialist Henry Ford built an internal-combustion engine from plans he saw in a magazine. In 1896 he used an engine to power a vehicle mounted on bicycle wheels and steered by a tiller. Early Electric Cars

For a few decades in the 1800s, electric engines enjoyed great popularity because they were quiet and ran at slow speeds that were less likely to scare horses and people. By 1899 an electric car designed and driven by Belgian inventor Camille Jenatzy set a record of 105.8810 km/h (65.79 mph). Early electric cars featured a large bank of storage batteries under the hood. Heavy cables connected the batteries to a motor between the front and rear axles. Most electric cars had top speeds of 48 km/h (30 mph), but could go only 80 km (50 mi) before their batteries needed recharging. Electric automobiles were manufactured in quantity in the United States until 1930.

Automobiles in the 20th century For many years after the introduction of automobiles, three kinds of power sources were in

common use: steam engines, gasoline engines, and electric motors.In 1900 more than 2,300 automobiles were registered in New York City; Boston, Massachusetts; and Chicago, Illinois. Of these, 1,170 were steam cars, 800 were electric cars, and only 400 were gasoline cars. Gasoline-powered engines eventually became the nearly universal choice for automobiles because they allowed longer trips and faster speeds than engines powered by steam or electricity.

Improvements in the operating and riding qualities of gasoline automobiles developed quickly after 1900. The 1902 Locomobile was the first American car with a four-cylinder, water-cooled, front-mounted gasoline engine, very similar in design to most cars today. Built-in baggage compartments appeared in 1906, along with weather resistant tops and side curtains. An electric self-starter was introduced in 1911 to replace the hand crank used to start the engine turning. Electric headlights were introduced at about the same time.

Most automobiles at the turn of the 20th century appeared more or less like horseless carriages. In 1906 gasoline-powered cars were produced that had a style all their own. In these new models, a hood covered the front-mounted engine. Two kerosene or acetylene lamps mounted to the front served as headlights. Cars had fenders that covered the wheels and step-up platforms called running boards, which helped passengers, get in and out of the vehicle. The passenger compartment was behind the

5

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive Enginesengine. Although drivers of horse-drawn vehicles usually sat on the right, automotive steering wheels were on the left in the United States.

6

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesIn 1903 Henry Ford incorporated the Ford Motor Company, which introduced its first

automobile, the Model A, in that same year. It closely resembled the 1903 Cadillac, which was hardly surprising since Ford had designed cars the previous year for the Cadillac Motor Car Company. Ford’s company rolled out new car models each year, and each model was named with a letter of the alphabet. By 1907, when models R and S appeared, Ford’s share of the domestic automobile market had soared to 35 percent.

Ford Model T

A Ford Model T rolls off the assembly line. Between 1908 and 1927, Ford built 15 million Model Ts.

Ford’s famous Model T debuted in 1908 but was called a 1909 Ford. Ford built 17,771 Model T’s and offered nine body styles. Popularly known as the Tin Lizzy, the Model T became one of the biggest-selling automobiles of all time. Ford sold more than 15 million before stopping production of the model in 1927. The company’s innovative assembly-line method of building the cars was widely adopted in the automobile industry.

Silver Ghost

7

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesOne of the highest-rated early luxury automobiles, the 1909 Rolls-Royce Silver Ghost’s features included a quiet 6-cylinder engine, leather interior, folding windscreens and hood, and an aluminum body. Generally driven only by chauffeurs, the emphasis of the luxury car was on comfort and style rather than speed.

By 1920 more than 8 million Americans owned cars. Major reasons for the surge in automobile ownership were Ford’s Model T, the assembly-line method of building it, and the affordability of cars for the ordinary wage earner.

8

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesImprovements in engine-powered cars during the 1920s contributed to their popularity:

synchromesh transmissions for easier gear shifting; four-wheel hydraulic brake systems; improved carburetors; shatterproof glass; balloon tires; heaters; and mechanically operated windshield wipers.

PhaetonCars of the 1920s exhibited design refinements such as balloon tires, pressed-steel wheels, and four-wheel brakes. Although assembly lines (which originated with Henry Ford in 1908) continued to bring the price of automobiles down, many cars in this time were one-of-a-kind vintage models, made to individual specifications. The 1929 Graham Paige DC Phaeton shown here featured an 8-cylinder engine and an aluminum body.

From 1930 to 1937, automobile engines and bodies became large and luxurious. Many 12- and 16-cylinder cars were built. Independent front suspension, which made the big cars more comfortable, appeared in 1933. Also introduced during the 1930s were stronger, more reliable braking systems, and higher-compression engines, which developed more horsepower. Mercedes introduced the world’s first diesel car in 1936.

Automobiles on both sides of the Atlantic were styled with gracious proportions, long hoods, and pontoon-shaped fenders. Creative artistry merged with industrial design to produce appealing, aerodynamic automobiles.

9

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive Engines

De Luxe SedanThe roomy interior and rear-hinged back door of this 1937 Pontiac De Luxe sedan represent a move toward a car more suited to the needs of families. With these consumers in mind, cars were designed to be convenient, reliable, and relatively inexpensive. Vehicles in the 1930s were generally less boxy and more streamlined than their predecessors.

Some of the first vehicles to fully incorporate the fender into the bodywork came along just after World War II, but the majority of designs still had separate fenders with pontoon shapes holding headlight assemblies. Three companies, Ford, Nash, and Hudson Motor Car Company, offered postwar designs that merged fenders into the bodywork. The 1949 Ford was a landmark in this respect, and its new styling was so well accepted the car continued in production virtually unchanged for three years, selling more than 3 million. During the 1940s, sealed-beam headlights, tubeless tires, and the automatic transmission were introduced.

Two schools of styling emerged in the 1950s, one on each side of the Atlantic. The Europeans continued to produce small, light cars weighing less than 1,300 kg (2,800 lb). European sports cars of that era featured hand-fashioned aluminum bodies over a steel chassis and framework.

10

www.jntuworld.com

www.jntuworld.com


StudebakerThis

1940 Studebaker Champion two-door sedan was designed by Raymond Loewy and built by Studebaker craftsmen. Features emerging in the 1940s include automatic transmission, sealed-beam headlights, and tubeless tires.

In America, automobile designers borrowed features for their cars that were normally found on aircraft and ships, including tailfins and portholes. Automobiles were produced that had more space, more power, and smoother riding capability. Introduction of power steering and power brakes made bigger cars easier to handle. The Buick Motor Car Company, Olds Motor Vehicle Company (Oldsmobile), Cadillac Automobile Company, and Ford all built enormous cars, some weighing as much as 2,495 kg (5,500 lb). The first import by German manufacturer Volkswagen AG, advertised as the Beetle, arrived in the United States in 1949. Only two were sold that year, but American consumers soon began buying the Beetle and other small imports by the thousands.

VW BeetleThe

Volkswagen Beetle dominated the market for several years, during which few modifications were made on the original design. Volkswagen’s name means “car for the people,” and the car served at least two important consumer needs. The rear-mounted engine and small, rounded, buglike shape of the European car represented an appealing combination of look and economy that remained popular for more than four decades.

That prompted a downsizing of some American-made vehicles. The first American car called a compact was the Nash Rambler. Introduced in 1950, it did not attract buyers on a large scale until 1958.

11

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesMore compacts, smaller in overall size than a standard car but with virtually the same interior body dimensions, emerged from the factories of many major manufacturers. The first Japanese imports, 16 compact trucks, arrived in the United States in 1956.

In the 1950s new automotive features were introduced, including air conditioning and electrically operated car windows and seat adjusters. Manufacturers changed from the 6-volt to the 12-volt ignition system, which gave better engine performance and more reliable operation of the growing number of electrical accessories.

By 1960 sales of foreign and domestic compacts accounted for about one-third of all passenger cars sold in the United States. American cars were built smaller, but with increased engine size and horsepower. Heating and ventilating systems became standard equipment on even the least expensive models. Automatic transmissions, power brakes, and power steering became widespread. Styling sometimes prevailed over practicality—some cars were built in which the engines had to be lifted to allow simple service operations, like changing the spark plugs. Back seats were designed with no legroom.

12

www.jntuworld.com

www.jntuworld.com


GullwingPowerful high-performance cars such as this 1957 Mercedes-Benz 300SL were built on compact and stylized lines. Also called the Gullwing because its doors open upward into the shape of a gull’s wings, the 300SL was capable of 230 kmh (144 mph), its on-road performance matching its racing capacity.

El Dorado

This 1957 Cadillac El Dorado convertible epitomizes the large cars of the “American Dream” era. Tail fins are an example of a trend in car design. Although the feature did little for the performance of the vehicle, consumers loved the look, and demanded fins of increasing size until the 1960s.

MustangMore than 100,000 Ford Mustangs sold during first four months the model was on the market in 1964, making it Ford’s best early sales success since the introduction of the Model T. A vehicle from the “muscle car” category, the Mustang’s popular characteristics included a small, fast design, excellent handling, a powerful engine, and a distinctive look.

13

www.jntuworld.com

www.jntuworld.com


In the 1970s American manufacturers continued to offer smaller, lighter models in addition to the bigger sedans that led their product lines, but Japanese and European compacts continued to sell well. Catalytic converters were introduced to help reduce exhaust emissions.

Digital speedometers and electronic prompts to service parts of the vehicle appeared in the 1980s. Japanese manufacturers opened plants in the United States. At the same time, sporty cars and family minivans surged in popularity.

Advances in automobile technology in the 1980s included better engine control and the use of innovative types of fuel. In 1981 Bayerische Motoren Werke AG (BMW) introduced an on-board computer to monitor engine performance. A solar-powered vehicle, SunRaycer, traveled 3,000 km (1,864 mi) in Australia in six days.

MR-2 Turbo

Modern cars like the Japanese 1992 MR-2 Turbo T-bar Toyota are generally light, aerodynamically shaped, and compact. Japanese imports changed the automobile industry significantly. The generally reliable, inexpensive cars increased competition between manufacturers dramatically, to the benefit of consumers.

New technologies

Gas-Electric Hybrids

The Toyota Prius,a four-seat hybrid electric vehicle (HEV), was the first HEV to be marketed when Toyota introduced it in Japan in 1997.

The Honda Insight,a two-seat HEV, followed in 1999 when it was sold in both Japan and the United States. The Prius had its U.S. debut in 2000.

14

www.jntuworld.com

www.jntuworld.com


Gas-Electric Hybrids The Toyota Prius, a four-seat hybrid electric vehicle (HEV), was the first HEV to be marketed when Toyota introduced it in Japan in 1997. The Honda Insight, a two-seat HEV, followed in 1999 when it was sold in both Japan and the United States. The Prius had its U.S. debut in 2000.

Pollution-control laws adopted at the beginning of the 1990s in some of the United States and in Europe called for automobiles that produced better gas mileage with lower emissions. In 1996 General Motors became the first to begin selling an all-electric car, the EV1, to California buyers. The all-electric cars introduced so far have been limited by low range, long recharges, and weak consumer interest.

Engines that run on hydrogen have been tested. Hydrogen combustion produces only a trace of harmful emissions, no carbon dioxide, and a water-vapor by-product. However, technical problems related to the gas’s density and flammability remains to be solved.

Diesel engines burn fuel more efficiently, and produce fewer pollutants, but they are noisy. Popular in trucks and heavy vehicles, diesel engines are only a small portion of the automobile market. A redesigned, quieter diesel engine introduced by Volkswagen in 1996 may pave the way for more diesels, and less pollution, in passenger cars.

While some developers searched for additional alternatives, others investigated ways to combine electricity with liquid fuels to produce low-emissions power systems. Two automobiles with such hybrid engines, the Toyota Prius and the Honda Insight, became available in the late 1990s. Prius hit automobile showrooms in Japan in 1997, selling 30,000 models in its first two years of production. The Prius became available for sale in North America in 2000. The Honda Insight debuted in North America in late 1999. Both vehicles, known as hybrid electric vehicles (HEVs), promised to double the fuel efficiency of conventional gasoline-powered cars while significantly reducing toxic emissions.

Computer control of automobile systems increased dramatically during the 1990s. The central processing unit (CPU) in modern engines manages overall engine performance. Microprocessors regulating other systems share data with the CPU. Computers manage fuel and air mixture ratios, ignition timing, and exhaust-emission levels. They adjust the antilock braking and traction control systems. In many models, computers also control the air conditioning and heating, the sound system, and the information displayed in the vehicle’s dashboard.

Expanded use of computer technology, development of stronger and lighter materials, and research on pollution control will produce better, “smarter” automobiles.

In the 1980s the notion that a car would “talk” to its driver was science fiction; by the 1990s it had become reality.

Onboard navigation was one of the new automotive technologies in the 1990s. By using the satellite-aided global positioning system (GPS), a computer in the automobile can pinpoint the vehicle’s location within a few meters. The onboard navigation system uses an electronic compass, digitized maps, and a display screen showing where the vehicle is relative to the destination the driver wants to reach. After being told the destination, the computer locates it and directs the driver to it, offering alternative routes if needed.

Some cars now come equipped with GPS locator beacons, enabling a GPS system operator to locate the vehicle, map its location, and if necessary, direct repair or emergency workers to the scene.Cars equipped with computers and cellular telephones can link to the Internet to obtain constantly updated traffic reports, weather information, route directions, and other data. Future built-in computer systems may be used to automatically obtain business information over the Internet and manage personal affairs while the vehicle’s owner is driving.

During the 1980s and 1990s, manufacturers trimmed 450 kg (1,000 lb) from the weight of the typical car by making cars smaller. Less weight, coupled with more efficient engines, doubled the gas mileage obtained by the average new car between 1974 and 1995. Further reductions in vehicle size are

15

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive Enginesnot practical, so the emphasis has shifted to using lighter materials, such as plastics, aluminum alloys, and carbon composites, in the engine and the rest of the vehicle.

Looking ahead, engineers are devising ways to reduce driver errors and poor driving habits. Systems already exist in some locales to prevent intoxicated drivers from starting their vehicles. The technology may be expanded to new vehicles. Anti-collision systems with sensors and warning signals are being developed. In some, the car’s brakes automatically slow the vehicle if it is following another vehicle too closely. New infrared sensors or radar systems may warn drivers when another vehicle is in their “blind spot.”

Catalytic converters work only when they are warm, so most of the pollution they emit occurs in the first few minutes of operation. Engineers are working on ways to keep the converters warm for longer periods between drives, or heat the converters more rapidly.

16

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesTypes of power plant

An engine is a device which transforms one form of energy into another form. However, while transforming energy from one form to another, the efficiency of conversion plays an important role. Normally, most of the engines convert thermal energy into mechanical work and therefore they are called 'heat engines'.

Heat engine is a device which transforms the chemical energy of a fuel into thermal energy and utilizes this thermal energy to perform useful work. Thus, thermal energy is converted to mechanical energy in a heat engine.

Heat engines can be broadly classified into two categories:(i) Internal Combustion Engines (IC Engines) (ii) External Combustion Engines (EC Engines)

Table 1.1 Classification of heat engines

Engines whether Internal Combustion or External Combustion are of two types, viz.,(i) Rotary engines (ii) Reciprocating engines

Of the various types of heat engines, the most widely used ones are the reciprocating internal combustion engine, the gas turbine and the steam turbine. The steam engine is rarely used nowadays. The reciprocating internal combustion engine enjoys some advantages over the steam turbine due to the absence of heat exchangers in the passage of the working fluid (boilers and condensers in steam turbine plant). This results in a considerable mechanical simplicity and improved power plant efficiency of the internal combustion engine.

Fig.1.1 Classification of heat engines

17

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesAnother advantage of the reciprocating internal combustion engine over the other two types is

that all its components work at an average temperature which is much below the maximum temperature of the working fluid in the cycle. This is because the high temperature of the working fluid in the cycle persists only for a very small fraction of the cycle time. Therefore, very high working fluid temperatures can be employed resulting in higher thermal efficiency.

Further, in internal combustion engines, higher thermal efficiency can be obtained with moderate maximum working pressure of the fluid in the cycle, and therefore, the weight of power ratio is less than that of the steam turbine plant. Also, it has been possible to develop reciprocating internal combustion engines of very small power output (power output of even a fraction of a kilowatt) with reasonable thermal efficiency and cost.

The main disadvantage of this type of engine is the problem of vibration caused by the reciprocating components. Also, it is not possible to use a variety of fuels in these engines. Only liquid or gaseous fuels of given specification can be efficiently used. These fuels are relatively more expensive.

Considering all the above factors the reciprocating internal combustion engines have been found suitable for use in automobiles, motor-cycles and scooters, power boats, ships, slow speed aircraft, locomotives and power units of relatively small output.External Combustion and Internal Combustion Engines

External combustion engines are those in which combustion takes place outside the engine whereas in internal combustion engines combustion takes place within the engine. For example, in a steam engine or a steam turbine, the heat generated due to the combustion of fuel is employed to generate high pressure steam which is used as the working fluid in a reciprocating engine or a turbine.In case of gasoline or diesel engines, the products of combustion generated by the combustion of fuel and air within the cylinder form the working fluid.

Principle of engine operation (4 stroke & 2 stroke operating cycles)In reciprocating engines, the piston moves back and forth in a

cylinder and transmits power through a connecting rod and crank mechanism to the drive shaft as shown in Fig1.2. The steady rotation of the crank produces a cyclical piston motion. The piston comes to rest at the top center (TC) crank position and bottom-center (BC) [These crank positions are also referred to as top-dead-center (TDC) and bottom-dead-center (BDC)] crank position when the cylinder volume is a minimum or maximum, respectively. The minimum cylinder volume is called the clearance volume.

The volume swept out by the piston, the difference between the maximum or total volume Vt and the clearance volume, is called the displaced or swept volume Vd. The ratio of maximum volume to minimum volume is the compression ratio rc. Typical values of rc are 8 to 12 for SI engines and 12 to 24 for CI engines.

Fig 1.2Basic geometry of the reciprocating internal combustion engine.Vc, Vd, and Vt, indicate clearance, displaced, and total cylinder volumes.

18

www.jntuworld.com

www.jntuworld.com


Fig.1.3 :-The f our-stroke operating cycle.

The majority of reciprocating engines operate on what is known as the four-stroke cycle. Each cylinder requires four strokes of its piston-two revolutions of the crankshaft-to complete the sequence of events which produces one power stroke. Both SI and CI engines use this cycle which comprises 1. An intake stroke, which starts with the piston at TC and ends with the piston at BC, which draws fresh mixture into the cylinder. To increase the mass inducted, the inlet valve opens shortly before the stroke starts and closes after it ends.2. A compression stroke, when both valves are closed and the mixture inside the cylinder is compressed to a small fraction of its initial volume. Toward the end of the compression stroke, combustion is initiated and the cylinder pressure rises more rapidly.3. A power stroke, or expansion stroke, which starts with the piston at TC and ends at BC as the high-temperature, high-pressure, gases push the piston down and force the crank to rotate. About five times as much work is done on the piston during the power stroke as the piston had to do during compression. As the piston approaches BC the exhaust valve opens to initiate the exhaust process and drop the cylinder pressure to close to the exhaust pressure.4 An exhaust stroke, where the remaining burned gases exit the cylinder: first, because the cylinder pressure may be substantially higher than the exhaust pressure; then as they are swept out by the piston as it moves toward TC. As the piston approaches TC the inlet valve opens. Just after TC the exhaust valve closes and the cycle starts again.

Though often called the Otto cycle after its inventor, Nicolaus Otto, who built the first engine operating on these principles in 1876, the more descriptive four-stroke nomenclature is preferred.

The four-stroke cycle requires, for each engine cylinder, two crankshaft revolutions for each power stroke.

To obtain a higher power output from a given engine size, and a simpler valve design, the two-stroke cycle was developed. The two-stroke cycle is applicable to both SI and CI engines.

Figure 1.4 shows one of the simplest types of two-stroke engine designs. Ports in the cylinder liner opened and closed by the piston motion, control the exhaust and inlet flows while the piston is close to BC. The two strokes are:

A compression stroke, which starts by closing the inlet and exhaust ports, and then compresses the cylinder contents and draws fresh charge into the crankcase. As the piston approaches TC, combustion is initiated.

19

www.jntuworld.com

www.jntuworld.com


Fig.1.4 The two-stroke operating cycle. A crankcase-scavenged engine

A power or expansion stroke, similar to that in the four-stroke cycle until the piston approaches BC, when first the exhaust ports and then the intake ports are uncovered. Most of the burnt gases exit the cylinder in an exhaust blow down process. When the inlet ports are uncovered, the fresh charge which has been compressed in the crankcase flows into the cylinder.The piston and the ports are generally shaped to deflect the incoming charge from flowing directly into the exhaust ports and to achieve effective scavenging of the residual gases.

Each engine cycle with one power stroke is completed in one crankshaft revolution. However, it is difficult to fill completely the displaced volume with fresh charge, and some of the fresh mixture flows directly out of the cylinder during the scavenging process. The example shown is a cross-scavenged design; other approaches use loop-scavenging or uniflow systems

20

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesEngine classifications

Fig.1.5IC engine classification

There are many different types of internal combustion engines. They can be classified by:

1. Application.Automobile, truck, locomotive, light aircraft, marine, portable power system, power generation

2 Basic engine designs Reciprocating engines (in turn subdivided by arrangement of cylinders: e.g., in-line, V, radial, opposed-ref, fig1.6.), rotary engines (Wankel and other geometries)

Fig1.6.Engine Classification by Cylinder Arrangements

21

www.jntuworld.com

www.jntuworld.com

(C)

Theory and Design of Automotive Engines3. Working cycle.

Four-stroke cycle: naturally aspirated (admitting atmospheric air), supercharged (admitting pre-compressed fresh mixture), and turbocharged (admitting fresh mixture compressed in a compressor driven by an exhaust turbine),Two-stroke cycle: crankcase scavenged, supercharged, and turbocharged,Constant volume heat addition cycle engine or Otto cycle engine -SI engine or Gasoline engine,Constant pressure heat addition cycle engine or Diesel cycle engine-CI engine or Diesel engine.

4 Valve or port design and location. Overhead (or I-head) valves, under head

(or L-head) valves, rotary valves, cross-scavenged porting (inlet and exhaust ports on opposite sides of cylinder at one end), loop-scavenged porting (inlet and exhaust ports on same side of cylinder at one end), through- or uni-flow scavenged (inlet and exhaust ports or valves at different ends of cylinder)

Fig1.7classification of SI engineby port/ valve location

(a)Cross, (b) Loop, (c) Uniflow Scavenging

22

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive Engines5. Fuel

Gasoline (or petrol), fuel oil (or diesel fuel), natural gas, liquid petroleum gas, alcohols (methanol, ethanol), hydrogen, dual fuel

6. Method of mixture preparation.Carburetion, fuel injection into the intake ports or intake manifold, fuel injection into the engine

cylinder

7. Method of ignition Spark ignition (in conventional engines where the mixture is uniform and in stratified-charge engines where the mixture is non-uniform), compression ignition (in conventional diesels, as well as ignition in gas engines by pilot injection of fuel oil)

8. Combustion chamber design.Open chamber (many designs: e.g., disc, wedge, hemisphere, bowl-in-piston), divided chamber

(small and large auxiliary chambers; many designs: e.g., swirl chambers, pre-chambers)

9. Method of load control.Throttling of fuel and air flow together so mixture composition is essentially unchanged, control

of fuel flow alone, a combination of these

10. Method of cooling.Water cooled, air cooled, un-cooled (other than by natural convection and radiation)

. All these distinctions are important and they illustrate the breadth of engine designs available from a fundamental point of view. The method of ignition has been selected as the primary classifying feature. From the method of ignition-spark-ignition or compression-ignition-follow the important characteristics of the fuel used, method of mixture preparation, combustion chamber design, method of load control, details of the combustion process, engine emissions, and operating characteristics. Some of the other classifications are used as subcategories within this basic classification. The engine operating cycle--four-stroke or two-stroke--is next in importance.

23

www.jntuworld.com

www.jntuworld.com


Table 1.2

24

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesTable 1.3 Engine characteristics Emphasized by Type of Service

References:1. Microsoft Encarta2. Fundamentals of IC Engines By J B Heywood3. Theory & Practice in IC Engines By C F Taylor4. I C Engines By M L Mathur & RP Sharma5. I C Engines By Ganesan

25

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesCHAPTER 2

FOUR-STROKE CYCLE S-I ENGINE - PRINCIPLE OF OPERATION

Fig: cross section of a SI Engine

In Four-stroke cycle engine, the cycle of operation is completed in four-strokes of the piston or two revolutions of the crankshaft. Each stroke consists of 180°, of crankshaft rotation and hence a cycle consists of 720°of crankshaft rotation. The series of operations of an ideal four-stroke. SI engine are as follows (see Fig.2.1 & 2.2)1. Suction stroke

Suction stroke 0-1 starts when the piston is at top dead centre and about to move downwards. The inlet valve is open at this time and the exhaust valve is closed. Due to the suction created by the motion of the piston towards bottom dead centre, the charge consisting of fresh air mixed with the fuel is drawn into the cylinder. At the end of the suction stroke the inlet valve closes.2. Compression stroke.

The fresh charge taken into the cylinder during suction stroke is compressed by the return stroke of the piston 1-2. During this stroke both inlet and exhaust valves remain closed. The air which occupied the whole cylinder volume is now compressed into clearance volume. Just before the end of the compression stroke the mixture is ignited with the help of an electric spark between the electrodes of the spark plug located in combustion chamber wall. Burning takes place when the piston is almost at top dead centre. During the burning process the chemical energy of the fuel is converted into sensible energy, producing a temperature rise of about 2000°C, and the pressure is also considerably increased.3. Expansion or power stroke.

Due to high pressure the burnt gases force the piston towards bottom dead centre, stroke 3-4, and both the inlet and exhaust valves remaining closed. Thus power is obtained during this stroke. Both pressure and temperature decrease during expansion.4. Exhaust stroke.

At the end of the expansion stroke the exhaust valve opens, the inlet valve remaining closed, and the piston is moving from bottom dead centre to top dead centre sweeps out the burnt gases from the cylinder, stroke 4-0. The exhaust valve closes at the end of the exhaust stroke and some 'residual' gases remain in the cylinder.

Each cylinder of a four-stroke engine completes the above four operations in two engine revolutions. One revolution of the crankshaft occurs during the suction and compression strokes, and second revolution during the power and exhaust strokes. Thus for one complete cycle, there is only one power stroke while the crankshaft turns by two revolutions. Most of the spark-ignition internal combustion engines are of the four-stroke type. They are most popular for passenger cars and small aircraft applications.

26

www.jntuworld.com

www.jntuworld.com


Fig.2.1-The four-stroke spark-ignition (SI) engine cycle (Otto cycle or constant volume cycle)

Fig.2.2-Ideal and actual indicator diagrams for four-stroke SI engine

27

www.jntuworld.com

www.jntuworld.com


Fig. 2.3 Four-stroke petrol engine valve timing diagram in relation to the pressure volume diagram

Actual Valve Timing Of Four-Stroke Petrol Engine.Valve timing is the regulation of the points in the cycle at which the valves are set to open and

close. As described above in the ideal cycle inlet and exhaust valves open and close at dead centres, but in actual cycles they open or close before or after dead centres as explained below. There are two factors, one mechanical and other dynamic, for the actual valve timing to be different from the theoretical valve timing.

(a) Mechanical factor.The poppet valves of the reciprocating engines are opened and closed by cam mechanisms. The

clearance between cam, tappet and valve must be slowly taken up and valve slowly lifted, at first, if noise and wear is to be avoided. For the same reasons the valve cannot be closed abruptly, else it will 'bounce' on its seat. (Also the cam contours should be so designed as to produce gradual and smooth changes in directional acceleration). Thus the valve opening and closing periods are spread over a considerable number of crankshaft degrees. As a result, the opening of the valve must commence ahead of the time at which it is fully opened (i.e., before dead centres). The same reasoning applies for the closing time and the valves must close after the dead centres. Fig.2.3 shows the actual valve timing diagram of a four-stroke engine in relation to its pressure-volume diagram.b) Dynamic factor;

Besides mechanical factor of opening and closing of valves, the actual valve timing is set taking into consideration the dynamic effects of gas flow.Intake valve timing.

28

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesIntake valve timing has a bearing on the actual quantity of air sucked during the suction stroke

i.e. it affects the volumetric efficiency. Fig.2.4 shows the intake valve timing diagram for both low speed & high speed SI engines.

Fig:2.4 Valve timing for low and high speed four-stroke SI engine

It is seen that for both low speed and high speed engine the intake valve opens 100 before the arrival of the piston at TDC on the exhaust stroke. This is to insure that the valve will be fully open and the fresh charge starting to flow into the cylinder as soon as possible after TDC. As the piston moves out in the suction stroke, the fresh charge is drawn in through the intake port and valve. When the piston reaches the BDC and starts to move in the compression stroke, the inertia of the entering fresh charge tends to cause it to continue to move into the cylinder. To take advantage of this, the intake valve is closed after BDC so that maximum air is taken in. This is called ram effect. However, if the intake valve is to remain open for too long a time beyond BDC, the up-moving piston on the compression stroke would tend to force some of the charge, already in the cylinder, back into the intake manifold. The time the intake valve should remain open after BDC is decided by the speed of the engine.

At low engine speed, the charge speed is low and so the air inertia is low, and hence the intake valve should close relatively early after BDC for a slow speed engine (say about 100 after BDC).

In high speed engines the charge speed is high and consequently the inertia is high and hence to induct maximum quantity of charge due to ram effect the intake valve should close relatively late after BDC (up to 600 after BDC).

For a variable speed engine the chosen intake valve setting is a compromise between the best setting for low and high speeds.

There is a limit to the high speed for advantage of ram effect. At very high speeds the effect of fluid friction may be more than offset the advantage of ram effect and the charge for cylinder per cycle falls off.

Exhaust valve timing The exhaust valve is set to open before BDC (say about 250 before BDC in low speed engines

and 550 before BDC in high speed engines). If the exhaust valve did not start to open until BDC, the pressures in the cylinder would be considerably above atmospheric pressure during the first portion of the exhaust stroke, increasing the work required to expel the exhaust gases. But opening the exhaust valve earlier reduces the pressure near the end of the power stroke and thus causes some loss of useful

29

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive Engineswork on this stroke. However, the overall effect of opening the valve prior to the time the piston reaches BDC results in overall gain in output.

The closing time of exhaust valve effects the volumetric efficiency, By closing the exhaust valve a few degrees after TDC (about 150 in case of low speed engines and 200 in case of high speed engines) the inertia of the exhaust gases tends to scavenge the cylinder by carrying out a greater mass of the gas left in the clearance volume. This results in increased volumetric efficiency.Note that there may be a period when both the intake and exhaust valves are open at the same time. This is called valve over-lap (say about 150 in low speed engine and 300 in high speed engines). This overlap should not be excessive otherwise it will allow the burned gases to be sucked into the intake manifold, or the fresh charge to escape through the exhaust valve.

Table2.1–Typical valve timings for four-stroke SI engines

Note. Valve timing is different for different makes of engines.b-before, a-after TDC-Top dead centre,BDC-Bottom dead centre.

30

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive Engines FOUR-STROKE CI ENGINES- PRINCIPLE OF OPERATION

The four-stroke CI engine is similar to four-stroke SI engine except that a high compression ratio is used in the former, and during the suction stroke, air alone, instead of a fuel-air mixture, is inducted. Due to high compression ratio, the temperature at the end of compression stroke is sufficient to ignite the fuel which is injected into the combustion chamber.

In the CI engine a high pressure fuel pump and an injector is provided to inject fuel into combustion chamber.

The carburettor and ignition system, necessary in the SI engine, are not required in the CI engine.

The ideal sequence of operation for the four-stroke CI engine is as follows:

Fig.2.5 Ideal P-V Diagram Fig.2.6 Cycle of Operation

1.Suction strokeOnly air is inducted during the suction stroke. During this stroke intake valve is open and

exhaust valve is closed.2.Compression stroke

Both valves remain closed during compression stroke.3. Expansion or power stroke

Fuel is injected in the beginning of the expansion .stroke. The rate of injection is such that the combustion maintains the pressure constant. After the injection of fuel is over (i.e. after fuel cut off) the products of combustion expand. Both valves remain closed during expansion stroke.4. Exhaust stroke.

The exhaust valve is open and the intake valve remains closed in the exhaust stroke.Due to higher pressures the CI engine is heavier than SI engine but has a higher thermal efficiency because of greater expansion. CI engines are mainly used for heavy transport vehicles, power generation, and industrial and marine applications.

The typical valve timing diagram for a four-stroke CI engine is as follows

IVO about 300 before TDCIVO up to 500 after BDC

EVO about 450 before BDC

EVO up to 300 after TDC

Injection about 150 before TDC

31

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesTWO-STROKE CYCLE ENGINE-PRINCIPLE OF OPERATION

In two-stroke engines the cycle is completed in two strokes, i.e., one revolution of the crankshaft as against two revolutions of four-stroke cycle. The difference between two-stroke and four-stroke engines is in the method of filling the cylinder with the fresh charge and removing the burned gases from the cylinder. In a four-stroke engine the operations are performed by the engine piston during the suction and exhaust strokes, respectively. In a two stroke engine suction is accomplished by air compressed in crankcase or by a blower. The induction of compressed air removes the products of combustion, through exhaust ports. Therefore no piston strokes are required for suction and exhaust operations. Only two piston strokes are required to complete the cycle, one for compressing the fresh charge and the other for expansion or power stroke.

Types of two stroke engines• Based on scavenging method

i) Crankcase & ii) Separately scavenged engine• Based on scavenging process (air flow)

i) Cross flow scavenging,ii) Loop scavenging (MAN, Schnuerle, Curtis type)

iii) Uni-flow scavenging (opposed piston, poppet valve, sleeve valve)• Based on overall port-timing

i) Symmetrical & ii) Unsymmetrical

Crankcase-scavenged two-stroke engineFigure 2.7 shows the simplest type of two-stroke engine – the crankcase scavenged engine.

Fig.2.8 shows its ideal and actual indicator diagrams. Fig.2.9 shows the typical valve timing diagram of a two-stroke engine. The air or charge is sucked through spring-loaded inlet valve when the pressure in the crankcase reduces due to upward motion of the piston during compression stroke. After the compression, ignition and expansion takes place in the usual way: During the expansion stroke the air in the crankcase is compressed. Near the end of expansion stroke piston uncovers the exhaust port, and the cylinder pressure drops to atmospheric as the combustion products leave the cylinder. Further motion of the piston uncovers transfer ports, permitting the slightly compressed air or mixture in the crankcase to enter the engine cylinder. The top of the piston sometimes has a projection to deflect the fresh air to sweep up to the top of the cylinder before flowing to the exhaust ports. This serves the double purpose of scavenging the upper part of the cylinder of combustion products and preventing the fresh charge from .flowing directly to the exhaust ports. The same objective can be achieved without piston deflector by proper shaping of the transfer port. During the upward motion of the piston from bottom dead centre, the transfer ports and then the exhaust port close and compression of the charge begins and the cycle is repeated.

Fig.2.7-Crankcase-scavenged two-stroke engine

32

www.jntuworld.com

www.jntuworld.com


Fig. 2.8 Ideal and actual indicator diagrams for a two-stroke SI engine

Fig.2.9. Typical valve timing diagram of a two-stroke engine

33

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesSeparately scavenged engine

In the loop-scavenged engine (Fig. 2.10) an external blower is used to supply the charge, under some pressure, at the inlet manifold. During the downward stroke of the piston exhaust ports are uncovered at about 65° before bottom dead centre. At about 100 later the inlet ports open and the scavenging process takes place.

The inlet ports are shaped so that most of the air flows to the top of the cylinder for proper scavenging of the upper part of the cylinder. Piston deflectors are not used as they are heavy and tend to become overheated at high output. The scavenging process is moreefficient in properly designed loop-scavenged engine than in the usual crank-case compression engine with deflector piston.

Fig.2.10. Loop-scavenged two-stroke engine (separately scavenged engine)

Opposed piston or end to end scavenged engine (uniflow scavenged) two stroke engine.

In this type of engine the exhaust ports or exhaust valves are opened first. The inlet ports give swirl to incoming air which prevents mixing of fresh charge and combustion products during the scavenging process. Early on the compression stroke the exhaust ports close. In loop scavenged engine the port timing is symmetrical, so the exhaust port must close after the inlet port closes. These timings prevent this type of engine from filling its cylinder at full inlet pressure. In the end-to-end scavenged engines counter flow within the cylinder is eliminated, and there is less opportunity for mixing of fresh charge and burnt gases. The scavenging should therefore be more efficient.

Fig. 2.11. 'End to end' scavenged or uniflow two-stroke engine

34

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesValvetiming for two-stroke engines

Fig. 2.12(a), (b) and (c) show typical valve timing diagram for a crankcase-scavenged two-stroke engine and supercharged two-stroke engine and a four-stroke engine, respectively.

Fig 2.12

In case of two-stroke engine the exhaust port is opened near the end of the expansion stroke. With piston-controlled exhaust and inlet port arrangement the lower part of the piston stroke is always wasted so as far as the useful power output is concerned; about 15% to 40% of the expansion stroke is ineffective. The actual percentage varies with different designs. This early opening of the exhaust ports during the last part of the expansion stroke is necessary to permit blow down of the exhaust gases and, also to reduce the cylinder pressure so that when the inlet port opens at the end of the blow down process, fresh charge can enter the cylinder. The fresh charge, which comes from the crankcase for scavenging pump, enters the cylinder at a pressure slightly higher than the atmospheric pressure. Some of the fresh charge is lost due to short-circuiting. For petrol engine this means a loss of fuel and high unburnt hydrocarbons in the exhaust.By comparing the valve timing of two stroke and four-stroke engines, (Fig. 2.12), it is clear that the time available for scavenging and charging of the cylinder of a two stroke engine is almost one-third that available for the .four-stroke engine. For a crankcase-scavenged engine the inlet port closes before the exhaust port whilst for a supercharged engine the inlet port closes after the exhaust port [Fig. 2.12 (b)]. Such timing allows more time for filling the cylinder.

35

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesScavenging process

At the end of the expansion stroke, the combustion chambers of a two-stroke engine is left full of products of combustion. This is because, unlike four-stroke engines, there is no exhaust stroke available to clear the cylinder of burnt gases. The process of clearing the cylinder of burned gases and filling it with fresh mixture (or air}-the combined intake and exhaust process is called scavenging process. This must be completed in a very short duration available between the end of the expansion stroke and start of the charging process.

The efficiency of a two-stroke engine depends to a great degree on the effectiveness of the scavenging process, since bad scavenging gives a low mean indicated pressure and hence, results in a high weight and high cost per bhp for the engine. With insufficient scavenging the amount of oxygen available is low so that the consequent incomplete combustion results in higher specific fuel consumption. Not only that, the lubricating oil becomes more contaminated, so that its lubricating qualities are reduced and results in increased wear of piston and cylinder liners. Poor scavenging also leads to higher mean temperatures and greater heat stresses on the cylinder walls.

Thus it goes without saying that every improvement in the scavenging leads to improvement in engine and its efficiency in several directions and hence, a detailed study of scavenging process and different scavenging systems is worthwhile.

The scavenging process is the replacement of the products of combustion in the cylinder from the previous power stroke with fresh-air charge to be burned in the next cycle. In the absence of an exhaust stroke in every revolution of the crankshaft, this gas exchange process for a two-stroke engine must take place in its entirety at the lower portion of the piston travel. Obviously, it cannot occur instantaneously at bottom dead centre. Therefore, a portion of both the expansion stroke and the compression stroke is utilized for cylinder blow-down and recharging.

The scavenging process can be divided into four distinct periodsFig. 2.13 show the pressure recordings inside the cylinder for a Flat 782 S engine. When the

inlet port opens the gases expanding in the main cylinder tend to escape from it and to pre-discharge into the scavenge air manifold. This process, called pre-blowdown, ends when the exhaust port opens. As soon as the exhaust ports are open, the gases existing in the cylinder at the end of expansion stroke discharge spontaneously into the exhaust manifold and the pressure of the main cylinder drops to a value lower than that existing in the scavenge air manifold. This process, called blowdown, terminates at the moment the gas pressure inside the cylinder attains a value slightly lower than the air-pressure inside the scavenge manifold. During the third phase, called scavenging, which starts at the moment the spontaneous exhaust gases from the cylinder terminates and ends at the moment the exhaust ports are closed; the scavenge air sweeps out all residual gases remaining in the main cylinder at the end of the spontaneous exhaust and replaces them as completely as possible with fresh charge. After scavenging is complete the fresh charge continues to flow till the scavenge ports are open and the pressure in the cylinder rises. This results in better filling of the cylinder. This last part of the scavenging process is called additional-charging.

36

www.jntuworld.com

www.jntuworld.com


Fig. 2.13 Fiat 782 S engine standard scavenging & typical valve timing diagram of a two-stroke engine

37

www.jntuworld.com

www.jntuworld.com


Fig.2.14shows, a typical pressure-volume diagram for a two-stroke engine. In this diagram the total piston stroke has been divided into power stroke and scavenging stroke (This division is arbitrary). The area of the p-v diagram for the power stroke depends very much on the scavenging efficiency. With proper scavenging efficiency the pressure rise due to combustion is lower and hence this area is smaller and lower thermal efficiency is obtained.

Fig. 2.14 Typical pressure-volume for a two-stroke engine.

Theoretical scavenging processes

Fig. 2.15 Three theoretical scavenging processes.

Fig.2.15 illustrates three theoretical scavenging processes. They are• Perfect scavenging,• Perfect mixing and• Complete shortcircuiting.

{ The delivery ratiomass reference

cycleper mixture)(or air delivered of mass=delR , compares the actual

scavenging air mass (or mixture mass) to that required in an ideal charging process.(If scavenging is done with fuel-air mixture, as in spark-ignition engines, then mixture mass is used instead of air mass.)

The reference mass is defined as displaced volume × ambient air (or mixture) density.Ambient air (or mixture) density is determined at atmospheric conditions or at intake conditions.

This definition is useful for experimental purposes. For analytical work, it is often convenient to use the trapped cylinder mass mtr as the reference mass. OR in other words the delivery ratio is a measure to the air (mixture) supplied to the cylinder relative to the cylinder content.

If Rdel = 1, it means that the volume of the scavenging air supplied to the cylinder is equal to the cylinder volume (or displacement volume whichever is taken as reference).

Delivery ratio usually varies between 1.2 to 1.5, except for closed crankcase-scavenged, where it is less than unity.

The scavenging efficiencychargecylinder trappedof mass

retained mixture)(or air delivered of mass=scη ,

indicates to what extent the residual gases in the cylinder have been replaced with fresh air.If 1=scη , it means that all gases existing in the cylinder at the beginning of scavenging have

been swept out completely.}

38

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive Engines(I)Perfect scavenging.

Ideally, the fresh fuel-air mixture should remain separated from the residual combustion products with respect to both mass and heat transfer during the scavenging process. Fresh air pumped into the cylinder by the blower through the inlet ports at the lower end of the cylinder pushes the products of combustion ahead of itself and of the cylinder through the exhaust valve at the other end. There is no mixing of air and products. As long as any products remain in the cylinder the flow through the exhaust valves consists of products only. However, as soon as sufficient fresh .air has entered to fill the entire cylinder volume (displacement plus clearance volume) the flow abruptly changes from one of products to one of air. This ideal process would represent perfect scavenging with no short -circuiting loss.

(ii) Perfect mixing.The second theoretical scavenging process is perfect mixing, in which the incoming fresh charge mixes completely and instantaneously with the cylinder contents, and a portion of this mixture passes out of the exhaust ports at a rate equal to that entering the cylinder. This homogeneous mixture consists initially of products of combustion only and then gradually changes to pure air. This mixture flowing through the exhaust ports is identical with that momentarily existing in the cylinder and changes with it. For the case of perfect mixing the scavenging efficiency can be represented by the following equation:

delRsc e−−= 1η , where scη and Rdel are scavenging efficiency and delivery ratio respectively.

This is plotted in Fig. 2.15. The result of this theoretical process closely approximates the results of many actual scavenging processes, and is thus often used as a basis of comparison.

(iii)Short-circuiting.The third type of scavenging process is that of short-circuiting in which the fresh charge coming from the scavenge manifold directly goes out of the exhaust ports without removing any residual gas. This is a dead loss and its occurrence must be avoided.The actual scavenging process is neither one of perfect scavenging nor perfect mixing. It probably consists partially of perfect scavenging, mixing and short-circuiting.Fig. 2.16shows the delivery ratio and trapping efficiency variation with crankangle for three different scavenging modes., i.e.,perfect scavenging (displacement), perfect mixing and intermediate scavenging.Fig. 2.17shows the scavenging parameters for the intermediate scavenging. This represents the actual scavenging process. It can be seen from this Fig. that a certain amount of combustion products is initially pushed out of the cylinder without being diluted by fresh air. Gradually, mixingand short circuiting causes the out flowing products to be diluted by more and more fresh air until ultimately the situation is the same as for perfect mixing, i.e., the first phase of the scavenging process is a perfect scavenging process which then gradually changes into a complete mixing process.

Fig,2.16 Delivery ratio and efficiency variation with ` Fig. 2.17 Scavenging parameters for crankcase for three different scavenging modes. intermediate scavenging

39

www.jntuworld.com

www.jntuworld.com


Scavenging parameters ..The delivery ratio - The delivery ratio represents the ratio of the air volume, under the ambient

conditions of the scavenge manifold, introduced per cycle and a reference volume. This reference volume has been variously chosen to be displacement volume, effective displacement volume, total cylinder volume or total effective cylinder volume. Since it is only the quantity or charge in the remaining total cylinder volume at exhaust port closure that enters into the combustion, the total effective cylinder volume should be preferred. The delivery ratio is mass of fresh air delivered to the cylinder divided by a reference mass,

i.e.,mass reference

cycleper mixture)(or air delivered of mass=delR ,

The delivery ratio compares the actual scavenging air mass (or mixture mass) to that required in an ideal charging process. OR The delivery ratio is a measure to the air (mixture) supplied to the cylinder relative to the cylinder content.

If Rdel = 1, it means that the volume of the scavenging air supplied to the cylinder is equal to the cylinder volume (or displacement volume whichever is taken as reference).

Delivery ratio usually varies between 1.2 to 1.5, except for closed crankcase-scavenged, where it is less than unity.

(If scavenging is done with fuel-air mixture, as in spark-ignition engines, then mixture mass is used instead of air mass.) The reference mass is defined as displaced volume × ambient air (or mixture) density.

Ambient air (or mixture) density is determined at atmospheric conditions or at intake conditions. This definition is useful for experimental purposes. For analytical work, it is often convenient to use the trapped cylinder mass mtr as the reference mass.

The trapping efficiency - The amount of fresh charge retained in the cylinder is not same as that supplied to the cylinder because some fresh charge is always lost due to short-circuiting. Therefore, an additional term, trapping efficiency, is used to indicate the ability of the cylinder to retain the fresh charge. It is defined as the ratio of the amount of charge retained in the cylinder to the total charge

delivered to the engine, i.e., (mixture)air delivered of mass

retained mixture)(or air delivered of mass=trη

Trapping efficiency indicates what fraction of the air (or mixture) supplied to the cylinder is retained in the cylinder. This is mainly controlled by the geometry of the ports and the overlap time.

The scavenging efficiency Scavenging efficiency is the ratio of the mass of scavenge air which remains in the cylinder at the end of the scavenging to the mass of the cylinder itself at the moment when the scavenge and exhaust ports of valves are fully closed. It is given by

chargecylinder trappedof mass


indicates to what extent the residual gases in the cylinder have been replaced with fresh air.If 1=scη , it means that all gases existing in the cylinder at the beginning of scavenging have

been swept out completely.

The purity of the charge:chargecylinder trappedof mass

chargecylinder in trappedair of masspurity = , indicates

the degree of dilution, with burned gases, of the unburned mixture in the cylinder.

The charging efficiency densityambient x volumedisplaced

retained mixture)(or air delivered of mass=chη , indicates

how effectively the cylinder volume has been filled with fresh air (or mixture)Relative cylinder charge.- The air or mixture retained, together with the residual gas, remaining

in the cylinder after flushing out the products of combustion constitutes the cylinder charge. Relative cylinder charge is a measure of the success of filling cylinder irrespective of the composition of charge. The relative cylinder charge may be either more or less than unity depending upon the scavenging pressure and port heights.

40

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesExcess air factor,λ - The value (Rdel-1) is called the excess air factor. If the delivery ratio is 1.4,

the excess air factor is 0.4.

Classification based on scavenging processThe simplest method of introducing the charge into the cylinder is to employ crankcase

compression as shown in Fig.2.7. This type of engine is classified as the crankcase scavenged engine.In another type, a separate blower or a pump (Fig.2.8) may be used to introduce the charge through the inlet port. They are classified as the separately scavenged engines.

Fig.2.16 Methods of Scavenging (a)Cross Scavenging (b) Loop Scavenging, M.A.N. Type(c)Loop Scavenging Schüürle Type, (d) Loop Scavenging, Curtis Type

Another classification of two-stroke cycle engines is based on the air flow.Based on a transversal air stream, the most common arrangement is cross scavenging, illustrated

in Fig.2.16 (a). Most small engines are cross-scavenged. The cross scavenging system employs inlet and exhaust ports placed in opposite sides of the cylinder wall. The incoming air is directed upward, to combustion chamber on one side of the cylinder and then down on the other side to force out the exhaust gases through the oppositely located exhaust ports. This requires that the air should be guided by use of either a suitably shaped deflector formed on piston top or by use of inclined ports. With this arrangement the engine is structurally simpler than that with the uniflow scavenging, due to absence of valves, distributors, and relative drive devices. The inlet and exhaust of gases is exclusively controlled by the .opening and closure of ports by piston motion. The main disadvantage of this system is that the scavenging air is not able to get rid of the layer of exhaust gas near the wall resulting in poor scavenging. Some of the fresh charge also goes directly into the exhaust port. The result of these factors is poor bmep of cross-scavenged engines.

Based on a transversal air stream, with loop or reverse scavenging, the fresh air first sweeps across the piston top, moves up and then down and finally out through the exhaust. Loop or reverse scavenging avoids the short -circuiting of the cross-scavenged engine and thus improves upon its scavenging efficiency. The inlet and exhaust ports are placed on the same side of the cylinder wall.In the M.A.N. type of loop scavenge, Fig.2.16(b), the exhaust and inlet ports are on the same side, the exhaust above the inlet.

In the Schnuerle type, Fig.2.16(c), the ports are side by side. the inlet ports are placed on both sides of the exhaust ports so that the incoming air enters in two streams uniting on the cylinder wall opposite the exhaust ports, flows upwards, turns under the cylinder head, then flows downwards the other side to the exhaust ports. Such a system of air deflection reduces the possibilities of short-circuiting to minimum. With this system flat-top pistons without deflectors are used. The speed of loop or reversed scavenged engine is not restricted by mechanical limitations because valves are not used, the charging process being controlled by the piston only. The speed can thus, exceed that of valve controlled two-stroke engines. Owing to the absence of cams, valves and valve gear, engines are simple and sturdy. They have a high resistance to thermal stresses and are, thus, well suited to higher

41

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive Enginessupercharge. The major mechanical problem with a loop scavenged two-stroke engine is that of obtaining an adequate oil supply to the cylinder wall consistent with reasonable lubricating oil consumption and cylinder wear. This difficulty arises because when the piston is at top dead centre there is only a very narrow sealing belt available to prevent leakage of oil from crankcase into the exhaust ports. Since for loop scavenging greater cylinder distance is necessary to accommodate scavenge-air passage between the cylinder, a strong connecting rod and crankshaft need for supercharged engine can be used.

The Curtis type of scavenging, Fig.2.16(d), is similar to the Schnuerle type, except that upwardly directed inlet ports are placed also opposite the exhaust ports.

The most perfect method of scavenging is the uniflow method, based on a unidirectional air stream. The fresh air charge is admitted at one end of the cylinder and the exhaust escapes at the other end flowing through according to parallel flow lines normally having a slight rotation to stabilize the vertical motion. Air acts like an ideal piston and pushed on the residual gas in the cylinder after the blowdown period and replaces it at least in principle, throughout the cylinder. The air flow is from end to end, and little short-circuiting between the intake and exhaust openings is possible. Due to absence, at least in theory, of any eddies or turbulence it is easier in a uniflow scavenging system to push the products of combustion out of the cylinder without mixing with it and short circuiting. Thus, the uniflow system has highest scavenging efficiency. Construction simplicity is, however, sacrificed because this system requires either opposed pistons, poppet valves or sleeve valve all of which increases the complication.

The three available arrangements for uniflow scavenging are shown in Fig.2.17 A poppet valve is used in (a) to admit the inlet air or for the exhaust, as the Case may be. In (b) the inlet and exhaust ports are both controlled by separate pistons that move in opposite directions. In (c) the inlet and exhaust ports are controlled by the combined motion of piston and sleeve. In an alternative arrangement one set of ports is controlled by the piston and the other set by a sleeve or slide valve. All uniflow systems permit unsymmetrical scavenging and supercharging.

Fig.2.17 Uniflow Scavenging (a) Poppet Valve(b) Opposed Piston(c) Sleeve Valve

42

www.jntuworld.com

www.jntuworld.com


Reverse flow scavenging is shown in Fig.2.17 In this type the inclined ports are used and the scavenging air is forced on to the opposite wall of the

cylinder where it is reversed to the outlet ports. One obvious disadvantage of this type is the limitation on the port area. For long stroke engines operating at

low piston speeds, this arrangement has proved satisfactory.

Fig2.17 Reverse Flow Scavenging

An interesting comparison of the merits of two cycle engine air scavenging methods is illustrated in Fig.2.18. In fact, specific output of the engine is largely determined by the efficiency of the scavenging system-and is directly related to the brake mean effective pressure. As shown in Fig.2.18 scavenging efficiency varies with the delivery ratio and the type of scavenging. In this respect cross scavenging is least efficient and gives the lowest brake mean effective pressure. The main reason for this is that the scavenging air flows through the cylinder but does not expel the exhaust residual gases effectively. Loop scavenging method is better than the cross scavenging method. Even with a delivery ratio of 1.0 in all cases the scavenging efficiencies are about 53, 67 and 80 per cent for cross scavenging, loop scavenging and uniflow scavenging systems with corresponding values of bmep as 3.5,4.5 and 5.8 bar.

Fig.2.18 Scavenging Efficiency

Comparison of different scavengingsystemsFig.2.19 compares the scavenging efficiencies of three different types of scavenging system.

The cross-scavenging system employs inlet and exhaust ports placed in opposite sides of the cylinder wall. In the loop scavenging system, inlet and exhaust ports are in the same side of the cylinder wall and in uniflow scavenging system, the inlet and exhaust port are at opposite ends of the cylinder.It can be seen that uniflow scavenging gives by far the best scavenging, that loop scavenging is good, and that in .general, cross-scavenging is the worst.

The scavenging curve for the uniflow scavenging is very near to that of perfect scavenging that for loop scavenging is near the perfect mixing. With good loop scavenging the scavenging curve is generally above the perfect mixing curve and that of cross-scavenging engines it is, generally, below the perfect mixing curve.

Table 2.2 compares the port areas available for different scavenging systems. Largest flow areas are available with uniflow system. In such a case the whole circumference of cylinder wall is available

43

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive Enginesand the inlet port area can be as high as 35 per cent of the piston area. Due to the use of exhaust valve the exhaust flow area is small - about 18 per cent. In cross-scavenging the size of the inlet and exhaust ports is limited to about 25 and 18 per cent of piston area respectively because the ports are located on the opposite sides of cylinder wall. Schurnle type of loop scavenging requires that both the ports must be located within about three-quarters of the cylinder circumference. This limits the size of inlet and exhaust ports to about 18 and 14 per cent of piston area only. The data for a typical four-stroke engine are also given for comparison. However, while comparing with the four-stroke engine it must be kept in mind that though the flow area is small, the time available for flow is almost three times more than that available for the two-stroke engine.

Fig. 2.19 scavenging efficiency, versus delivery ratio of different scavenging system.

Table 2.2 Typical values for areas for different scavenging systems

Loop or cross-scavenged engines with their inlet ports limited half of the cylinder circumference fall in low speed category. Uniflow scavenged engines with adequate air inlet port are and limited exhaust port areas fall in medium speed category, whilst the opposed piston engine takes on to high speeds because of its high rate of exhaust port opening, freedom from valve gear speed limits, good scavenging and perfect balancing. Un-supercharged uniflow engine has a considerable higher mean effective pressure than the loop-scavenged engine. There is more freedom in design of combustion chamber for loop scavenging. This results in low fuel consumption and the engine is simple to make and easy to produce. Table 2.3 compares the typical bmep values obtainable with different types of

44

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive Enginesscavenging systems. The output of both uniflow and loop scavenged engines is limited 'by the thermal stresses imposed. But the loop scavenged engine due to its simple cylinder head can better withstand the thermal stresses.

45

www.jntuworld.com

www.jntuworld.com


Table 2.3 Typicalvaluesof bmep for the C.I. two-stroke oil engines

Table 2.4compares the representative port timings for different types of two-stroke engines.

Table 2.4. Port timings for different two-stroke engines

Port design

The Design of the inlet and exhaust ports for two stroke engines depends on various parameters. Some of the important basic parameters are;

a) Scavenging methodb) Shape, inclination & width of portsc) Amount of air/charge deliveredd) Scavenging pressuree) Mean inlet velocity –fn. Of pr. Ratio, temp. of scavenging & scavenging factorf) Duration(crank angle) of port opening & average port height uncovered by piston

Blowdown time area (for exhaust)–[which is a fn. of temperature of exhaust Gas, expansion end volume(fn. of displacement volume), exhaust Gas pr., scavenging pr., & indicated mean effective pressure]

g) Inlet duration, exhaust lead* & hence exhaust durationh) Number of ports & height of ports

*during exhaust Lead, only exhaust port is kept open, & during super charging only inlet port is kept open.

46

www.jntuworld.com

www.jntuworld.com


• THE DIFFERENT SCAVENGING METHODS ARE AS FOLLOWS

BASED ONSCAVENGING PROCESS( AIR FLOW )

I. CROSS FLOW -for low power o/p engines eg. Two wheelers,Simple, but more short circuiting, hence more charge loss, super charging is not possible. It is found that port position is limited with in 50% of circumference.

II. LOOP FLOW -for medium o/p engines.Air takes loop, less short circuiting, hence less charge loss

A. MAN type -intake & exh. ports positioned one below the other. -GoodB. SCHNURLE type-intake & exh. ports positioned side by side. -BetterC. CURTIS type -intake on one side & exhaust on the other side. -Best

III. UNIFLOW (BEST) –for very High o/p enginesEx. large power marine engines, locomotive engines etcAs intake port is on one side & exhaust port on the other side. & the flow is uni-directional, ports can be wider. Residual gases are low. Ports can be located all around the circumference. Opposed piston engines also use this type. Ports with poppet valves & Sleeve valves have been used.

BASED ON SCAVENGING METHOD

I. CRANKCASE SCAVENGED ENGINE (crank case compression)-petroil lubrication is adopted. Hence lubricating oil is also burnt. So pollution is more. Compression is bad, more petrol consumption, and more residual gases. Generally used along with symmetrically scavenged engine, but lower delivery ratio (generally 0.7), Simple and suitable for small engines. Suitable for low o/p engines (5-20bhp)

II. SEPARATE BLOWER / PUMP SCAVENGED ENGINE-higher scavenging pressure & delivery ratio is possible. Residual gases are low. Used in bulky arrangements i.e. above 100 hp engines

BASED ON OVERALL PORT TIMING

I. SYMMETRICAL PORT TIMING - EPO-IPO-IPC-EPC-Opening and closing of the ports by the piston is symmetrical.Advantage-arrangement of the mechanism is very simple.Disadvantage- more short circuiting, hence more charge loss, super charging is not possible. Suitable for low power o/p engines up to 5bhp i.e. scooters / moped engines.

II. UN-SYMMETRICAL PORT TIMING - EPO-IPO-EPC-IPC-Opening and closing of the ports by the piston is un-symmetrical. Mechanism is complex. Advantages- super charging is possible - by the following ways

Supercharging valve-rotary valves,Poppet valves by suitably designing the cam mechanism,Using sleeve /slide valve, but it is mechanically complicated,

47

A c

ompa

riso

n

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive Engines& using opposed piston

• The common different Shapes of ports are as follows

Rectangular -BEST

With rounded corners, which gives maximum flow area & smooth edges reduce friction &

Rhomboidal & Oblong -good w.r.to ring entrance avoidance

Circular-only some applications (only for intake)

Inclination -is given for better mixing, scavenging, turbulence, swirl and combustion.Width -for Uniflow scavenging -0.6πD (entire circumference available for porting

-for lLoop scavenging -0.2πD (both ports are on same side of the wall)-for Crossflow scavenging -0.3πD (50% of circumference is available for porting)

Ports should be sufficiently wider for max. flow area, But should not create problem of piston ring entrance into it.

• Amount of air/charge delivered The delivery ratio is a measure of the air (mixture) supplied to the cylinder relative to the cylinder content.

The delivery ratiomass reference

cycleper mixture)(or air delivered of mass=delR ,

If Rdel = 1, it means that the volume of the scavenging air supplied to the cylinder is equal to the cylinder volume (or displacement volume whichever is taken as reference).Delivery ratio usually varies between 1.2 to 1.5, except for closed crankcase-scavenged, where it is less than unity.Rdel = 0.7 to 0.8 – for crank case scavengingRdel = 1.4 –normal valueRdel = 1.3 –for fuel economyRdel = 1.5 –for high o/p

The scavenging efficiencychargecylinder trappedof mass


Indicates to what extent the residual gases in the cylinder have been replaced with fresh air.If 1=scη , i.e. all gases existing in the cylinder at the beginning of scavenging have been swept out completely}

• Scavenging pressure Proper scavenging pressures to be adopted for the respective scavenging method

• Mean inlet velocity Mean inlet velocity to be calculated, which is a function of pressure ratio, temp. of scavenging & scavenging factor.

• Duration(crank angle) of port opening & average port height uncovered by piston With Duration (crank angle) of port opening, average port height & port timing can be calculated.

48

For separately scavenged engines

www.jntuworld.com

www.jntuworld.com


• Number of ports & height of ports. No. of ports are selected to ensure enough (max.) width, with sufficient bridge to sustain mechanical and thermal load & to avoid piston ring failure i.e. entering in port area. After selecting no. of ports, width of the ports may be calculated and adopted. The height of ports is a major factor in timing of ports.

The flow of gases through a two-stroke cycle engine is diagrammatically represented in fig. The hatched areas represent fresh air or mixture and the cross hatched areas represent combustion gases. The width of the channels represents the quantity of the gases expressed by volume at NTP condition.

Fig. Scavenging Diagram for Two-stroke Cycle SI Engine

49

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesScavenging pumps

Since the pumping action is not carried out by the piston of a two-stroke engine, a separate pumping mechanism, called the scavenging pump, is required to supply scavenging air to the cylinder. Different types of scavenging pumps used range from crankcase compression, piston type blowers to roots blower. The design of a two-stroke engine is significantly affected by the type of scavenging pump used; hence a careful selection of the scavenging pump is a pre-condition to good performance. Crankcase Scavenging. The most obvious and cheapest in initial cost is the use of crankcase for compressing the incoming air and then transferring it to the cylinder through a transfer port. Fig.2.20 shows such a system. This system is, however, very uneconomical and inefficient in operation. This is because the amount of air which can be used for scavenging is less than the swept volume of the cylinder due to low volumetric efficiency of the crankcase which contains a large dead space. Thus, the delivery ratio of a crankcase scavenged engine is always less than unity.

Since the delivery ratio is less than unity it is not possible to scavenge the cylinder completely of the products of combustion and some residual gases always remain in the cylinder. This results in low mean effective pressure for the crankcase scavenged engine. Typical values are 3 to 4 bar. The output of the engine is strictly limited because the amount of the charge transferred through the transfer port is only 40-50% of the cylinder volume.

(a) two ports (b) three portsFig. 2.20 Two-stoke crankcase scavenged engines

A further disadvantage is that the oil vapors from the crankcase mixes with the scavenging air. This results in high oil consumption. Because of these disadvantages the crankcase scavenging is not preferred and for high output two-stroke engines a scavenging pump is a must.Piston, Roots, and Centrifugal blowers

Piston type blowers as shown in Fig.2.21(a) are used only for low speed and single or two cylinder engines. For all other type of engines either roots or centrifugal blowers are used. The roots blower is preferred for small and medium output engines. While the centrifugal blower, is preferred for large and high output engines. From Fig. 2.22 it is clear that the centrifugal blower has a relatively flat characteristic curve compared to the steep characteristic curve of the 'roots blower. An increase in the flow-resistance due to deposits, etc., thus, has a much greater effect on the scavenging air; output of a centrifugal blower than on that of a roots blower. If deposits accumulate, an engine having a centrifugal blower will start smoking earlier than that having a roots blower. Therefore, roots blower is preferred due to its lower sensitivity to flow resistance changes for systems where space for exhaust ports is limited.The control of air delivery of centrifugal blowers can be done by throttling the air on the intake side. This, however, would not reduce the scavenging power required by the centrifugal blower. In the roots blower the air delivery is controlled by a throttle-actuated by-pass valve between blower inlet and outlet. Such a control divides the air-flow into two parts and only half the flow passes through the engine. This saves a substantial amount of scavenging power and hence results in lower specific fuel consumption.

50

www.jntuworld.com

www.jntuworld.com


Fig. 2.21 Scavenging-pump types.

Fig. 2.22 Pressure characteristics of centrifugal and roots blower.

51

www.jntuworld.com

www.jntuworld.com


Comparison of two-stroke SI and CI engines

The two-stroke SI engine suffers from two big disadvantages-fuel loss and idling difficulty. The two-stroke CI engine does not suffer from these disadvantages and hence CI engine is more suitable for two-stroke operation.

If the fuel is supplied to the cylinders after the exhaust ports are closed, there will be no loss of fuel and the indicated thermal efficiency of the two-stroke engine will be as good as that of four-stroke engine. However, in an SI engine using carburettor, the scavenging is done with fuel-air mixture and only the fuel mixed with the retained air is used for combustion. To avoid the fuel loss instead of carburettor fuel injection just before the exhaust port closure may be used.

The two-stroke SI engine runs irregularly and may even stop at low speeds when mean effect pressure is reduced to about 2bar. This is because large amount of residual gas (more than in four-stroke engine) mixing with small amount of charge. At low speeds there may be back firing due to slow burning rate. Fuel injection improves idling and also eliminates backfiring as there is no fuel present in the inlet system.

In CI engines there is no loss of fuel as the charge is only air and there is no difficulty at idling because the fresh charge (air) is not reduced.

Advantages and disadvantages of two-stroke enginesTwo-stroke engines have certain advantages as well as disadvantages compared to four-stroke

engines. In the following sections the main advantages and disadvantages are discussed briefly.

Advantages of Two-stroke Engines(i) As there is a working stroke for each revolution, the power developed will be nearly twice that of a four-stroke engine of the same dimensions and operating at the same speed.(ii) The work required to overcome the friction of the exhaust and suction strokes is saved.(iii) As there is a working stroke in every revolution, a more uniform turning moment is obtained on the crankshaft and therefore, a lighter flywheel is required.(iv) Two-stroke engines are lighter than four-stroke engines for the same power output and speed.(v) For the same output, two-stroke engines occupy lesser space.(vi) The construction of a two-stroke cycle engine is simple because it has ports instead of valves. This reduces the maintenance problems considerably.(vii) In case of two-stroke engines because of scavenging, burnt gases do not remain in the clearance space as in case of four-stroke engines.

Disadvantages of Two-Stroke Engines(i) High speed two-stroke engines are less efficient owing to the reduced volumetric efficiency.(ii) With engines working on Otto cycle, a part of the fresh mixture is lost as it escapes through the exhaust port during scavenging. This increases the fuel consumption and reduces the thermal efficiency.(iii) Part of the piston stroke is lost with the provision of the ports thus the effective compression is less in case of two-stroke engines.(iv) Two-stroke engines are liable to cause a heavier consumption of lubricating oil.(v) With heavy loads, two-stroke engines get heated due to excessive heat produced. Also at light loads, the running of engine is not very smooth because of the increased dilution of charge.

52

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesSI and CI Engine application

We have seen that both SI and CI engines have certain advantages and disadvantages. The selection of a type of engine for particular application needs consideration of various factors.The SI engine offers the following advantages:(1) Low initial cost.(2) Low weight for a given power output.(3) Smaller size for a given power output.(4) Easy starting.(5) Less noise.(6) Less objectionable exhaust gas odor and less smoke.

The SI engine finds wide application in automobiles because passenger comfort and in small airplanes because of low weight. Two stroke petrol engines finds extensive use in motor cycles, scooters, mopeds, pleasure motor boats, etc., because of simplicity and low cost. The SI engine is also used for light mobile duty like lawn movers, mobile generating sets, water pumps, air compressors, etc...

The CI engine offers the following advantages.(1) Low specific fuel consumption at both full load and part load conditions.(2) Utilizes less expensive fuels.(3) Reduced fire hazard,(4) Long operating life.(5) Better suited for supercharging.(6) Better suited for two-stroke cycle operating, as there is no loss of fuel in scavenging.Because of fuel economy the CI engine finds wide usage in buses, trucks, locomotives, stationary generating plants, heavy duty equipment such as bulldozers, tractors and earthmoving machinery. Because of the reduced fire hazard the CI engine is also used for confined installations and marine use. The great advantage of the CI engine is lower fuel consumption which counteracts the disadvantage of higher initial cost, if the engine is used for long duties. (Table 2.6a gives complete comparison of the two types of engines.)

Comparison of two-stroke and four-stroke- engines (table 2.5)The two-stroke engine was developed to obtain valve simplification and a greater output from

the same size of engine. Two-stroke engines have no valves but only ports (some two-stroke engines are fitted with conventional exhaust valve). This simplicity of the two-stroke engine makes it cheaper to produce.Theoretically a two-stroke engine will develop twice the power of a comparable four-stroke engine because of one power stroke every revolution (compared to one power stroke every two revolutions of four-stroke engine). This makes the two-stroke engine cheaper and more compact than a comparable four-stroke engine.

In actual practice power is not exactly doubled but is only about 30% extra because of (a) reduced effective stroke, and (b) due to increased heating caused by increased power strokes. The maximum speed is kept less than 4-stroke engine. The other advantages of the two-stroke engine are more uniform torque on crankshaft and complete exhaust of products of combustion.

However, when applied to spark-ignition engine the two-stroke cycle has certain disadvantages which have restricted its use to only small engines suitable for motor cycles, scooters, mopeds, lawn mowers, out-board engines, etc. In spark-ignition engine (petrol engine) the charge consists of a mixture of air and fuel. During scavenging, as both inlet and exhaust ports are open simultaneously for some time, some part of the fresh charge containing fuel escapes with exhaust. This results in high fuel consumption and hence lower thermal efficiency. The other drawback of two-stroke SI engine is the lack of flexibility- the capacity to run with equal efficiency at any speed. If the throttle is closed below the best point, the amount of fresh mixture entering the cylinder is not enough to clear out all the exhaust, some of which remains to contaminate the fresh charge. This results in irregular running of the engine.

53

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesThe two-stroke diesel engine does not suffer from these defects. There is no loss of fuel with exhaust gases as the intake charge in diesel engine is air only. The two-stroke diesel engine is therefore used quite widely. Many of the biggest diesel engines work on this cycle. They are generally bigger than 60cm bore and are used in marine propulsion.

A disadvantage common to all two-stroke engines, petrol as well as diesel, is greater cooling and lubrication requirements due to one power stroke in each revolution of crankshaft. Consumption of lubricating oil is also high in the two-stroke engine due to higher temperatures.

Table 2.5 Comparison of four-stroke and two-stroke cycle engines

54

www.jntuworld.com

www.jntuworld.com


Fundamental differences between SI and CI enginesBoth SI and CI engines are internal combustion engines and have much in common. However,

there are also certain fundamental differences that cause their operation to vary considerably. These are given in Table 2.6

Table 2.6 Comparison of SI and CI engines

55

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive Enginestable 2.6a detailed comparison of SI & CI engines

56

www.jntuworld.com

www.jntuworld.com


References-6. Theory & Practice of I C Engines By C F Taylor7. Fundamentals of I C Engines By J B Heywood8. I C Engines By M L Mathur & RP Sharma9. I C Engines By Ganesan

57

www.jntuworld.com

www.jntuworld.com


Chapter-3

Cylinder BlockForms the basic frame work of the engine it houses engine cylinders, where combustion take

place & serves as a bearing & guide for piston reciprocating in it. It carries lubricating oil to various components through drilled passages.

At lower end the crank case is cast integral with the block. At the top, is attached the cylinder head. Besides, other parts like timing gear, water pump, ignition distributor, flywheel, fuel pump etc. are also attached

Around cylinders, there are passages for circulation of cooling water

Cylinder heads, Cylinders & linersMost modern automotive engines have all of their cylinders and the greater part of their

crankcase poured in a single casting, so that cylinders and crankcase form a single unit. However, cylinders and crankcase perform different functions.

Separate Vs. Integral Cylinder Heads. Cylinder heads now almost always are made separate castings, which are secured to the cylinder

block with studs and nuts, with a gasket in between to ensure a gas-tight joint. The cylinder head can be cast integral with the block, and at one period in engine development that was the predominant practice.

With integral cylinder heads there is, of course, no machining of joint surfaces and no need for a gasket, but the cylinder casting is much more difficult to produce, and. besides, with the design which was usually employed, cooling of the combustion-chamber walls was less effective-the wall temperature of each combustion chamber being less uniform-than in an engine with a detachable head.

58

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesIn the case of L-head engines with integral cylinder heads, the valves were introduced through

openings in the head which were closed by threaded plugs generally referred to as "valve caps." These plugs presented to the hot gases in the cylinder a considerable surface which was not water-cooled, and which therefore formed "hot spots." It was customary to screw the spark plug into one of these "valve caps." Since the insulator of the plug naturally is a poor conductor of heat, and the additional threaded joint also formed an obstruction to heat flow, this further aggravated the situation with respect to "hot spots" and made it necessary to keep the compression quite low.

With the valve-in-head type of cylinder there are two alternate designs of integral heads. With one of these, exemplified in Fig, 1, the valves seat directly on the metal of the head, but this has the disadvantage that when they are to be reground, the whole block has to be removed from the car. With the other, use is made of so-called valve cages, that is, cylindrical sleeves which are set into bores in the cylinder head and retained therein between a shoulder and a ring nut. The valve seat is fom1ed on the inner end of the cage, and there is a port in the wall of the latter through which the gases flow from or into a valve passage cast in the cylinder head. The objection to valve cages is that they add another "joint" to the path for heat flow from the valve head to the jacket water, and therefore result in higher valve temperatures (particularly of the exhaust valve), which promotes detonation and makes the construction unsuitable for high speed, high-compression engines.

Fig.1. Cylinder with integral head

When the cylinder head is a detachable casting, the cylinder and jacket cores can be more securely supported in the mold, and the cylinder castings are likely to be more nearly true to pattern, with the result that after the cylinder is finished, its walls will be more nearly uniform in thickness.

With an engine having a removable head it is possible to thoroughly clean the combustion chamber of carbon, by scraping, after the head has been removed. If it is desired to locate the valves in the head, they may be seated directly on a water-cooled surface.

One reason for the continued, limited use of integral heads is that they avoid trouble due to distortion of the upper or outer end of the cylinder bore due to the drawing up of the cylinder-head retaining nuts. Such trouble is experienced occasionally, with detachable cylinder heads (blow-by past piston rings, leakage past valves, and excessive oil consumption), but it can be guarded against by performing the final finishing operation on the bore with a dummy cylinder head in place~ This produces a bore which is true when the retaining nuts are tightened.

GasketsCopper-Asbestos Gaskets.

Separate cylinder heads were rendered practical by the introduction of the copper-asbestos gasket. This consists of an asbestos sheet cut or stamped to the required form, which is armored with thin sheet copper. There is a copper sheet on each side of the asbestos sheet, and the two copper sheets lap along the outer edges of the asbestos sheet, so that the latter is completely encased. Copper grommets are inserted in the waterway openings and sometimes also in the combustion-chamber openings. In heavy duty engines the combustion-chamber grommet of the gasket may be reinforced by a copper-wire loop or a copper washer. In these copper-asbestos gaskets the copper provides the tenacity and the asbestos the compressibility needed in a packing. A gasket for a four-cylinder L-head engine is shown in Fig.2.

59

www.jntuworld.com

www.jntuworld.com


Steel-Encased and Other Gaskets.Cylinder-head gaskets are made also of asbestos sheet encased in steel instead of copper. Cold-

rolled, deep-drawing steel is used, and is rust-proofed to prevent trouble from corrosion. Among the rust-proofing processes applied to sheet steel for gaskets are tinning, electro-galvanizing, and terne-plating. Steel, being harder, does not have as good sealing properties as copper, and a sealing coat of some heat-resistant, non-hardening material is generally applied to the gasket, either in the manufacturing process or during installation. The edges of the steel sheet, of course, are not rust-proofed, and some steel-encased gaskets are fitted with copper grommets at the waterways. The principal advantage of steel- over copper-encased gaskets is that the production cost of the former is about 20 per cent less.

Another type of gasket comprises a central steel core with a layer of .coated and graphited asbestos on each side thereof, the asbestos being bonded to the core by means of integral steel tangs clinched into it. These gaskets, which are used chiefly in the engines of low-priced passenger cars, generally are provided with steel grommets at the combustion-chamber and waterway openings, one manufacturer is using a cylinder head gasket consisting of a sheet of SAE No. 1010 steel 0.015 in. thick, which is corrugated around the openings therein, including those for the cylinder-head studs. The corrugations have a spring action. and the sealing properties of the gasket are further improved by applying a coating of a heat-resistant lacquer to both sides.

Cylinder-Head Studs.To obtain a gas-tight permanent joint with a cylinder-head gasket it is necessary to make

provision for an adequate number of studs distributed as nearly uniformly as possible. With L-head cylinders from 16 to 20 studs are used for a four-cylinder block, from 24 to 26 for a six-cylinder, and from 30 to 32for an eight-cylinder. With , valve-in-head cylinders only two rows of studs are required, instead of three, and the total number therefore is less, viz., 12 for a four-cylinder block, 16 for a six- cylinder, and 20 for an eight-cylinder. To prevent distortion of the casting by drawing up the nuts, there must be plenty of metal in the bosses for the studs, and the studs must not be too near the valve seats. In the design of the heads careful attention must be given to the avoidance of pockets which might form steam traps. It is not necessary to use very large water ports. Moderate-sized ports judiciously distributed, are better, as they make it easier to prevent leaks.

Cylinder Material.In the past automobile-engine cylinders have been generally cast of close-grained gray iron

approximating the following composition.Percent

Silicon 1.9 to 2.2Sulphur not over 0.12Phosphorus not over 0.15Manganese 0.6 to 0.9Combined carbon 0.35 to 0.55Total carbon 3.2 to 3.4

The SAE has standardized five grades of cast iron, of which four are recommended for cylinder blocks and cylinder heads as follows: No. 111 for small cylinder blocks; No. 120 for cylinder blocks generally. No.121 for truck and tractor-, and No. 122 for diesel engine cylinder blocks. Pistons also are cast of these irons.

It was determined from tests conducted, that to obtain the better physical properties the total carbon & silicon contents must be reduced and the phosphorus content held to a lower limit.

Among other points usually covered in specifications for cylinder castings arc the following: Castings must be smooth, well cleaned and free from shrinkage cavities, cracks and holes, large inclusions, chills, excess free carbides and any other defects detrimental to machinability, appearance, or performance. They must finish to the size specified. When tensile tests are provided for, the portion of the casting from which the test piece is to be machined is usually specified. .

60

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesThe use of steel for cylinders has often been suggested, and for racing and aircraft engines,

cylinders are sometimes made from hollow steel forgings. Several American manufacturers use cylinder castings of semi-steel, more properly called high-test cast iron. This material is made by adding a certain percentage of scrap steel to the melt of cast iron, which results in a finer grain and in somewhat better tensile properties.

To make it possible to successfully cast a multiple-cylinder block with thin walls, the iron must pour well and have a "long life" (as the foundry men call it). These characteristics are strengthened, by high phosphorus content, but, unfortunately, this element tends to make the iron soft and less resistant to wear.

Nickel-Chromium irons.Certain iron ore mined in Cuba contains small percentages of nickel and chromium, and the

metal made from this are, known as Mayari iron, is sometimes added to gray iron for cylinder castings: Mayan iron therefore is a natural alloy. It is claimed that it is free from oxidation & has a lower solidification point, and that the "longer life" of the iron improves the "feeding" of castings when they are properly gated, in spite of low phosphorus content. Castings when sectioned -show sound metal even where there are heavy bosses and thick sections. Cylinder castings made of a mixture containing 10 per cent of Mayari iron showed a tensile strength of 36,740 psi, according to makers of the iron; a transverse strength of 4250 lb, and a Brinell hardness of 223-229. The same iron is also used for cylinder heads and pistons. Results similar to those from Mayari iron are being obtained by the addition of small quantities of nickel and chromium, and such alloy irons are now used not only for cylinder blocks, but also for pistons, particularly for heavy duty, commercial-vehicle engines.

The chief advantage of alloyed irons is that they possess greater hardness and wear resistance, and that without being harder to machine. The machinability of grey iron is dependent upon the absence of excess iron carbide of chilled or hard spots. Nickel acts to eliminate both, and so to improve machinability. In many cases the alloyed iron, although having a Brinell hardness from 30 to 40 points greater, is actually easier to machine than ordinary gray iron.

When nickel is used alone as an alloying element, the content usually ranges between 1.25 and 2.5%, whereas if it is used in combination with chromium, the nickel content ranges between o.50 and 1.50 % and that of chromium between 0.25 and 0.50 % it is claimed that a combed content of nickel and chromium of 1 per cent will give cast iron with a Brinell hardness of 207-217; of 2 per cent, 223-235, and of 3 per cent, 241-255.

Chromium and nickel, however, are not the only alloying elements purposely added to cylinder irons; others added to improve the fluidity of the molten iron, the resistance of the iron to wear, its machinability, or both of the latter qualities, include, molybdenum, vanadium and titanium.

Copper and Molybdenum Additions.Copper is of value in cylinder irons in that it tends to prevent chill in thin sections and to give a

finer grain structure in the heavier sections, thus acting the part of a stabilizer, It also increases the fluidity of the iron and acts as a "graphitizer"; it hardens and tightens up the matrix so that “sponginess” is reduced. The improvement due to copper is well shown in transverse tests, and these additions are particularly effective in the presence of high manganese and of nickel or chromium.

Molybdenum increases the resistance to wear of cast iron, especially at higher temperatures. This results from the refining action it has on the grain, and from the finer division of graphite which it brings about. It increases the Brinell hardness-although in this respect it is not as effective as an equal proportion of chromium and it accomplishes this without rendering the metal less machinable. It also increases the tensile strength and the toughness of the metal. Where there is a tendency for the castings to crack owing to faults in either the design or the foundry technique-molybdenum is often of benefit. It is mostly used in combination with either chromium alone or with both nickel and chromium.

Heat Cracks in Cylinder Walls.Cracks in L-head cylinder castings (especially in large ones) sometimes start at the sharp edge

formed by the cylinder bore and the valve-passage wall. This edge reaches a very high temperature,

61

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive Enginesbecause the hot gases pass over it during the exhaust period, and a crack naturally starts easily at a sharp edge. Rounding off this edge has been found a good preventative against heat fatigue cracks. Cracks may start also at either the inlet- or exhaust-valve seat. It was shown that such cracks usually are the result of pre-ignition. The latter causes local overheating of the combustion-chamber wall, and the crack forms when the overheated metal cools again. By installing a "hot" spark plug in one cylinder and then running the engine under full load at from 3000 to 3500 rpm, cracks could be produced at will. The "hot" plug causes pre-ignition, and usually one 10-minute run under these conditions resulted in the formation of a crack, though sometimes several such runs were required.

Cylinder Wear.The characteristic which is most important in judging cylinder irons is their resistance to wear

under engine- operating conditions. As the cylinder bore wears, the engine loses power, consumes excessive quantities of oil, and gives off smoke in the exhaust. In fact, the rate of oil consumption is usually taken as an index of the state of wear of the cylinder bore.It was observed many years ago that the wear of cylinder bores is very non-uniform. It is greatest at the top end of piston travel (under the topmost ring with the piston at the end of its up-stroke), and decreases rather rapidly from there down. (Fig. 3.) It has been pointed out that cylinder wear is due to three separate causes, viz.,• Abrasion , which is due to foreign particles in the oil film;• Erosion , which is due to metal-to-metal

contact between the cylinder wall on the one hand and the piston and rings on the other; and

• Corrosion , which results from chemical action on the cylinder walls by the products of combustion.The order of importance of the three

causes varies with conditions of operation. That corrosion may play an important part in the wear of cylinder bores, it was found that accelerated cylinder wear occurs at low cylinder temperatures and is attributable to corrosion resulting from deposition of acid-bearing moisture on the cylinder walls. The reasons for assuming corrosion to be responsible were briefly as follows:

1. The pitted and discolored appearance of the cylinder walls and piston rings after low-temperature operation.

2. The fact that increased wear begins just below the calculated dew point.3. The detection of acids in the water of combustion.4. A large reduction in the rate of wear obtained with hydrogen fuel.5. A reduction in wear obtained when using corrosion-resisting materials.

The research work showed that corrosion is largely due to carbonic acid formed by the solution of carbon dioxide, a product of combustion, in water condensed from the gases of combustion. When hydrogen is used as fuel there is no carbon dioxide in the exhaust, so that no carbonic acid can form.

Effect of Cylinder Material on Rate of Bore Wear.The result of the Brinell test is generally

regarded as bearing some relation to the rate of cylinder wear. That hardness is a factor in wear resistance is indicated by the fact that heat-treated liners of alloyed iron with a Brinell hardness of slightly over 500, have been found to require reconditioning of the bore (by re-grinding) only one third as often as the bores of gray-iron cylinder blocks with a Brinell hardness of around 200. Cylinders with soft or "porous" spots which are readily detected by the Brinell test, usually show a high rate of wear, but differences in

62

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive Engineshardness within the usual range specified for gray-iron cylinder castings, say. 180 to 230 Brinell, have little effect on the resistance to wear.

Cylinder Stress and Wall Thickness.With the usual compression ratio of between 7 and 8 (for passenger-car engines) a maximum

explosion pressure of about 700 psi may be figured with. Now consider a section of a cylinder of b in. bore and 1 in. long, as represented in Fig. 4. The pressure developed in the cylinder by the explosion tends to rupture the wall along lines parallel with the cylinder axis and at opposite ends of a diameter. With a maximum combustion pressure of 700 psi the rupturing force on the section of the cylinder considered is 700b lb. If the wall has a thickness t and the material has a tensile strength of 35,000 psi, the resistance to rupture of the two sections 1 in. long and t in. thick is 70.000t lb and the. factor of safety then is

f =70000t/700b =100t/bFor a factor of safety of 4 the ratio of wall thickness to bore then evidently must be 1/25

This rule when applied to cylinders of small bore gives values for the cylinder-wall thickness which, while large enough so far as withstanding the stresses of a normal explosion is concerned, would be too small from the standpoint of shop production. If the water jacket is cast integral, as it usually is, the cylinder can be machined only on the inside, and the minimum thickness of the wall then depends upon the accuracy with which the cores are set. Some allowance must be made for inaccurate core work, and a good value for the wall thickness is

t = (b/25) +0.10inThis formula can be safely applied to the whole range of sizes of automotive engines with cast-

iron cylinders.The cylinder head must be quite stiff in order to resist the stresses of detonation. The wall itself

is usually made slightly thicker than the cylinder wall. In the case of an overhead-valve engine, the Wall is normally stiffened by the vertical walls of the valve pockets. A similar stiffening effect is usually obtained in the heads of L-head cylinders from the walls of spark-plug wells, but if there are any extended flat surfaces in these heads, they should be stiffened by ribbing.

Details of Water Jacket.For a long time it was the general practice to extend the water jacket down the cylinder wall

only to the level of the top of the piston when at the bottom of the stroke. As the lower part of the cylinder is not contacted directly by the hot gases, it does not reach an excessive temperature, and therefore does not seem to require water-jacketing. However, in modern high-speed engines the crankcase oil often reaches an excessive temperature, which reduces the load-carrying capacity of the oil film in the bearings, and may cause the latter to fail in hard service. It has been found that by extending the water jacket all the way down the cylinder, the temperature of the oil in the crankcase under extreme conditions may be lowered by as much as 50 Fahrenheit degrees, as compared with an engine with "half-length" jackets, and "full-length" jackets have come into general use.

Some designers taper the jacket down from the top to the lower end, so as to place a larger body of water around the compression chamber, where most of the heat must be absorbed. In most engines, however, the depth of the water jacket is uniform from top to bottom. This depth varies somewhat in different designs, but usually is equal to about one-eighth the cylinder bore. Certain parts of the jacket which directly affect the over-all dimensions of the block can be made smaller in depth, including the space between adjacent cylinders and that between a cylinder and a valve pocket or a tappet housing. Liberal water spaces have the advantage that the core sand can be more effectively removed from the casting. In engines of special design, such as those with "wet" liners, the jacket depth can be made less.

The jacket wall generally is made as thin as the foundry process permits. It can be made thinner, of course, in a small cylinder than in a big one, because in the former the area is smaller. Average practice with regard to jacket-wall thickness is as follows:

Cylinder bore, inches 3 4 5 6Thickness of jacket wall inches 5/32 3/16 7/32 1/4

63

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesJacket walls must be made heavier when cylinder liners (especially the "wet" kind) are used and

the tensile stresses due to the force of explosion are sustained chiefly or wholly by these walls.On the cylinder head the water jacket is usually made of somewhat greater depth than around the

cylinders, so as to provide adequate heat-storage capacity over the area where most of the waste heat enters the cooling water. There should be water spaces between all adjacent valve pockets (instead of common walls), and the water should come quite close to the valve seats, as it is only in this way that uniform cooling of the valve seat can be assured, and distortion and consequent leakage prevented. Cylinder heads must be so designed that no steam pockets can form in them; that is, it must be possible for the water to flow from any part of the jacket to the outlet along a continuously rising path. Trouble from overheating is most likely to arise at the exhaust-valve seats, and it is therefore desirable that the cooling effect of the circulating water be most intense at the valve pockets. This can be assured by inserting a distributing header in the water jacket, the header connecting with the water entrance to the jacket at the front of the block and having an outlet adjacent to each exhaust-valve pocket. The header is usually made of sheet metal and set into the mold. Two arrangements are illustrated in Fig. 5.

With valve-in-head cylinders the location of the water outlet presents some difficulty: because the valve mechanism on top of the engine is usually provided with a cover. One solution of the problem consists in forming a number of outlet bosses on the head over to one side, so they come outside the valve cover, and using a water-return manifold. While this tends to promote uniformity of circulation, it makes for dissymmetry of appearance, which is the more objectionable because the manifold is located very prominently on top of the engine. The more common plan is to have an outlet at the front end of the head, just outside the valve cover, and usually oblong in form, with the long diameter across the engine, so as to minimize the overhang.

In cylinders provided with "full-length" jackets, the central portion of the barrel lacks the reinforcement which with "half-length" jackets is provided by the flange that forms the bottom of the jacket. If the barrel also happens to be of minimum thickness its central portion will have very little rigidity and will distort easily, particularly if during machining operations the tool strikes a “hard spot:” This makes it almost impossible to obtain a true cylindrical bore. Conditions can be improved in this respect by providing the barrels of such engines with one or two circumferential ribs at intermediary points of their length.

While the flange around the cylinder at mid-length in engines with half-length water jackets has the advantage of affording the rigidity of structure desirable during machining operations it is detrimental under certain operating conditions. For instance, when an engine is being run under full load immediately after a cold start, the piston heats up much more rapidly than the cylinder block and is apt to get tight in the cylinder and scuff. It has been observed that in engines with half-length jackets such scuffing occurs particularly at the level of the water-jacket bottom flange, which latter prevents the cylinder from expanding.

64

www.jntuworld.com

www.jntuworld.com


Guarding Against Cylinder Distortion.It has been pointed out already that a frequent source of trouble in operation is distortion of the cylinder bore which results in blow-by overheating and excessive cylinder wear. Cylinder distortion may he due to either mechanical or thermal causes. Mechanical distortion is most likely to result from tightening of the cylinder-head nuts, if the anchorages for the cylinder head studs are not properly supported. It .has been suggested that these anchorages be either located in a wall which extends straight down to the cylinder bottom flange so that the pull of the stud produces pure tensile stresses on the material of the block, or else be cast on the jacket wall rather than on the cylinder wall, as illustrated in Fig. 6. To further reduce cylinder-wall distortion, this wall is thickened near the top, while the thickness of the deck around the cylinder wall is reduced.

In valve-in-head engines the bases for the brackets carrying the rocker arms must he well supported, so they will not yield unduly under load which would make the engine noisy.

Removable Liners.In most engines the pistons hear directly on walls forming part of the cylinder block, hut in

some-and particularly in engines with large cylinders-removable liners are used. There are two types of these liners:A "dry" liner is one which is in contact with metal of the block over its whole length, or nearly its whole length, while a "wet" liner is one which is supported by the block over narrow belts only, and is surrounded by cooling water between these belts.

In the United States "wet" liners came into use first, especially in the engines of farm tractors and commercial vehicles. Aside from the fact that any liner when worn or damaged can be replaced at relatively low cost, the construction offers the advantage that because of their uniform wall thickness (being machined inside and lout) and because they are very little affected by the tension of cylinder-head studs, separate liners distort less in service than the integral barrels of conventional cylinder blocks.

Fig. 7 "Wet" cylinder liner with packing rings.

65

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesAt first the liners were made of the same gray iron that was used for cylinder blocks, but in the

course of time materials of greater wear resistance were developed, and as most of these were more expensive than ordinary gray iron, they lent themselves particularly to use in liners. One method of installing a removable "wet" liner in a cylinder block is illustrated in Fig. 7. At the top the liner is provided with an external flange which enters a counter bore in the cylinder. The top of the liner is flush with the top of the block, and the joint is sealed by the cylinder-head gasket. In some cases and especially in Diesel engines-the hole in the gasket is made slightly larger than the cylinder bore, and a ring or loop of copper is inserted to reduce the pressure on the gasket. At the bottom the liner is enlarged in diameter and has three grooves for packing rings cut in it. Instead of in the liner, the grooves may be cut in the block. These packing rings are made of synthetic rubber, which is more resistant to mineral oil and other petroleum products than natural rubber. The packing rings may be made of circular section, of a diameter slightly larger than the width of the grooves, and insertion of the liner then will deform them so that they substantially fill the grooves. To permit easy insertion of the liner, either it or the bore of the block is chamfered, depending on which part contains the packing rings.

Inaccuracies in the section diameter of these packing rings are said to have been the cause of some trouble. If the diameter is too small there may be leakage, whereas if it is too large the pressure exerted when the liner is forced into place may crack it. To overcome this difficulty, a cork-synthetic rubber composition of greater elastic compressibility has been developed. Packing ring of this material are molded with a square section, and when inserted project slightly above the surface of the part in which the grooves are cut. Insertion of the liner compresses them flush with that surface. Single and two packing rings also are used, and in the case of two rings, a third groove sometimes is cut between the two containing the packing rings, to collect any oil or water that may seep past the rings and allow it to drain off.

"Dry" liners, which in Great Britain were used practically exclusively from the beginning, seem to have gained the ascendancy over the "wet" type in this country after World War II. A typical "dry-liner" installation (in a GMC engine) is shown in Fig. 8. In this engine the cylinder block and crankcase are separate castings, and the liner extends some distance into the crankcase. It is held in position by a flange. at the top. In some other engines with dry liners and a separate crankcase the retaining flange on the liner is near the bottom and is held between the cylinder block and the crankcase. A British manufacturer of Diesel truck engines (Albion) copper-plates the dry liners on the outside. The copper is said to act as a lubricant, facilitating the insertion of the liner, and also to improve the heat flow from liner to cylinder wall. Fig. 8 "Dry" cylinder

liner in position.

66

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesMaterials for Cylinder Liners

For the engines of public-service vehicles, which latter run up enormous mileages in the course of a year, it has been found advisable to use alloy iron for the liners and to heat-treat them. General Motors Truck & Coach Division, for instance, uses such hardened liners in all of its larger engines, the material being a nickel-chromium iron of the following composition:

PercentTotal carbon 3.10-3.40Combined carbon 0.75-0.90Manganese 0.55-0.75Phosphorus 0.20 max.Sulphur 0.10 max.Silicon 1.90-2.10Nickel 1.80-2.20Chromium 0.55-0.75In the "as cast" condition the liners show a Brinell hardness of 212-241, a transverse strength of

2400 lb on A.S.T.M. arbitration bars (bars of 1.2 in. diameter and 18 in. between supports), a transverse deflection of 0.20-0.30 in., and a minimum tensile strength of 37,000 psi on test bars machined from-the casting. A hydrostatic test also is applied to the liners, which must withstand 1500 psi for a wall thickness of l/8 in. and. bores of 4-5 in. To increase their wear resistance, the liners are hardened, by being heated to. 1540- 1560 F for 30 to 40 minutes and quenched in still oil. After this they must show a Brinell hardness of at least tensile 512 while the strength must range between 28,000 and 36,000 psi and the transverse strength between 2700 and 2900 lb for the arbitration bar. With these liners the mileage between cylinder overhauls is said to be practically trebled, as compared. With solid cylinders of gray cast iron showing from 230 to 240 Brinell. A minor disadvantage is that it takes up to 5000 miles for the piston rings to wear in fully, hence the oil consumption is rather high during the early part of the life of the liner.

Nitrided Cylinder Liners.A process for nitrogen hardening or “nitriding” cast iron was developed in Europe. The process

consists in exposing cast-iron objects to be case- hardened to a current of ammonia vapor at about 900 F for a considerable length of time, and then quenching. At this high temperature the ammonia breaks up into its constituents. Nitrogen and hydrogen, and the nitrogen penetrate into the surface of the casting & combines chemically with the metallic elements, forming very hard nitrides.

A Special alloy iron containing aluminum must be used. The liners are exposed to the ammonia vapors for 65 hours at 950 F and then have a hardened case of 0.015 in. depth, the hardness tapering off from the outside, where it is somewhere between 800 and 1000 Brinell.

A slight "nitride fuzz" produced on the surface of the liners during the process is removed before they are shipped to engine builders. Some distortion is caused, and the effects of this are eliminated by honing after the liners are inserted into the block, for which purpose an allowance of 0.002 in. on the diameter is made. Nitriding also produces a slight "growth," of the order of 0.001 in., and this, too, is allowed for in advance. Liners are installed in blocks with a press fit, an interference of 0.0015 to 0.0025 in. being allowed, depending on the bore.

Chromium Plating.Another method of reducing the rate of wear consists in chromium plating the bore. The process

differs radically from that of chromium plating for ornamental purposes. .It gives a "porous" coating which holds oil, while the so called bright plating process gives a dense coating to which oil will not adhere & which for this reason is readily is scored in service. From 200 to 500 times as much chromium as in conventional decorative plating is deposited per unit of area. If slightly too much should be deposited, so that the bore is undersize by from 0.0005 to 0.001 in., the excess can be removed by honing. Wear tests made on a plain gray-iron cylinder of 241 Brinell hardness and a similar cylinder plated indicated that chromium plating reduces the rate of cylinder wear approximately in the proportion of 7:1 and that the wear on the top piston ring is coincidentally reduced about 4:1.

67

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesSuch methods as nitriding and chromium plating of cylinder bores are applicable particularly to

bus and railcar gasoline engines and to Diesel engines, which have a much longer service life than passenger-car engines. Cylinder bores in plain cast iron must be reconditioned about every 50,000 miles, and with either a nitrided or chromium-plated bore, if reconditioning is required at all, it will be required only after a much longer interval.The primary function of a cylinder of an IC Engine is to maintain the working fluid & the secondary function is to guide the trunk piston.

Advantages of D ry liner 1. simpler to replace2. no danger of water leakage either in to crankcase or the combustion chamber3. due to absence of heavy flanges at the top of the liner, cylinder centres can be reduced4. better cooling of upper part of the liner

Disadvantages of D ry liner 1. Complicated casting2. decreased heat flow through the composite wall

Advantages of Wet liner 1. the foundry problem is considerably eased, since the large internal cores of the cylinder block

can be properly supported2. cylinder block is relieved of the stresses due to longitudinal expansion of the liner

Disadvantages of Wet liner 1. difficult replacement2. Danger of water leakage in to the crankcase& the combustion space if the casting is defective

Qualities of a good liner1. Strength to resist the gas pressure2. Sufficiently hard to resist wear3. Strength to resist the thermal stresses due to the heat flow through the liner wall4. Anticorrosive5. Capable of taking a good bearing surface6. Should be symmetrical in shape to avoid unequal deflection due to gas load & unequal

deflection due to thermal load7. no distortion of inner surface due to restraining fixings

68

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesValve Seats

Water jackets should be carried close to and all around the valve seats. It has been shown that the heat absorbed by the valve heads through their contact with the burning gases passes off chiefly through the seats, and if the water comes close to the seats the heads will be cooled more effectively, while if it extends all around, the heads will be cooled more nearly uniformly and will not warp.

Particularly effective cooling of the valve seats and valve guides is claimed for the arrangement shown in Fig. 9. Here a water distributor is cast in the block, and has discharge openings in both the top and the side adjacent to the valve guide. The outlets in the top discharge into the space between the exhaust valve pocket and the cylinder wall, and the water discharged there is induced to flow completely around the valve pocket, by scallops directly above the valve passage, through which it passes into the cylinder-head jacket. The outlets at the side discharge against the bottom of the valve pocket adjacent to the upper part of the valve guide.

In laying out the valve pocket, enough clearance must be allowed all around the valve seat so the gases will pass fairly uniformly through all sections of the valve port.

Fig. 9Valve-Seat Inserts.

In high-speed, heavy duty engines the exhaust-valve seats, if directly on the cast iron of the block, are likely to erode or wear way rather rapidly, causing the valve to sink deeper into the seat, reducing the valve clearance, and necessitating valve-stem adjustment. To eliminate the necessity for frequent adjustments, valve-seat inserts of heat-resistant material were introduced in 1931, first for commercial-vehicle engines and shortly thereafter also for passenger-car engines. Such inserts had long been used on engines having the part containing the valves made of aluminum, as the ordinary aluminum alloys are far too soft to sustain the pounding of the valve heads. In that case the inserts are made of aluminum bronze -(90 percent copper, 10 per cent aluminum), which has about the same coefficient of heat expansion as the aluminum alloys used for cylinder heads. Aluminum-bronze inserts are forced into counter bores in the head with a shrink fit.

One of the requirements of a valve-insert material therefore is that it must have substantially the same coefficient of heat expansion as the material of the block or head; another is that it must be sufficiently hard to withstand the pounding of the valve head at high temperatures over long periods. The materials commonly used include nickel-chromium iron with moderate alloy contents, and the high-percentage tungsten steel known as high-speed steel. Where the conditions are too severe for these materials the seat of the insert can be provided with a facing or veneer of a nonferrous, heat-resistant alloy. Alloys available for the purpose include "Eatonite" (chromium, tungsten, nickel and cobalt), "Elkonite" (tungsten and copper), and "Stellite" (cobalt, chromium and tungsten). These alloys are applied to the seat portion of the inserts by "puddling" with a welding torch.In most applications the inserts are shrunk in place, and to get the necessary shrink fit without the use of too great pressure (which might cause distortion), the inserts are cooled to about -100 F in dry ice or -220 F in liquid air, while the blocks are heated in water to about 200 F. The interference is made about 0.0015 in. Per inch in the case of steel inserts, and 0.003 in. per inch in the case of cast iron.

As a rule, the inserts are chamfered at both top and bottom-at the bottom to facilitate entering them in the counter bore, and at the top so that the block material can be rolled over the edge of the insert to help retain it. In addition to rolling the block material over the top chamfer, the insert sometimes is provided with a number of axial grooves into which metal of the block is forced by the rolling process.

69

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesFig. 10 shows a Stellite-faced, threaded insert. The Stellite is puddled onto the steel base with an acetylene torch. The insert is provided with splines in its throat, to take a tool for screwing it in place, and a 0.014-in. washer of extra-soft iron is placed underneath it to assure a good path for heat flow. After the insert is screwed home, it is-locked in place by rolling the metal of the block around it. The shape of the rolling tool and the method of rolling are illustrated in Fig. 11.

Valve inserts have been standardized by the S.A.E. (Fig. 12). The standard includes two series, one intended for- passenger-car, the other for heavy-duty engines. It specifies the diameter and depth of the bore in the cylinder or head, and the thickness of the insert. This leaves

the diameter of the insert-which determines the interference-to be set by the manufacturer.Valve-seat inserts shrunk in place sometimes come loose in service, and this is particularly likely

to occur if the interference is relatively large. This is due to the fact that in severe service such high temperatures may be reached that the resulting stresses exceed the elastic limit of the metal and produce a permanent set. Then, when the: engine cools down, the insert will be loose. It is therefore recommended that the interference be made no greater than needed to firmly hold the- insert in place when the engine is cold.

An insert specially designed to prevent trouble from distortion and loosening. in severe service is illustrated in Fig. 13. The main portion, which has-a section similar to that of the S.A.E. standard insert, is given a clearance of 0.004 in. around its circumference, so that the metal around-it can distort freely without subjecting the insert to undue stress. In addition there is an extension or skirt, which is made an interference fit in the head or block. Owing to its greater distance from the valve seat, this portion will not reach as high a temperature, and therefore is not likely to take a permanent set. The considerable length of valve port or throat required seems to be a disadvantage of this type.

Length of BoreIn most modem engines of both the L-head and I-head type the combustion chamber is formed

in the cylinder head and at the end of the up-stroke the top of the piston is flush with the finished top surface of the cylinder block. One reason for not making the piston overrun the end of the bore is that that would bring the top ring beyond the upper end of the water jacket at the end of the up-stroke, where it would not be so effectively cooled, in the ring groove. The lower end of the piston generally is made to overrun the end of the bore slightly.

The total length of the finished bore evidently is equal to the length of stroke plus the length of the piston minus any overrun of the piston at both ends, the overrun being considered negative when the piston does not come quite to the end of the bore. To facilitate getting the piston rings into the cylinder, the bore is chamfered at the end from which the piston is entered

70

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesLocation of Spark-Plug Bosses

In L-head cylinders the sparkplug bosses usually are located over the passage between the valve chamber and the cylinder barrel, as this location reduces the tendency to detonation to a minimum. The spark points must never be directly in line with the cylinder wall, where they would become fouled by oil thrown off by the piston.

The length of thread of standard spark plugs is considerably less than the average thickness of the cylinder head, and to compensate for this difference, a conical depression is formed in either the inner or outer wall of the head, as shown in Fig. 14. There appears to be one objection to each arrangement, which is probably the reason designers have not yet agreed on one or the other of these designs. If there is a depression in the outer wall, any water getting onto the top of the engine will collect in it and tend to cause rusting of the spark-plug shell and its thread. A conical depression in the inner wall adds to the cooling surface of the combustion chamber. Besides, the mixture at the spark points, near the bottom of the depression, may be less ignitable.

In valve-in-head engines the depth of the head is altogether too great to permit of having the spark-plug bosses extend through it vertically, and in such engines they extend through the head from the side at an angle, a recess being formed in the side of the head to obtain a square seat (Fig. 15). In larger cylinders, of course, the vertical depth of the compression space is sufficient to allow of the plug being screwed horizontally into the compression chamber wall.

Fouling of spark plugs by lubricating oil is most likely to occur in valve-in-head engines, where the plugs are located directly over the pistons. One method of combating trouble from this source consists in reducing the diameter of the spark-plug hole by about one-half at the inner end (see Fig. 15), and making the thread of the plug slightly shorter than the depth of the threaded hole, so that a small chamber is formed at the inner end. In engines which do not have this provision the same effect can be obtained by the use of "adapters," which screw into the spark-plug hole and have an internal thread which will take the plug.

Optimum Combustion-Chamber Form.It was found that the combustion chambers illustrated in Figs. 16 and 17 work out satisfactorily, and that with these the small compression volumes required can be obtained without difficulty. The various combustion chambers represented by Fig. 16 differ from each other with respect to slope over the piston and with respect to clearance ahead of and behind the valves, as shown in Fig. 17. A clearance of 1/16 to 3/32 in. is satisfactory above the valve, and 3/32 to 1/4 in. behind the valve, while as much clearance as possible should be provided ahead of the valves. The width of the valve passage should be about 75 per cent of the bore, and the ratio between swept volume and passage area should be between 15 and 20. The "masked" area of the piston may vary considerably; a satisfactory angle of slope over the piston-should be aimed at, and the "masked" area allowed to come what it will; in the design shown it is about 50 per cent of the piston-head area.Although this study of the best combustion-chamber form was made on engines with aluminum heads, the results obtained and the conclusions reached are equally applicable to high-compression engines with cast-iron heads.

71

www.jntuworld.com

www.jntuworld.com


Fig. 16 Forms of combustion chamber used in experimental aluminium cylinder head.

Fig.17 Clearance ahead & behind valves.

Production of Engine BlocksIn the design of engine block or cylinder block it is well to consult with the foreman of the

pattern shop, because a casting of this kind is a difficult piece of mold, and the advice of an experienced mechanic may obviate trouble later.

Cylinders must be molded with the head downward, for the reason that blowholes, porous spots, etc are most likely to occur near the top of the casting, & the head of the cylinder, which is the working end, must of necessity be of sound metal.

When the castings have cooled the core sand is removed, the seams etc., are chipped off, & the castings are then put through a cleaning process. [Either by pickling & neutralizing or by blast cleaning (blast cleaning by sand or small granules of chilled iron or steel) & then normalizing & cleaning]

Further the cast & cleaned blocks would undergo other operations in sequence like Milling, Drilling, Cylinder boring, Precision boring, Finish of bore, Honing, Lapping, followed by measurement of quality of surface finish, Water test Finishing of valve seats & guides & surface broaching.

Transfer machines were adopted since world war-II to perform the operations automatically.

72

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesDESIGN OF CYLINDER AND CYLINDER HEAD

Cylinder should be- designed to withstand the high pr. & temp. conditions. - be able to transfer the unused heat effectively so that metal temp. does not approach the dangerous limit.

The Cylinder wall is subjected to gas pressure & the piston side thrust.-Piston side thrust tends to bend the wall but the stress in the wall due to side thrust is very small & can be neglected.

-The gas pressure Produces 2 types of stresses;-longitudinal and circumferential - which act at right angle to each other & the net stress in each

direction is reduced. The longitudinal stress is usually small & can be neglected.

lf =longitudinal stress=area

force= ( )

4

422

max

2

DD

pD

O −π

π

cf =circumferential force=t

Dp

2max ×

D=cylinder diameter, DO= cylinder outside diameter,

maxp =max. gas pr.

Netlf

=lf- m

f c

& Netcf

=cf- m

f l

,

where=

m

1

poision’s ratio= 4

1

73

www.jntuworld.com

www.jntuworld.com


CYLINDER WALL THICKNESSThe Wall Thickness is usually calculated by applying the formula for a thin cylinder,

thus kf

Dpt

c

+×=2

max

Where t=wall thickness, mm,

maxp = max. gas pr.,N/mm2 (3.1 to 3.5N/mm2),D=cy. bore, mm,

cf =max. hoop stress and is equal to 35 to 105 N/mm2 depending on the size and material, larger values are used for smaller bores,

Cylinder bore, mm 75 100 150 200 250 300 350 400 450 500k =reboring factor, mm 1.5 2.3 4.0 6.0 7.5 9.5 10.5 12.5 12.5 12.5

The thickness of the cylinder wall usually varies from 4.5mm to more than 25mm, depending upon the cylinder size.According to an empirical relation,For liners of oil engines,

t ≥15

D near the top portion & through 20% of the stroke.

For dry liners,The total thickness ‘t’ is the thickness of the liner & that of the cylinder wall. The thickness of the Dry liner is given as 't =0.03D to 0.035D

The thickness of the inner walls of the automobile engine cylinders is usually given empirically ast =0.045D+1.6mm

The thickness of Jacket wall is given as =3

1 to

4

3t , larger ratio for smaller cylinder

or =0.032D+1.6mmThe water space between the outer cylinder wall & inner jacket wall is =10mm for a 75mm cylinder to about 75mm for a 750mm cylinder

or =0.08D+6.5mm

CYLINDER DIAMETER AND LENGTH

The o/p of a given cylinder can be written as - Power= WpLAn

,60

Where L=stroke in m,A=piston area, mm2,n=no. of working strokes per minute

= N for 2 stroke engines and 2

Nfor 4 stroke engines

p=imep-if power is indicated &bmep if o/p is in brake power, N/mm2

* As a guide, the max. gas pr. can be taken as 9 to 10 times the bmep

74

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesCYLINDER FLANGES AND STUDSThe cylinder is either cast integral with the upper half of the crankcase or attached to it with the help of flanges, studs and nuts.The cylinder flange is made thicker than the wall of the cylinder.Flange thickness should not be less than 1.1 to 1.25 tCommon value for flange thickness = 1.2 to 1.4 t

Or =1.25 to 1.5 d where d =bolt diameter, nominalThe distance of the end of the flange from the center of the stud or bolt should not be < d +6mm, and not > 1.5 d .The use of studs decreases the bending stress at the flange root since the moment arm can be made very small.The material of the studs or bolts is usually nickel steel with a yield point of 630 to 945MPa.The diameter of the bolt or stud is calculated by equation of the gas load to the area of all the studs at the root of the threads multiplied by the allowable fibre stress.

,∴ 4

π.max

2 pD × = tc fdz ×× 2

4

π

∴ .max2 pD × = tc fdz ×× 2

∴ cd =tfz

pD

×max ,

where tf = allowable fibre stress, 35 to 70 N/mm2,

cd = core diameter

Low value of tf is taken since there is already high stress in the studs due to tightening of the nuts.

The number of studs '' z may be taken as

+

+ 4

504

100

Dto

D, D in mm

Or the pitch of the bolts may be taken as d19 to d5.28 , where d is in mm.

In practice d generally varies from (4

3to 1) times the thickness of the flange.

In no case d should be < 16mm

75

Outside

Diameter

Core

Diameter

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesCYLINDER HEAD

Usually a separate cylinder cover or head is provided with all but the smallest engines. A box type section is employed of considerable depth to accommodate ports. The general design of the cover is governed by the following factors along with the strength consideration.

Air and gas passages Accommodation of valves and their gear Accommodation of the atomizer at the centre of the cover in the case of the diesel engines.Cylinder head is the most difficult part to be designed and manufactured. The cylinder heads are

usually made of close grained cast iron or alloy cast iron containing nickel, chromium and molybdenum, for small and medium sized engines, while for large engines, the material is low carbon steel.

The thickness of the cylinder wall ranges from about 6.5mm for small engines to proportionately larger values for large engines. The thickness depends on the shape of the head. If the cylinder head is approximately a flat circular plate, the thickness can be determined by the relation:

tf

CpDt max=

Where C=const., in this case equal to 0.1, tf =allowable stress, taken to be 35 to 56 N/mm2

A low value of '' tf is taken because both pr. & temp. stresses are induced in the cylinder head and the above equation is based upon only the cylinder pressure. The heat transfer through the head is about 5 to 13 times as much heat per unit area as the cylinder walls, depending on the design and amount of cooling.

76

www.jntuworld.com

www.jntuworld.com


77

3.5N/mm2 pr. gas max. 300mm,borecylinder D

Given

:Solution•

3.5N/mm of pressure gas maximum awith

engine, petrol 300mm afor stresses the&allcylinder wiron cast a of thickness theDetermine

1-Example•

max

2

==== p

bores,smaller for usedare ueslarger val material, and size theon depending

2N/mm 105 to35 toequal is and stress hoop max.fc

),23.5N/mm to(3.1 2N/mmpr., gas max. maxmm, bore, cy.D,

,2

maxp tThickness, WallThus

cylinder, thin afor formulaapplyingby calculatedusually is Thickness Wall

=

==

+×

=

pwhere

kcf

D

mmkf

DpThicknessWall

mmNfAssume

mmktableaboveFrom

c

c

t 5.215.9452

3005.3

2,

,/45

,5.21

max

2

=+××=+

×=

==

12.5 12.5 12.5 10.5 9.5 7.5 6.0 4.0 2.3 1.5 mm factor, Reboring

500 450 400 350 300 250 200 150 100 75 mm bore,Cylinder

( ) ( )

( )2

22

2

maxo

22

2max

22max

2

/45.11300343

3005.3 stress allongitudinApparent

343mm 21.5)(2 3002t 3002t D Do Now

pr. gas max. &diameter outsidecylinder D

diameter,cylinder D Where,

4/][

])4/[(

stress, allongitudinapparent Now

mmNf

p

DD

Dp

DD

pD

area

forcef

l

ool

=−

×=

=×+=+=+===

=−×

=−

×==

ππ

2

2

c

2max

/54.2186.24.24 4

11.45-24.4 =Net &

/35.51.645.114

24.4-11.45=Net

4

1=ratio spoision'

m

1 where,

m

f- = Net

/4.245.212

3005.3

2

stress, ntinalcircumfereapparent Now

mmNf

mmNf

ff

mmNt

Dp

area

forcef

c

l

ll

c

=−=

=−=

=

=××=×==

www.jntuworld.com

www.jntuworld.com


78

WattrpmnmmAmLmmΝP

PowerIndicated

kWoη

PowerBrakePowerIndicated

PowerIndicated

PowerBrakeηSince

Solution

cylinder.theofdimensionstheDetrmine

0.80η,mm0.35Νimep1200rpm,Speed4.5kW,powerΒrake

ions:specificatfollowingthehasΕngineCIstroke4verticalΑ

2Example

22imep

mech

mech

mech2

60

/

625.58.

5.4

:

./

×××=

===∴

=

====

DLorasratioDL

eiBore

StrokeassumingNow

mmD

mLor

mmAmLor

ALwatt

Enginestrokeactingsingleforn

n

s

mNWatt

35.1,35.1..

10608.14

10608.160035.0

6010625.5

60

60035.010625.5

322

33

2

3

6002

12004

2

]11[

=

×=××

×=×

××=×

×××=×∴

===

=

π

mm

CylinderofLength

stroketheoftoStroke

sidesbothonclearanceStrokeCylinderofLengthNow

mmDLLengthStroke

mmDDiameterBoreor

mmD

mmD

or

mmD

mD

5.178

)15.0155(155

%1510

15511535.135.1

,115

10608.141000

35.1

10608.14

35.1

322

322

=×+=∴

+=+=

=×==∴=

×=××

×=××∴

π

π

www.jntuworld.com

www.jntuworld.com


References:1. High Combustion Engines – P M Heldt2. M/C Design –Sharma & Agarwal

79

mmt

ironcastgradegoodformmNfassuming

mmNtostressfibreallowablefconstantCmmNpmmwhereD

f

pCDtcovercylinderofThicknessSolution

covercylindertheforalsostudstheDesign.3.2N/mmelyapproximatispressuregasmaximumThe

cylinder.0.3mforheadcylinderplainaofthicknesstheDetermine

Example

t

t

t

2

2.2642

2.31.0300

,/42

}/5635&,1.0{/2.3,300

:

.

3

2

22max

max

=×=∴

=

======

×==

pressuregasD

studtheonLoad

mmddDDdiametercirclePitch

diametercoreddiameterboltnominald

cylindertheofwallinnerthethefromdtodbeshouldstudtheofcentre

thethatpracticecommonaisitOrdiametercylindertheactingbetoassumedbemaypressurethe

initiallyknownnotisdiameterstudtheasbutstudstheofdcptheuponactactuallywillgasThe

Studs

p

p

c

.max4

33003

),(

.5.125.1

,.

,,...

2

××

=∴

+=+=∴==

π

2/63

.,&2/7035

&8

8.08.0,,

,4

2.34

)3300(

,107450

3004

100

3004

504

100.

2

2

mmNtf

studsofNoZdiametercorecdmmNtotfwhere

ZLet

ddiameternominalddiametercorelet

fd

ZloadBut

d

totoD

toD

ZstudsofNoNow

c

tc

=

===

=

×=×=

×××=

×+×=

=++=++=

π

π

satisfiedareconditionsBothmmtommei

dtodbetweenliespitchmaximumandmmdbeshouldpitchminimumNow

mmZ

DstudstheofPitch

mmddDD

mmdgetweerrortrialBy

dd

p

p

∴=×=

=×=×

=∴

=×+=+=+=∴=

×××=×+×∴

,1875.124.,.

5.28191294333

5.1688

429

42943330033003

43,,&

634

)8.0(82.3

4

)3300( 22

ππ

ππ

www.jntuworld.com

www.jntuworld.com


Chapter-8VALVE AND VALVE MECHANISM

Types of valve operating mechanisms, valve springs, guides, push rods, rocker arms, tappets, valve timing diagrams, design

80

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesValve and valve mechanism

To admit the air-fuel mixture in the engine cylinder and to force the exhaust gases out at correct timings, some control system is necessary, which is provided by the valves.

The engine valves may be broadly divided into 3 main categories:1. Poppet valve2. Sleeve valve3. Rotary valve

Out of these three, poppet valve is the one which is being universally used for automobile engines.

The conventional automotive engine is fitted with mechanically operated poppet valves for both inlet and exhaust. A poppet valve consists of a disc of metal with a coaxial stem on one side which closes a circular opening in a wall separating two chambers, against which wall it is drawn by a spring. To open the valve, a force must be applied to it in, a direction contrary to that of the spring pressure. In the earliest automotive engines, the inlet- valves were opened automatically by the suction in the cylinder during the inlet stroke,. Automatic valves cannot be used in engines that must operate over a wide speed range, as they close too early at low and too late at high speeds to permit of good volumetric efficiency. These valves, moreover, are troublesome in service, because gum in the gasoline may cause them to stick.

Poppet valves are lifted from their seats by means of cams, and are closed by springs. The rate at which the valve is opened and closed depends on the cam outline and on the type and size of cam follower employed. From the standpoint of gas flow it is, of course, desirable that the valve should open and close very quickly, and remain fully open for the greatest possible length of time. However, the valve gear must operate quietly, and in order to do this it must lift and drop the valves more or less gradually. Cams, therefore, usually are so designed that the valve begins to close as soon as it

has attained its full lift, and there is no "dwell" in the full-open position

81

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesConstruction of Poppet Valves

The poppet valve derives its name from its motion of popping up and down. This is also called "mushroom valve" because of its shape which is similar to a mushroom. It consists of a head and a stem as shown in Fig. It possesses certain advantages over the other valve types because of which it is extensively used in the automotive engines:

1. Simplicity of construction2. Self-centering.3. Free to rotate about the stem to new position.4. Maintenance of sealing efficiency is relatively easier.Generally inlet valves are larger than the

exhaust valves, because speed of incoming air-fuel mixture is less than the velocity of exhaust gases which leave under pressure. Further, because of pressure, the density of exhaust gases is also comparatively high. Moreover, smaller exhaust valve is also preferred because of shorter path of heat flow in this case and consequent reduced thermal loading.

Generally inlet valves and exhaust valves are 45% and 38% of the cylinder bore respectively. Further, to improve heat transfer to the cylinder head, the stem diameter of the exhaust valve is generally 10 to 15% greater than that of the inlet valve. Moreover, the valve lift in both inlet and exhaust valves should be at least equal to 25% of the valve head diameter which would provide the annular valve-opening area equal to the port throat area. If the valve lift is less, the volumetric efficiency of the engine will be decreased. On the other hand if it is excessive, the inertia of the valve actuating mechanism would be unduly large resulting in excessive noise and wear.

The valve face angle (with the plane of the valve head) is generally kept 45° or 30°. A smaller face angle provides greater valve opening for a given lift, but poor sealing because of the reduced seating pressure for a given valve spring load. Due to this reason in some engines, the inlet valve face angle may be kept 30° or 45° whereas the exhaust valve face angle is only 45°, as this increases its heat dissipation. In some cases, a further differential angle of about 1/2 deg to 1 deg is provided between the valve and its seating (Fig.), which results in better sealing conditions.

The machined surface of the block or the cylinder head on which the valve rests when closed is known as the valve seat. This surface usually forms a truncated cone whose generatrices make an angle of either 45° or 30° with the plane of the valve head. During the early years of the industry flat-seated poppet valves were used to a certain extent, which have the advantage that for a given port diameter and lift, the flow area is considerably greater than with conical valves. A disadvantage of flat-seated valves, which led to their abandonment-is that they are not self centering, and therefore are more likely to leak, especially after the guides have become worn.

In the analysis it has been assumed that the flow through the valve is parallel to the seat elements. This is substantially correct at small lifts, when the distance between valve and seat is only a fraction of the width of the seat, but with increase in the lift the direction of flow changes. The gases naturally seek the path of least resistance, and in turning a corner they approach the inner boundary of the flow path.

82

www.jntuworld.com

www.jntuworld.com


In an L-head engine the direction of flow on the side of the valve toward the cylinder, where the gases can flow off freely, is somewhat different from that on the opposite side, where there is only a moderate clearance between valve and valve-chamber wall. There the gases must describe nearly a semi-circle, and in seeking the path of least resistance, they approach the edge of the valve head. The best measure of valve capacity evidently is the minimum sectional area normal to the direction of flow.

From Fig., where the dashed lines are meant to represent the center lines of the flow paths, it can be seen that the direction of flow relative to the seat elements varies around the circumference of the valve. In modern engines the valve seats are made comparatively narrow, and in Fig., which closely represents actual proportions, the line BC connecting the inner edge of the valve seat at full lift with the outer edge of the seat on the block, makes an obtuse angle with the elements of the seats, instead of a right angle as in flat-seated valve Fig.. It has been suggested that the area of the conical frustrum of which BC, is an element be taken

as a measure of the valve capacity, but in view of the fact that the direction of flow is not normal to that line this plan is of doubtful value. The valve lift generally is slightly more than one-fourth the port diameter in the case of 45-deg, and slightly less in the case of 30-deg valves. Valve-stem diameters are made equal to one-fourth the valve diameter. The outside diameter of the valve head will be about 1.175 times the port diameter.

83

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesValve-Operating Conditions

The heads of the valves are subjected to the high temperature of the burning gases, and it is essential that they should not warp under the influence of the heat, and that their seats should not scale or corrode, as in either case they would become leaky. Occasionally small particles of scale will get onto the valve seats, and the valve heads must be of sufficient hardness at the high temperature at which they operate so they will not pit under this condition.

Lubrication of the valve stems is hard to effect, and the stems must not wear too rapidly in their guides, even though poorly lubricated or not lubricated at all. That portion of the stem immediately below the head is subject also to the heat of the burning gases which, when the exhaust opens rush by it at a velocity of up to 300 fps; and to the corrosive action of unconsumed, hot oxygen and intermediate products of combustion.

Trouble of a rather serious nature is sometimes caused by valves breaking a short distance below the head, at the point where their working temperature is the maximum. This is probably due to corrosion fatigue; in other words, it results from repetitive applications of mechanical stress combined with corrosive action by the exhaust gases or certain constituents thereof. Air-hardening properties of the valve steel sometimes have been blamed for such breakages, but these latter occur also with valves made of a steel having no such properties. High resistance to corrosion fatigue is therefore desirable in valve steels.

Finally, the tip of the stem receives a quick succession of blows from the tappet as the clearance is being taken up, and it must be sufficiently hard to withstand these blows without undue wear. Hardening of the tips is sometimes effected by the so-called cyaniding process (dipping in a bath of molten potassium ferro-cyanide).

84

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesValve-Operating Temperatures

Tests have .shown that under continued full-load conditions, exhaust valves may reach a temperature of 1475 F-a cherry red. Valves of large diameter run hotter than smaller ones, and the valve temperature increases with engine speed. An increase in the compression ratio, as a rule, lowers the valve temperature, but if the compression is carried too high and detonation sets in, the effect is reversed. It is usually assumed that exhaust valve temperatures are highest with retarded ignition and

weak mixtures, probably because the exhaust pipe is hottest under these conditions, but a large number of tests carried out on a particular engine showed that the reverse holds true, the exhaust-valve temperature being lower with a weak mixture and retarded ignition. The explanation is that the temperature of the valve depends not only on that of the exhaust gases, but also on the temperature of combustion, which latter is lowered by weakening the mixture and retarding the spark.

It was found that the exhaust valve ran cooler when a long valve guide (Fig.) was used; that is, when the valve guide was carried closer to the valve head. The guide then has the effect of protecting the valve stem from the hot gases passing through the valve immediately after opening. One objection to such a long valve guide is that it is difficult to lubricate, and as a result wear on both the valve stem and guide is rapid. The experiment was therefore tried of enlarging the bore of the guide 0.016 in., by counter boring from the valve-head end as far as the wall of the valve pocket, and this was found to result in decreasing the valve temperature (27 deg at 1500 rpm and 72 deg at 4500 rpm). An increase in the valve-stem diameter from 0.343 to 0.405 in. lowered the temperature of the valve about 40 deg throughout the speed range.

If an exhaust valve becomes leaky, as, for instance, through "dishing" of its head by reason of loss of strength at high temperature, through improper adjustment, or through excessive warping, the head will be destroyed very quickly, as the burning gases will then blow by it.

85

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesExhaust valves operate under relatively

more severe conditions on account of higher temperatures involved. An exhaust valve is subjected to:

1. Longitudinal cyclic stresses due to the return spring load and the inertia response of the valve assembly.

2. Thermal stresses in the circumferential and longitudinal directions due to the large temperature gradient from the centre of the head to its periphery and from the crown to the stem. A typical variation of temperature in an exhaust valve is given in Fig.

3. Creep conditions due to operation at very high temperatures, particularly in case of valve head.

4. Corrosion conditions.

Exhaust Valve Material RequirementsOn account of operating conditions described above the material for exhaust valve should have the following requirements.

1. High strength and hardness to resist tensile loads and stem wear.

2. High hot strength and hardness to combat head cupping and wear of seats.

3. High fatigue and creep resistance.4. Adequate corrosion resistance.5. Least coefficient of thermal expansion

to avoid excessive thermal stresses in the head.6. High thermal conductivity for better

heat dissipation.

86

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesMaterials for Valves

Owing to the expensive character of the material necessary for the exhaust valves, the inlets are now generally made of more common and cheaper material. This practice is encouraged also by the fact that the inlet valves now are generally made of somewhat larger diameter, so they would not be interchangeable with the exhaust valves even if they were made of the same material.

Both the mechanical and the thermal stresses on engine valves increase with the speed of operation, and as engine speeds have increased continuously, there has been a constant search for better materials, especially for the exhaust valves. Silicon-chromium (Silcrome) steel containing 3-3.5 per cent silicon and 8-9 per cent chromium came into use during the early twenties, and was considered an excellent exhaust-valve material at the time. This steel possessed good workability and good machining qualities, but it left something to be desired with respect to hot strength. While at normal temperature it showed a tensile strength in excess of 200,000 psi, at 1200 F this dropped to 42,000 psi, and at 1600 F it was only 4600 psi. The steel began to scale at 1800 F. Its resistance to warpage and corrosion at high temperatures was poor.

In the middle thirties specific outputs had increased so much that a better material was needed for heavy-duty bus and truck engines. What was called for particularly was higher hot strength and a higher scaling temperature. These properties could be obtained by a more liberal use of alloying elements, particularly chromium, and a new type of valve steel was then introduced of which Silcrome XB, developed by Thompson Products, Inc., is representative. This has a higher carbon content than the original Silcrome steel, viz., 0.60-0.86 per cent; less silicon, 1.25-2.75; but more than twice as much chromium, 19.00-23.00, and in addition from 1.00 to 2.00 per cent nickel. This steel resists warping much better, and it also has greater resistance to heat corrosion. At 1600 F its tensile strength is 7625 psi, and its scaling temperature is 2150 F.

Austenitic Valve SteelsMore recently so-called austenitic, non hardening steels have been introduced as a

material; for exhaust valves. They excel silicon-chromium steel with respect to hot strength, impact value, hot hardness, and resistance to oxidation and corrosion. These steels, which contain high percentages of chromium and nickel-the combined contents of these two elements usually ranging between 25.00 and 30.00 per cent-in addition to being non-responsive to heat treatment, are non-magnetic.

An austenitic valve steel contains 0.30-0.45 carbon, 0.80-1.30 manganese, 2.50-3.25 silicon, 17.50-20.50 chromium, 7.00-9.00 nickel and not over 0.03 phosphorus and sulphur each.

It has a hot strength of 17,500 psi at 1600 F and a scaling temperature of 2200 F. But while these austenitic valve steels possess many advantages, they also have some undesirable qualities. One thing against them is that their coefficient of heat expansion is materially greater than that of silicon-chromium steel (0.000011 as compared with 0.0000078). This calls for a slightly greater clearance between the valve stem and its guide and between the valve and its tappet. For heavy-duty engines a valve-stem clearance of 0.010 to 0.015 in. per inch of stem diameter is recommended. The hardness of austenitic steel is rather low (about 45 Rockwell C, as compared with 55 for the original Silcrome steel) ,and it does not resist the hammering action on the tip very well, especially where there is line or point contact, as with rocker arms contacting the tip. To meet this condition, valves of heavy-duty engines sometimes have tips of Stellite or tool steel applied by either electric or acetylene welding. The wear of austenitic valve stems' in the guides also is somewhat more rapid than that of other steels. This difficulty may be overcome by nitriding the stems, but the injurious effect of tetraethyllead on nitrided surfaces would seem to discourage this practice. Cold-working (rolling) of the stems to increase their hardnessalso has been suggested. The various processes by which nonmetallic coatings are formed on wearing surfaces to keep them from scoring may be applied also to valve stems. Carbon-steel stems may be. welded to heads of austenitic steel, which has the further advantage that the carbon steel is much lower in cost. This process of building up valves by welding has been carried to its logical conclusion by welding heads of a material resistant to scaling and pitting, to a stem of a material having good bearing

87

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive Enginesqualities in cast iron under conditions of poor lubrication, and to the latter a tip of a material which is capable tappet action well of withstanding the

Precipitation-Hardening SteelThe latest addition to the list of exhaust-valve materials is a steel intermediate between

the ferritic and the austenitic types; it shares the property of harden ability with the ferritic steels, and high hot strength with the austenitic. This steel, Silcrome XCR, contains 0.40-0.50 carbon, not more than 1.00 manganese, 23.25-24.25 chromium, 4.50-5.00 nickel, 2.50-3.00 molybdenum, and not more than 0.035 phosphorus and sulphur each. At 1600 F it shows a tensile strength of 20,000 psi, which is a tremendous improvement over the 4600 psi of the original Silcrome. With respect to heat expansion it is intermediate between the ferritic Silcromes and the austenitic type. Its oxidation and corrosion resistances are excellent, but its workability is only fair, and its machinability definitely poor. Valves of Silcrome XCR are hardened to 48-58 Scleroscope all over, and owing to the relatively great hardness, both the seat and the stem wear well. This steel must be forged within a narrow temperature range; if overheated it loses its hardenability; while if forged at too lower temperature, it is likely to shatter, its impact value being quite low. In spite of these drawbacks and its rather high cost, this steel is being used extensively for the exhaust valves of heavy-duty engines.

Materials for Inlet Valves

The inlet valve does not present nearly so difficult a materials problem as the exhaust valve, as the temperature attained by it in service is always considerably lower. Two types of low alloysteel are used extensively for inlet valves. Nos. 3140 and 8645. The former is a chrome-nickel steel- containing 1.0 to 1.5 percent of nickel and 0.50-0.80 percent chromium (besides 0.37-0.45 per cent carbon and 0.60-0.95 per cent manganese); the latter a chromium-nickel-molybdenum steel containing 0.35-0.75 percent nickel, 0.35-0.65 per cent chromium, and 0.12- 0.25 per cent molybdenum, besides normal amounts of carbon and manganese. Some-use has been made of a medium-alloy chromenickel- silicon steel-with(8 to 9 per cent nickel, 12 to 13 per cent chromium, and 2.5 to3 per cent silicon. This CNS steel, which has low carbon and manganese contents, is said to be immune to the corrosive influences of tetra-ethyl lead.

Miscellaneous Considerations

1. An adequately designed valve with proper material can also fail due to local stress concentrations if there is any unevenness around the valve-seat interface on account of distortion of valve heads or seats, bending of valve stem or trapping of carbon particles between the valve and the seat.

2. Excessive surface finish of the valve stem will result in loss of lubricating oil film, while excessive roughness of the stem would increase the guide wear. A thin layer of chromium giving the surface finish of about 0.5 μm would provide the optimum condition.

3. As engine thermal efficiency is increased with increase of compression ratio, lower valve temperatures would result in case of higher compression ratio.

4. Arranging the inlet and the exhaust ports in the cylinder head alternately would increase the transfer of heat from the exhaust to the inlet valves, compared to the case when the like valves are placed together. This would result in decreased exhaust valve temperatures. However, this would also complicate the design of the inlet and the exhaust manifolds in case both are to be on the same side of the engine.

88

www.jntuworld.com

www.jntuworld.com


Form and Dimensions of Head The valve head should be made of the minimum thickness consistent with strength requirements.

In American practice, the top of the head is usually made spherical and the bottom surface conical. This gives a head which is much thicker at the center (where the bending moment is the maximum) than at the edge. Poppet valves have been standardized by the.S.A.E.

The seat on the valve head must project slightly beyond the seat in the cylinder casting at both top and bottom, in order that no shoulder will be formed on the-casting when the valve is ground in. A fairly wide seat is an advantage, as it helps to keep down the temperature of the head. The heat absorbed from the burning gases by the valve head has only two paths through which to flow off-down the valve stem to the valve guide and thence into the cylinder block and jacket, and through the valve seat directly into the block-and with the- conventional design by far the greater portion passes off through the seat.

In Fig. is shown the form of head generally used for steel valves in American practice, as fixed by the S.A.E. standard.

Fig. is the so-called tulip valve, used to a considerable extent in aircraft and racing engines, which is thought to facilitate How through the valve port.

Fig. shows a form of valve that is intermediate between the S.A.E. standard and the tulip type, which has come into use in recent years. It has a Hat-top head and a rather large fillet between head and stem, which latter tends to improve How conditions and to add to the strength of the stem near its junction with the head. Sometimes that part of the stem which does not enter the guide is made to taper slightly from the fillet down.

To ensure good seating of the valve, -it should be made with an interference angle of 1 deg. That is to say, the included angle of the valve face should be made 1 deg greater than that of the seat face, so that initially the valve seats only on the outer edge.

89

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesSodium-Cooled Valves

The exhaust valve temperatures in modern engines reach very high values of the order of 750°C. In heavy duty engines, it may be still higher. Therefore, cooling of exhaust valves becomes very important. To do this cooling water jackets are arranged as near the valve as possible. In many cases, nozzles are directed towards the hot spot caused by the exhaust valve.

Large valves of heavy-duty engines can be kept at a reasonably low temperature by sodium-cooling, which is now employed extensively for aircraft-engine valves, and occasionally for bus- and truck-engine valves.

Originally a mixture of potassium nitrate and lithium nitrate was used, which melts at about260 F, but later metallic sodium -was substituted for these salts.The advantages of sodium are a low specific gravity (0.97), a high specific heat, a low melting point (207 F), and a high boiling point (1616 F). Fig. shows a section of a sodium-cooled valve designed for use in aircraft engines. The stem is of somewhat larger diameter than usual, and is drilled out from the end & the chamber thus formed, after being nearly closed at the end by swaging process, is filled about half full with metallic sodium. Assuming the valve to be positioned as in an L-head vertical engine, the sodium will be at the bottom (tip end) of the chamber when the valve is closed. it may be seen that in normal operation the valve is alternately accelerated and decelerated at rates many times that due to gravity, with the result that the sodium is thrown violently from one end of the chamber to the other. When at the top end, it absorbs heat from the hot wall, which it gives up to the cooler, lower end of the stem when next it drops to the bottom of the chamber, whence the heat passes to the valve guide and into the cylinder block. These sodium-cooled valves are sometimes furnished with an inner lining of copper, which latter has four times the heat conductivity of valve steels. The end of the stem is sealed with a steel plug, over which is welded a cap of hard steel.

In the sodium-cooled valve shown in Fig., which is of an earlier design, only the stem is hollow and partly filled with sodium. Later it was found possible to make both the head and the stem hollow, as in Fig., which shows a valve designed for installation in the cylinder head. In operation the highest temperatures are reached by the center portion of the top surface of the valve and a point on the stem some distance below the head. In comparative tests under similar conditions with a conventional "solid" valve, a sodium cooled valve with hollow stem, and a sodium-cooled valve with hollow stem and head, the maximum temperatures reached by the center portions of the heads were approximately 1380 F, 1240 F, and 1170 F, respectively. At the seat the temperatures of the valves in these cases ranged between 1000 F and 1100 F. In solid valves the higher temperature of the center portion of the head sometimes results in the formation of cracks at the seat.

There can be no doubt as to the great operating advantages of sodium-cooled valves, and the only reason they are not widely used in automotive engines is that they are rather expensive to produce. Considerable effort has been devoted in recent years to the development of improved processes of production. In one process a piece of steel tubing of slightly more than the diameter of the finished stem, after being cut off to the right length, is upset at one end and then spun to what may be called "tulip shape." A disc of steel is then welded on to close the opening in the head, while the end of the stem is closed in the same way as in the case of a valve in which only the stem is hollow.

90

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesValve Seats

The valve seats must be faced very accurately, so that there is complete contact between the valve and the valve seat when the former closes. Valve seat face is thus ground to the same angle to which the valve face is ground. This may have any value from 30° to 45°. For cylinder blocks or heads made of grey iron, the inlet valve seats are directly machined on the cylinder blocks or heads as the case may be because working conditions are not severe. These are called integral seats. However, where aluminium blocks or heads are used, separate valve seat inserts are employed even for inlet valves. For the exhaust valves, always the separate valves seat inserts are used, the operating conditions being very severe. Insert seats are also used as salvage procedure when badly damaged integral seats are reconditioned. Valve seat inserts are simply rings made of alloy steel consisting of chromium, silicon, tungsten or cobalt with a conical seat on one of the inside edges. These are force-fitted in the recesses machined in the cylinder head. When worn, these inserts can be easily replaced.

91

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesValve Springs

Helical springs are used to keep the valve in constant contact with the tappet and the tappet with the cam. Since the spring is subject to compressive loads, it is ground flat at each end to ensure even distribution of pressure. The coil ends are also placed diametrically opposed to avoid the bending tendency of the spring under compression. The arrangement for the retention of the springs is simple. A ring split into two halves with internal projection to fit into the valve spring retaining groove and the outer surface tapered is employed. Over the split ring another ring is inverted which supports the spring. The valve springs are subject to heavy service. These are, therefore, made from high grade spring steel wire, the materials being generally hard-drawn carbon steel or chrome-vanadium steel. Valve springs are often shot peened to make them fatigue resistant.

92

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesValve Rotators

Ordinarily the temperature is not uniform around the circumference of a valve head, being highest where the greatest mass of hot gases passes over it, as on the side nearest the cylinder in an L-head engine. By actual test it was found that in an engine of this type, under certain operating conditions, the exhaust valve- seat insert reached a temperature about 280 F higher on the "near" than on the "far" side, and there can be little doubt that even greater temperature differences exist between opposite sides of the valve head. Such temperature differences cause distortion and leakage. If while in operation the valve could be made to rotate on its seat, that would tend to equalize temperatures and keep down their maximum value. It would tend to keep the seat clean and if leakage should start at any point of the circumference, the resulting damage to the valve would be reduced. A number of so-called valve rotators have been brought out, but so far they have not come into extensive use, probably because they have not always been reliable. Some merely "free" the valve of the restraining effect of the friction due to the spring pressure, while others, in addition, convert some of the axial force producing the opening or the closing motion into a tangential force. The problem of a simple mechanism that will positively rotate the valve on its seat evidently is not an easy one, and complicated and delicate mechanisms can hardly be tolerated in the valve gear. However, according to one valve specialist, positive rotation is more effective in prolonging valve life than any other known means.

Fig. shows a valve-rotating mechanism. The valve spring rests on a seating collar which transmits the spring pressure to the retainer cap through a conical spring washer. When the valve is closed (left view) the pressure of the valve spring is relatively light, hence the spring washer is distended and bears with its inner edge on the retainer cap at 2. As the valve is being lifted (right view) the pressure of the spring increases, the spring washer flattens out, and its point of support is transferred from 2 on the retainer cap to 3 on the steel balls, which latter rest on inclined surfaces on the retainer

cap. The effect of the incline is to create a horizontal force component tending to produce relative angular motion between spring washer and retainer cap. Both parts are subject to friction, but the restraining moment on the spring washer is much greater than that on the valve, and as a result an angular motion is imparted to the assembly consisting of valve, retainer cap and retainer lock. During each valve lift each ball moves down the incline, and in between lifts it is returned to the top of the incline by a light spring.

93

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesValve Guides

In low-cost engines the valve guides are sometimes cast integral with the cylinder block or head, but the more general practice is to make them in the form of a bushing which is pressed into a hole drilled in the cylinder or head casting. Separate guides have the advantage that they can be renewed when worn. A fairly close fit for the valve stem in the guide is necessary, particularly in the case of inlet valves, because if there is excessive clearance between the stem and guide of these valves, air will be drawn into the cylinder through this clearance during the inlet stroke and dilute the charge received by the cylinder. If the clearances of the valve stems of an engine differ, the charges received by the different cylinders will be unequally diluted, a condition that cannot be corrected by carburetor adjustment. In passenger-car engines the clearance between inlet-valve stem and guide ranges between 0.002 and 0.003 in. It is very difficult to lubricate the exhaust valve guides effectively, owing to the high temperatures reached by them, and these, therefore, are subject to comparatively rapid wear. Exhaust-valve stems should have a clearance of between 0.002 and 0.004 in. in their guides.

Separate guides are usually made of cast iron, of 1/8 to 3/16-in. wall thickness, and are made a force fit in the hole in the cylinder or head casting. Sometimes the guide is provided with a flange or shoulder which abuts against a finished surface on the cylinder or head casting as the guide is pressed into position, but more generally this is omitted.

The exhaust-valve guide is preferably made to extend substantially up to the point of the stem where the fillet under the head begins, as it has been found that this keeps the valve head cooler than a design which leaves more of the stem exposed to the action of the hot gases during the exhaust period. A slight further reduction of the valve-head temperature can be achieved by counter boring the upper part of the valve guide, or, alternatively, undercutting the upper part of the valve stem, so that there is no

contact between stem and guide over this portion of the length of the latter, which then serves merely as a shield for the valve stem, protecting it from the hot gases rushing by during the exhaust period. The guides of inlet valves are preferably made shorter, so that they project into the valve pocket only very slightly, as this reduces the resistance to flow. As indicated in Fig., in the case of the exhaust valve it is advantageous to water-jacket the whole length of the boss for the valve guide, as this keeps down the valve temperature. Lengths of valve guides are usually between 2 and 3 times the valve-port diameter.

An unusual type of valve guide (Fig.) was used in early Ford engines. The valves of this engine were formed with an enlargement on the end of their stem, which supported a horseshoe-shaped spring retainer. Owing to this enlargement, it was impossible to insert the valve into a one-piece guide. The guide therefore was split through its axis, and valve and guide were inserted and with drawn together. The guide was held in position in the bore in the cylinder block by a horseshoe-shaped stamping which entered a groove turned in the guide and rested against a machined surface on the block.

94

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesDesign of Valve Tappets

In L-head engines the valves are operated from the cams through the intermediary of tappets, which latter usually consist of a cylindrical steel part moving in a cast-iron guide formed on or secured to the crankcase. The tappet carries the cam follower at its lower end and is provided with clearance adjusting means at its upper end.

In the design of these members, lightness is an important consideration, since the strength of the valve spring required and the shock and noise produced by the lifting action are directly proportional to the weight of the parts moved by the cam, which include the tappet. In Fig. is shown a section of a roller-type tappet. Its body A is drilled out from the top for the sake of lightness, and its lower end is slotted to receive the roller B carried on

roller pin C. A roller-type cam follower must be held in alignment with the cam, and to this end the guide is extended downwardly and slotted to receive the roller. At its upper end the tappet is threaded internally to receive the adjusting screw E, which latter also is drilled out. After a clearance adjustment has been made by means of screw E, the latter is locked by check nut F.

Two tappets for use with mushroom-type cams are shown in Fig.. The one on the left is a very light design which provides ample bearing surface. The other one is of tubular stock and has the foot welded to it. A foot of cast iron sometimes is welded to a tubular shank and is hardened by

chilling. In other cases the entire tappet is made of cast iron. Tappets have been made also in which the wearing surface of the foot was provided with a veneer of Stellite or some other hard alloy.

95

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesTappets are used also in valve-in-head engines, but in that case

the clearance-adjusting means are located on the top of the engine, where they are more accessible. Two designs of tappets for this type of engine are shown in Fig.. The one on the left is thimble shaped and formed on the inside with a spherical seat for the ball end of the side rod. Solid side rods are used where the distance between the tappet and the rocker lever is comparatively short, as where the camshaft is located on the side of the cylinder block; with the camshaft in the crankcase, tubular side rods are preferable, because of their greater resistance to buckling. It will be seen that the side rod is shown to make a small angle with the axis of the tappet. This has the advantage that its reaction on the tappet has a small horizontal component which can be made to counteract the friction between cam and tappet, thus reducing the friction encountered by the latter. In the design shown at the right the spherical seat for the tubular side rod is at the top

The tappet bearing surface must be brought down as close to the cam as feasible, because the load on it is an overhanging load, and if the overhang were great the pressure near the lower end of the bearing would be quite intense. With mushroom type cam followers it is a good plan to offset the follower lengthwise from the middle of the cam, so it will rotate in. its bearing, and no groove will wear in its foot. Such an arrangement is shown in Fig..

In the past it has been customary to make the contact surface of the tappet foot flat. However, the cam usually is located closer to one supporting bearing than to the other, and when the camshaft flexes under the lifting impact, its contact element or surface will be inclined toward the flat tappet surface, which results in stress concentration at one side of the cam. To prevent such stress concentration, tappet-foot surfaces now are sometimes made spherical, with a radius of 30 in. or more. To cause

tappets with spherical contact surface to rotate, the cams are made to taper slightly in the axial direction.

Clearance RequiredThe amount of valve clearance required has increased in the course of time, because modern

engines, on account of their much higher speeds, operate at higher temperatures. In passenger-car engines of the L-head type the average clearance between pushrod and valve stem with the engine cold is about 0.010 in. for the inlet, and 0.012 in. for the exhaust valves. For service purposes the "hot" clearances usually are specified, and are somewhat smaller than the figures given. Valve-in-head engines require greater clearances, especially where the cam motion is amplified by the tappet levers.

96

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesAutomatic "Zero-Clearance" Tappets

By means of a hydraulic device supplied with oil from the engine lubricating system (or from a separate source), it is possible to take up clearance between the tappet and valve automatically as soon as it develops. A hydraulic tappet is used extensively by engine manufacturers. A sectional view of this tappet is shown in Fig.. The adjusting means is a separate hydraulic unit which is set into the valve lifter body during the process of assembly. The lifter body is of substantially the same design as the conventional valve lifter, hence only the hydraulic unit needs to be described.

Oil from the engine lubricating system is delivered to a chamber adjacent to the tappet guide, and is fed into the lifter from a point near the bottom of the chamber. It enters the annular groove in the tappet body and passes through the radial hole therein, into the cavity below the hydraulic unit. In order that the hydraulic unit may function properly, the oil must be free from air bubbles, and suitable means are provided to de-aerate it before it enters the tappet. The tube extending down from the hydraulic unit also assists somewhat in the separation of air, and it assures that practically all of the oil in the cavity forms a reserve supply for starting. Oil from the cavity passes through the ball-type check valve into the adjusting chamber below the plunger. A light spring holds the plunger against the valve stem, and the face of the lifter in constant contact with the cam. The bottom of the plunger

clears the bottom of the bore by about 1/I6 in. In this manner the air is worked out at the time of installation, the lifter is always filled with oil, and the clearance is entirely taken up by the oil column below the plunger. Hydraulic tappets of this type assure silent operation, eliminate the need for valve adjustment, and increase valve life. They obviate the need for a quieting ramp on the cam.

97

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesRocker Arm and Rocker Shaft

The function of the rocker arm is to reverse the upward motion of the push rod to downward motion of the valve and vice versa. The rocker arm may be either solid or hollow.

A stationary hollow rocker shaft serves as a pivot to the rocker arms and provides passage for lubricating oil simultaneously. Rocker arm is made of steel (forged or stamped) or iron (cast). Cast rocker arms are comparatively cheaper but are not as strong as forged or stamped ones. However, these give satisfactory service in cars. Stamped rocker arms have been found to be light, very strong, yet cheapest of all the types. Rocker shafts are made from hollow steel tubing. A typical material for these would consist of 0.55% carbon, 0.2% silicon, 0.65% manganese and the remainder iron. After machining the shaft is case-hardened. It is mounted on cast iron or aluminium pedestals placed between each pair of rocker arms.

98

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesPush Rod

It serves to transmit the reciprocating motion of the valve lifter to the rocker arm. Since the valve lifter moves in a straight line whereas the rocker arm end moves in an arc about its pivot, to provide compatibility of the two, the push rod forms part of ball and socket joints on both ends. Push rods are made of carbon-manganese steel. A typical push rod steel contains 0.35% carbon, 0.2% silicon and 1.5% manganese. After hardening and tempering, a hardness of about 250 B.H.N. is obtained. The push rod may be either solid or hollow. A hollow push rod is lighter, resulting in decrease of inertia forces. Further, it may also serve as a passage for oil for lubrication of the valve actuating mechanism.

99

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesDual ValvesLarge valves are more troublesome than small ones, because a large disc will be warped more by

the heat, and, besides, the weight of the valve increases rapidly with the linear dimensions, hence the stress on the valve and its mechanism due to rapid opening and closing becomes very great for large diameters. For this reason it has become customary in high-speed engines with large cylinder bores to use two inlet and two exhaust valves per cylinder.

An experimental investigation of the relative capacities of large and small valves was made in connection with the development of the Liberty aircraft engine, and the conclusions reached were that at the same pressure drop, one valve of diameter D and lift h is equal in capacity to, first, a pair of valves of diameter O.707D (equal port area) and lift 0.70h and, second, a pair of valves of diameter 0.6D and lift h, for values of h not exceeding about O.25D. Engines with two, three, four & five valves are shown in the following Figures.

100

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesValve Timing The valves are operated by cams on a shaft which turns at one-half the speed of the crankshaft,

so that each valve is opened and closed once during two revolutions of the crankshaft. In order that an engine may operate satisfactorily at high speeds, it is necessary that the exhaust valve open before the end of the power stroke and close after the completion of the exhaust stroke; and that the inlet open before the end of the exhaust stroke and close after the completion of the inlet stroke. This involves an overlapping of the exhaust and inlet periods, which is made necessary in part by the very slow opening and closing motions now employed for the sake of quiet operation. If the inlet began to open only after the exhaust had closed, the effective valve opening during a considerable part of the inlet stroke would be so small that the incoming charge would be seriously throttled. For an engine which is intended to "peak" at 3000-4000 rpm the valve timing shown in Fig. should prove satisfactory. That there is considerable latitude with respect to the different valve functions may be seen from the following table which applies to 1953 passenger-car engines:

101

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesOverhead Valves

Valves in the head are operated either by tappet rods extending up the side of the cylinders, or by means of an overhead camshaft. When tappet rods are used, they extend up through an enclosed space and the rocker arms, etc., at the top of the engine are enclosed by a valve cover, which is usually of pressed steel. When the whole valve mechanism is thus enclosed, not only are any noises produced by it muffled, but the joints can all be effectively lubricated and all bearings are protected against dust. In multi-cylinder overhead-valve engines the rocker arms are mounted on a hollow shaft, to which is connected a lead from the pressure lubricating system. At the center of each rocker arm bearing an oil hole is drilled through the wall-of the hollow shaft, so that oil will feed to the bearing surface, and sometimes a hole is drilled lengthwise of the rocker arm from the rocker bearing surface to the point of contact with the valve stem. By placing the breather pipe on the valve cover and establishing communication with the crankcase by means of the tappet-rod passages, an oil-misty atmosphere is created in the valve chamber, and the lubrication of all bearings is provided for.

With tappet rods extending up the side of the cylinder block, as shown in Fig., the weight of the moving parts naturally is greater than in L head engines, and every effort should be made to lighten these parts. The tappet rods preferably are made of steel tubing of an outside diameter of about 3/8 in. for the size of cylinder used in passenger-car engines. Into the lower end of the tube can be fitted a thrust pin with a half-round head which has a bearing in a socket formed on a thrust block set into the hollow tappet. At the top an internally threaded sleeve with a hexagon on it is fitted over the tube, and receives the clearance-adjusting screw, which latter has a spherical head nested in a socket formed on the end of the rocker-lever arm. Owing to the fact that all contact surfaces are very liberal in size, and of such shape that they naturally retain any oil getting onto them, there is usually very little wear on these surfaces. The rocker levers usually are drop forgings, though one manufacturer has made them of steel pressings.

102

www.jntuworld.com

www.jntuworld.com


In determining the weight of the valve-reciprocating parts where rocker levers are used, it is necessary to first find the radius of gyration of the rocker lever, which can be done either by calculation or experimentally by the pendulum method. The actual weight of the rocker is then reduced in the proportion of the radius of gyration to the length of the rocker arm bearing on the side rod, and the value thus obtained is added to the weight of the tappet and side rod. The weight of the valve and parts moving with it is reduced in the proportion of valve motion to tappet motion, and then added to the other weights.

103

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesOverhead Camshahs

The simplest and most direct method of actuating valves in the cylinder head is by means of an overhead camshaft, Fig.. It reduces the necessary weight of valve reciprocating parts, thus making it possible to get along with lighter springs

and to increase the maximum speed.It appears that one reason for the less quiet operation of the engine is that the source of the noise (the cam gear) is directly underneath the hood, and the noise is therefore more readily transmitted to the passengers ears than when it originates down in the crankcase. It has been found that a contributing factor to noise in the valve gear is the discontinuity of the torque; that is, when the nose of a cam has passed a cam follower, the pressure of the valve spring causes the cam and its shaft to snap forward, thereby taking up the clearance between gear teeth. Trouble from this source may be guarded against by "burdening" the camshaft with additional load, such as the fan, water pump, or generator.

104

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesValve Actuating Mechanisms

In all the valve actuating mechanisms a cam driven at half the crankshaft speed is used to operate each valve inlet or exhaust. However, there are different methods of operating the valves from the cam.

These may be broadly divided into two types viz.,

1. Mechanisms with side camshaft and 2.The mechanisms with overhead camshaft.

105

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive Engines1. Mechanisms with side camshaft

In these the camshaft is on the side of the engine and the valves are operated either directly by the cams or through the push rods and rocker arms.

These may be further classified as:

(a)Double row side valve mechanism (T-head)This is the oldest type of valve actuating

mechanism and is shown in Fig. In this the inlet and the exhaust valves are operated by separate camshafts which makes the mechanism complicated. Moreover the shape of the combustion chamber provides poor combustion and low engine performance due to which this type of mechanism is obsolete.

(b) Single row side valve mechanism (L-head)

In this the inlet and the exhaust valves are all arranged in a single row and operated from the same camshaft (Fig.). This method was once quite popular on account of the following advantages:

(i) Low engine height.(ii) Low production cost.(iii) Quiet operation.(iv) Ease of lubrication

This mechanism is, however, no more in use, because it was found to be very inefficient on account of the complicated shape of the combustion chamber which is more prone to detonation. There were also restrictions of space on the size of inlet valves that would be used. Moreover, difficulties were experienced in cooling the exhaust valves.

106

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive Engines(c) Overhead inlet and side exhaust valve mechanism (F-head)

This is a combination of the two systems described above. Overhead valve mechanism is used for the inlet valve operation and the side valve mechanism for the exhaust valve. It is used in F-head engines. This mechanism is simpler than the overhead camshaft operated types and allows the use of larger inlet valves, but larger valves being heavier, there is also a limitation on the maximum speed of the engine that could be allowed. F-head engines were found to be less efficient and were also more expensive due to which these have also become obsolete.

(d) Single row overhead valve mechanism (I-head)This type is used quite extensively these days and is

shown in Fig.. The cam operates the valve lifter which in turn actuates the push rod. The push rod further operates the rocker arm, which actuates the valve.

This type of mechanism is having the following advantages:

(i) Higher volumetric efficiency than the side valve design.

(ii) Higher compression ratios can be used.(iii) Leaner air-fuel mixtures can be burnt.(iv) The rocker-arm leverage makes it possible to

impart desired cam profile lift multiplication to the system and hence use smaller cam lobes compared to the side valve mechanism.

The above advantages are, however, accompanied by some drawbacks of the mechanism.These are,(i) The valve operation, on account of the elasticity of the system and the resulting vibrations, is

not very precise while accelerating or operating at high engine speeds.(ii) Larger valve lifter clearances are required.(iii) Noisy operation.(iv) Greater maintenance required due to more wear at more joints.

107

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive Engines2. Mechanisms with overhead camshaft

The valve operating mechanisms with overhead single or double camshafts are highly efficient. However, with these considerably more lubricating oil is needed to flood the cam profiles as compared to the overhead valves operated by side camshafts. Moreover, they have the disadvantage of higher initial costs.

Figure. shows single row valves operated by a single overhead camshaft and an inverted bucket type follower. With this type of follower, the camshaft is arranged directly over the valve stems. This type of mechanism is direct and very rigid so that valve movement follows precisely the designed cam-profile lift. Moreover, valve stems are not subjected to side-thrust which means less wear. Tappet clearances are also quite small and do not require adjustment very often. However, drive to the camshaft is quite

complicated, positive lubrication is required and adjustment of valve lifter clearance is relatively more difficult.

A similar valve-operating mechanism with end-pivoted rocker arm is shown in Fig.. The rocker arm provides leverage ratio, which enables the designer to provide smaller cam profile. Moreover, the inertia of rocker arm follower is less compared to the sliding bucket type described earlier and adjustment of tappets is easy. However, due to the elastic bending of the rocker arm, the stiffness of the system and hence precision of valve operation is decreased, a side-thrust is produced to the valve stem and guide and more wear and noise occur.

108

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesIn Fig. is depicted a mechanism for inlet and exhaust valves in separate rows, but operated by a

single overhead camshaft with inverted bucket type follower and the pivoted rocker arm.

However, quite often the double-row valves are operated by two separate overhead camshafts as shown in Figures.

109

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesComparison of the Side Camshaft and the Overhead Camshaft Mechanisms

The overhead camshaft type valve actuating mechanisms is generally preferred over the side camshaft type mainly because of its greater rigidity since the camshaft directly operates the valve instead of operating through push rod and rocker arm. Due to this the valve is opened and closely quicker with decreased vibrations and undesirable oscillation. This means in case of high-lift, high-acceleration cam profile, the valve operation in case of overhead camshaft is much more precise and smooth than in case of the side camshaft valve system.

However, in case of overhead camshaft with inverted bucker follower, the valve lift is equal to the cam lift, whereas in case of the side camshaft, the valve lift can be adjusted by suitable design, for a given cam lift. This means that in case of overhead camshaft, the cam size has to be relatively larger for the same valve lift, which leads to higher cam-to-follower velocities and relative rubbing velocities resulting in side-thrust reaction caused by the cam action.

110

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesDesmodromic

A form of valve actuation that uses mechanical means to open & close the valves, thus eliminating valve springs & the resulting bounce at high speeds. They featured in the Mercedes racing cars of the mid-50’s but are now associated with Ducati road & racing motor cycles

111

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesValve Production

The forgings for conventional poppet valves may be produced by several processes, including drop-forging, upsetting extruding, and electric gathering. In each case the stock used comes in rod or bar form. The extrusion process employs stock of a diameter approximately 70 per cent that of the finished valve head and forms the stem by forcing some of the material through a die while at a red heat. To prevent fracture during the shearing operation, the bars are first brought up to red heat either by induction or in an open furnace. Burrs on the slugs are removed by tumbling.

The average valve forging is completed in one heating on a 500- ton or 750-ton press operating at about 45 rpm. Valve slugs are placed in a hopper which feeds them into an induction-heating coil. The rate of feed can be varied by the operator from 8 to 20 slugs per minute, by means of a variable-speed motor drive. The average automotive valve requires a 1-in. slug, and of these slugs 16 are heated to about 2000 F per minute, the power consumption amounting to 50kw.

After the Forging Process, next process is Annealing.Most exhaust-valve and all intake-valve forgings are annealed to relieve forging stresses. The

treatment varies with the material, annealing temperatures ranging from 1200 F to 1800 F, and annealing periods from one hour to six hours. The process usually is carried on in a chain-type conveyor furnace, a dual control system permitting of changing the temperature during the annealing cycle. Another operation in the heat-treating department consists in grit-blasting the forgings in a tumble blast unit, to remove scale formed in the forging operation and to improve the appearance of the forgings.

The next process in valve production is Machining and Grinding OperationsMachining and grinding operations are performed on machine tools set up to make possible

progressive line production. After passing through this line and having been subjected to the final inspection, the valves go directly to the shipping room for oiling and packing. The lay-out of the line for a particular valve depends on such factors as size, type, material, and quantity.

All automotive valves have the following operations carried out on them: Roll straightening of stem and head, inspection for straightness, center-less rough-grinding of stem, hardening tip, finish grinding stem, finish grinding seat, inspection

The valve forging is straightened on a machine of the Waterbury- Farrell thread-roller type, the valves are heated to 1425 F in a Surface Combustion chain-type furnace, which is loaded through a hopper and discharges into a chute feeding the straightening machine. The center-less grinders used to grind the stems are pro vided with an infeed attachment which causes the valves to drop into position between the wheels, and with a "kicker" which removes the valves at the end of the grinding cycle. All valves are forged with a flash on the periphery of the head. Most automotive valves are "forge-finished" on top and under the head, but the heads must be finished on the outside diameter and on the seat. That operation, which is usually performed on multiple-spindle automatics with carbide tools is followed by machining of the tip of the valve stem and of the ,keeper groove chamfering of the tip, and facing the valve to length.

Heat Treatment

Valves made of XCR and chrome-manganese. steel usually are age-hardened after the semi-finish machining operation, and many intake valves of SAE 3140 steel also are hardened in the semi-finish stage. If the valve is to be heat-treated, semi finish grinding of the stem follows the heat treatment; otherwise it precedes machining of the retainer groove. Heat-treated valves usually are grit-blasted to remove surface scale, and then are hand straightened before any grinding is done on them. The semi-finish grind is performed on a Cincinnati centerless grinder.

112

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesHardening and Grinding of Tip

Tips of high-production valves usually are hardened by induction heat, those of others by the conventional flame-hardening process. A 20-kw high-frequency generator supplies the current for induction hardening. Induction hardening permits much better control of the hardness than flame hardening. It is usually specified that tips shall be ground to 15 RMS surface finish, and square with the valve stem to within 0.0015 in.. Large-production valves have the tip ground on automatic grinder equipped with a rotating fixture in which the valves are located from the seat face.

Grinding of the seat is the last operation on the valve, and usually is done on a hydraulic grinder. The total indicator reading of seat runout to stem can be held to less than 0.001 in. in large-volume production. For this operation most valves are located from the tip end.

InspectionAll valves are visually inspected for surface defects, possible operations missed, etc., at 'the end

of the line. Scleroscope and Rockwell machines are used to check the hardness. Stellited and welded valves are inspected 100 per cent. Magnaflux inspection is made to discover seams, subsurface stringers, and other defects difficult to recognize with the naked eye. Standard gauges are used throughout the line, masters being provided to check the dimensions from seat to tip, from seat to groove, and from groove to tip, and determinations are made of the runout of the tip and the seat relative to the stem. The machined retainer grooves are checked for form, radii, etc., in Comparators with a magnification of 25 to 1.

113

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesSLEEVE VALVES

As the term indicates, sleeve valves are cylindrical in shape. They surround the piston and actually form the working cylinder. There are two types of sleeves valves, viz., the single sleeve and the double sleeve types.

A single sleeve valve is shown in Fig.. The sleeves are made of steel.The advantages of sleeves are:I. Simplicity of construction.2. Silent in operation, because there are no valve cams, tappets, valves, etc. which make noise.3. A longer period of running before decarbonization becomes necessary (50,000 km as compared to about 10,000 km for poppet valves).4. Reduced tendency to denote, because there are no hot spots, path of flame travel is short and combustion chamber is of symmetrical shape.5. Higher thermal efficiency is attained.

However there are certain disadvantages also because of which it has become obsolete:I. High oil consumption, because larger area of sleeve surface has to be lubricated.2. Gumming.

ROTARY VALVES

Many types of rotary valves have been developed. Fig. shows a disc type rotary valve. It consists of a rotating disc, which has a port. While rotating, it communicates alternately with inlet and exhaust manifolds. The main advantage of rotary valves is their uniform and noise- free motion. However, there are many difficulties in pressure sealing. Economical valve lubrication is another problem.

References• High speed Combustion Engines-P M Heldt• Automobile Engineering-Dr. Kirpal Singh• Machine Design-Abdulla sheriff• Theory & Practice in I C Engines-C F Taylor• Autocar India Illustrated Automotive Glossary

114

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesVALVE DESIGN

• Material -For valve -Nickel steel for inlet valves; & High nickel chromium steel for exhaust valves

(due to high temp. and corrosive action)

For valve seat-Cast iron or bronze – Replaceable, (Cast iron-for big engines – due to economic reasons.)

• Size of valve ports

Vg×a=Ap×Cp ave.

Where Vg=velocity of gas ≈2300 to 3300m/min-for stationary/marine engines

≈3300 to 5000m/min-for automobile engines

a=port area=4

2portdπ

; Ap=area of piston=4

2Dπ; Cp ave= average piston velocity=2LNm/min

*Fix vel. of gas, calculate port area & port diameter

( )βαη

++×

=180520

1807.14'

P

TVV chg

g

Where Vg=gas velocity – fixed – in ft/min (180+α+β) =duration of valve opening

T=temp. in Rankine - ToF=1.8ToC+32, and T(R)= ToF+459.67

Intake temp.≈20oC = 68oF = 528Rankine; Exhaust temp.≈300oC=1032Rankine

chη =charging efficiency for NA Engine≈85%; and SC Engine≈95 to 100%

P=pr. of gas in psi =14.7psi - for intake=1atm.

=29psi to 35psi – for exhaust=2 to 4atm.

Inlet valve - α=opening advance – generally = 10o, & β=closing delay – generally = 20 to 30o

Exhaust valve -α= opening advance – generally – 40 to 50o, & β=closing delay – generally – 10 to 20o

Hence calculate '

gV -For stationary engines ≤ 12000ft/min – for intake valve & ≤ 18000ft/min – for exhaust valve

-For automobile engines ≤ 18000ft/min – for intake valve & ≤ 27000ft/min – for exhaust valve

• d1=dport=port diameter=valvegasthroughvelocityof

dpistonspeeD mean)(

• fix αv=valve face angle=30o or 45o

• valve lift h

Angular area of opening4

cos2

11

dhd v

παπ = =port area orαcos

25.0 1dh =

But this gives hammering effect (h=0.1d1 to 0.2d1)Therefore empirical relation h=0.2d1 may be adopted

• Thickness of valve disc = S

Pdkt 11=

115

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesWhere k1=0.54-for cast iron; 0.42-for carbon steel & high grade steel

S =4000psi-forcast iron; 8000psi-forcarbon steel & 15000psi-for high grade steel

P =max. gas pressure; d1=dport=port diameter

[Orσmax

15.0P

dt = , where, σ =allowable stress=420ksc for carbon steel & 700 to 800ksc for high grade steel]

• )]90sin([212 vtdd α−×+= or=d1+2b

0.7854 ( ) 21

22

23 7854.0 ddd ≥−

• d3= ( )22

21 dd +

• b=0.5(d2-d1) = 0.5d1

−−

1maxPS

S

b

b

,

Where Sb=safe bearing pressure=4000psi for cast iron

b=0.05d1 to 0.07d1 an empirical formula

or b=αtan

t=0.1d1+4mm

• do=diameter of valve stem = inchd

16

3

181 +

bearing pressure = 1

12

2

)(d

ddload

abearingare

load

××−=π

116

www.jntuworld.com

www.jntuworld.com


( ) ( )62.27mm

50.9635.8dddareaopeningheadvalveofDiameter

8.5mm4mm8

35.84mm

8

ddstemvalveofDiameter

7.58mm)b(assuming,

7.58235.82bddheadValveofDiamter

7.58mm4mm35.80.14mm0.1dbAlso,

10.74mmtan30

6.2

tanα

tbseatingofWidthAnd

6.2mm425

35.80.5σ

pd0.5theadvalveofThickness

hLiftValveMax.

35.8mm1500

30080

V

SDddiameterPort

DNDNm/min300m/minS,30

42N/mm1500m/min,V

80mm,D,5N/mmPGiven,Solution;

,30 AngleSeat Valve42N/mmStress Allowable

300m/min,Speed Piston Mean1500m/min, VelocityGas

80mm,Diameter Bore Cylinder,5N/mmPressure Gas Max.

data; following the from engine an of dimensions valve & lift valve the Determine

1-Example

2222

213

10

12

1

max1

1

0

2

2max

02

2

=+=+==

=+=+==

==×+=+==∴

=+×=+=

====

=×=×==

=×

===

====

====

==

====

====

mm

mmd

m

96.50

33.1030cos4

8.35

cos4

sec/60

,

,

1

α

ππα

σ

117

www.jntuworld.com

www.jntuworld.com


Firing Order

Every engine cylinder must fire once in every cycle. This requires that for a four-stroke four-

cylinder engine the ignition system must fire for every 180 degrees of crank rotation. For a six-cylinder

engine the time available is only 120 degrees of crank rotation.

The order in which various cylinders of a multi cylinder engine fire is called the firing order.

The number of possibilities of firing order depends upon the number of cylinders and throws of the

crankshaft. It is desirable to have the power impulses equally spaced and from the point of view of

balancing this has led to certain conventional arrangements of crankshaft throws. Further, there are three

factors which must be considered before deciding the optimum firing order of an engine. These are:

(i) Engine vibrations

(ii) Engine cooling and

(iii) Development of back pressure

Consider that the cylinder number 1 of the four-cylinder engine, shown in Fig., is fired first. A

pressure p, generated in the cylinder number 1 will give rise to a force equal to {pA × [b/(a + b)]} and

{pA × [a/(a + b)]} on the two bearings A and B respectively. The load on bearing A is much more than

load on bearing B. If the next cylinder fired is cylinder number 2, this imbalance in load on the two

bearings would further aggravate the problem of balancing of the crankshaft vibrations & would result

in severe engine vibrations. If we fire cylinder number 3 after cylinder number 1, the load may be more

or less evenly distributed.

Further, consider the effect of firing sequence on engine cooling. When the first cylinder is fired

its temperature increases. If the next cylinder that fires is number 2, the portion of the engine between

the cylinder number 1 and 2 gets overheated. If then the third cylinder is fired, overheating is shifted to

the portion between the cylinders 2 and 4. Thus we see that the task of the cooling system becomes very

difficult because it is then, required to cool more at one place than at other places and this imposes great

strain on the cooling system. If the third cylinder is fired after the first the overheating problem can be

controlled to a greater extent.

Next, consider the flow of exhaust gases in the exhaust pipe. After firing the first cylinder,

exhaust gases flow out to the exhaust pipe. If the next cylinder fired is the cylinder number 2, we find

that before the gases exhausted by the first cylinder go out of the exhaust pipe the gases exhausted from

118

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive Enginesthe second cylinder try to overtake them. This would require that the exhaust pipe be made bigger.

Otherwise the back pressure in it would increase and the possibility of back flow would arise. If instead

of firing cylinder number 2, cylinder number 3 is fired. then by the time the gases exhausted by the

cylinder 3 come into the exhaust pipe, the gases from cylinder 1 would have sufficient time to travel the

distance between cylinder 1 and cylinder 3 and thus, the development of a high back pressure is avoided

It should be noted that to some extent all the above three requirements are conflicting and

therefore a trade-off is necessary.

For 4-Cylinder engines the possible firing orders are: 1-3-4-2 or 1-2-4-3

The former is more commonly used in the vertical configuration of cylinders.

For a 6-Cylinder engine firing orders can be: 1-5-3-6-2-4 or 1-5-4-6-2-3 or 1-2-4-6-5-3 or 1-2-3-6-5-4

The first one is more commonly used.

Other Firing Orders

For 3 Cylinder engine 1-3-2

8 Cylinder in-line engine 1-6-2-5-8-3-7-4

8 Cylinder V shape engine 1-5-4-8-6-3-7-2, 1-8-4-3-6-5-7-2, 1-6-2-5-8-3-7-4,

1-8-7-3-6-5-4-2, 1-5-4-2-6-3-7-8

Cylinder No. 1 is taken from front of the in-line engines whereas in V shape front cylinder on

right side-bank is considered cylinder No.1 for fixing H.T. leads according to engine firing order.

119

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesVibration Damper

The power impulses tend to set up a twisting vibration in the crankshaft. When a piston moves

down on its power stroke, it thrusts through the connecting rod, against a crankpin with a force that may

exceed 2 tons. This force tends to twist the crankpin ahead of the rest of the crankshaft. Then, as the

force against the crankpin recedes, it tends to untwist, or move back into its original relationship with

the rest of the crankshaft. This twist-untwist action, repeated with every power impulse, tends to set up

an oscillating motion in the crankshaft. This is called Torsional vibration. If it were not controlled, it

could cause the crankshaft to break at certain speeds. To control torsional vibration, devices which are

called vibration dampers, or harmonic balancers, are used. These dampers are usually mounted on

the front end of the crankshaft and the drive-belt pulleys are incorporated into them.

A typical damper is made in two parts, a small inertia ring or damper flywheel and the pulley.

They are bonded to each other by a rubber insert about 4-inch [6-mm] thick. The damper is mounted to

the front end of the crankshaft. As the crankshaft speeds up or slows down, the damper flywheel has a

dragging effect. This effect, which slightly, flexes the rubber insert, tends to hold the pulley and

crankshaft to a constant speed. This tends to check the twist-untwist action, or torsional vibration, of the

crankshaft.

120

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesEngine Bearings

Bearings are placed in the engine wherever there is rotary motion between engine parts. These

engine bearings are called sleeve bearings because they are shaped like sleeves that fit around the

rotating shaft. The part of the shaft that rotates in the bearing is called a journal. Connecting-rod and

crankshaft (also called main) bearings are of the split, or half, type. The upper half of a main bearing is

installed in the counter bore in the cylinder block. The lower half is held in place by the bearing cap.

The upper half of a connecting rod big end (or crankpin) bearing is installed in the rod. The lower half is

placed in the rod cap. The typical bearing half is made up of a steel or bronze back, with up to five

linings of bearing material. The bearing material is soft therefore, the bearing wears, and not the more

expensive engine part. Then, the bearing, and not the engine part, can be replaced when it has worn too

much.

121

www.jntuworld.com

www.jntuworld.com


122

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesThrust bearing

The crankshaft has to be kept from moving back and forth in the block. To prevent back-and-

forth movement, one of the main bearings is a thrust, or end-thrust, bearing. This bearing has flanges on

its two sides. Flanges on the crankshaft fit close to the flanges on the thrust bearing. If the crankshaft

tends to shift forward or backward, the crankshaft flange comes up against the thrust-bearing flange.

This prevents endwise movement.

Bearing Lubrication

Oil from the engine oil pump flows onto the bearing surfaces. The rotating shaft journals are

supported on layers of oil. The journal must be smaller than the bearing so that there is a clearance

(called oil clearance) between the two. In the engine oil moves through this clearance. The lubricating

system feeds oil to the main bearings. It enters through the oil holes and the rotating journals carry it

around to all parts of the bearings. The oil works its way to the outer edges of the main bearings. From

there, it is thrown off-and drops back into the oil pan. The oil thrown off helps lubricate other engine

parts, such as the cylinder walls, pistons, and piston rings. The connecting-rod bearings are lubricated

through the oil holes drilled in the crankshaft. As the oil moves across the faces of the bearings, it also

helps to cool them. The oil is relatively cool as it leaves the oil pan. It picks up heat in its passage

through the bearings. This heat is carried down to the oil pan and released to the air around the oil pan.

The oil also flushes and cleans the bearings. It flushes out particles of grit and dirt from the bearings.

The particles are carried back to the oil pan by the oil. They then settle to the bottom of the oil pan, or

are removed from the oil by the oil screen or filter.

123

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesBearing Oil Clearances

The greater the oil clearance, the faster oil flows through the bearing. Proper clearance varies with

different engines, but 0.0015 inch [0.037mm] is a typical clearance. As the clearance becomes greater

(owing to bearing wear, for example), the amount of oil

flowing through and being thrown off increases. With a

0.003inch [0.076-mm] clearance only twice 0.0015 inch

[0.037 mm], the oil throw off increases as much as five

times. A 0.006inch [0.152-mm] clearance allows25 times as

much oil to flow through and be thrown off. As bearings

wear, more and more oil is thrown onto the cylinder walls.

The piston rings cannot handle so much oil. Part of it works

up into the combustion chambers, where it burns and forms

carbon. Carbon deposits in the combustion chambers

reduce engine power and cause other engine troubles.

Excessive oil clearances can also cause some bearings to fail from oil starvation. An oil pump

can deliver only a certain amount of oil. If the oil clearances are excessive most of the oil will pass

through the nearest bearings. There won't be enough for the more distant bearings. Then these will

probably fail from lack of oil. An engine with excessive bearing oil clearances usually has low oil

pressure: The oil pump cannot build up normal pressure because of the large oil .clearances in the

bearings. If bearing oil clearances are too small, there will be metal to- metal contact between the

bearing and the shaft journal. Very rapid wear and quick failure will result. Also, there will not be

enough oil throw off to lubricate cylinder walls, pistons, and rings.

124

www.jntuworld.com

www.jntuworld.com

Theory and Design of Automotive EnginesBearing Requirements

Bearings must be able to do other things besides carry loads. Some of these are listed below.

1. Load Carrying Capacity-Modern engines are lighter and more powerful. They have higher

compression ratios which impose greater bearing loads. Only a few years ago, bearing loads were

around 1600 to 1800 psi [11,032 to 12,411kPa]. Today, connecting-rod bearings carry loads of up to

6000 psi [41,369 kPa].

2. Fatigue Resistance-When a piece of metal is bent back and forth, over and over, it hardens

and finally breaks. This is called fatigue failure. You have probably done this with a piece of wire or

sheet metal. Bearings are subject to such loads and must withstand them without failing from fatigue.

3. Embedability This term refers to the ability of a bearing to permit foreign particles to embed

in it. Dirt and dust particles enter the engine despite the air cleaner and oil filter. Some of them work

onto the bearings and are not flushed away by the oil. A bearing protects itself by letting such particles

sink into, or embed in, the bearing lining material. If the bearing were too hard to allow this, the

particles would lie on the surface. They would scratch the shaft journal and probably gouge out the

bearing. This would cause overheating and rapid bearing failure. Therefore, the bearing material must

be soft enough for adequate embedability.

4. Conformability This is associated with embedability. It is the ability of the bearing material to

conform to variations in shaft alignment and journal shape. For example, suppose that a shaft journal is

slightly tapered. The bearing under the larger diameter will be more heavily loaded. If the bearing

material has high conformability, it will "flow" slightly, from the heavily loaded areas to the lightly

loaded areas. This slight flow evens the load on the bearing. A similar action takes place when foreign

particles embed in the bearing. As they embed, they displace bearing material, producing local high

spots. However with high conformability, the material flows away from the high spots. This prevents

local heavy loading that could cause bearing failure.

5. Corrosion resistance - the by-products of combustion may form corrosive substances harmful

to some metals. Bearing materials must be resistant to corrosion. Unleaded gasoline, required on cars

using catalytic converters, changes the chemistry of the engine oil. Catalytic converters, are installed in

the exhaust systems to reduce the pollutants coming out the tail pipe. The unleaded gasoline, in

changing the chemistry of the oil, tends to increase bearing corrosion. Therefore, the composition of

engine bearings has been changed. For example, instead of the copper-lead bearings used for years,

some engines now have aluminum-lead bearings. These appear to withstand corrosion better.

6. Wear Rate The bearing material must be so hard and tough that it will not wear too fast. At the

same time, it must be soft enough to permit good embedability and conformability.

125

www.jntuworld.com

www.jntuworld.com


126

www.jntuworld.com

www.jntuworld.com


127

www.jntuworld.com

www.jntuworld.com