ENGINE COMPONENTS AND OPERATION

A Seminar report Submitted

In Partial Fulfilment of the Requirements

for the Degree of

BACHELOR OF TECHNOLOGYIn

Mechanical Engineering

By

Arshad Ali(1313340030)

Under the Supervision of

Ms. Juthika Das

To the

Faculty of Mechanical Engineering

APJ Abdul Kalam Technical University

October, 2015

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CERTIFICATE

Certified that Arshad Ali (1313340030) has carried out the work presented in this seminar entitled “Engine Components and Operation” under my supervision.

Signature

(Ms. Juthika Das,

Mechanical Engineering)

Date:

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Abstract

The Spark Ignition (SI) engines work on the principle of  cycle of operations invented

by Nicolaus A. Otto in the year1876. The Compression Ignition (CI) engines work on the

principle founded by Rudolf Diesel in the year 1892. For the engine to work properly it has to

perform some cycle of operations continuously. The principle of operation of the spark

ignition (SI) engines was invented by Nicolaus A. Otto in the year 1876; hence SI engine is

also called the Otto engine. The principle of working of compression ignition engine (CI) was

found out by Rudolf Diesel in the year1892, hence CI engine is also called the Diesel engine. 

The principle of working of both SI and CI engines are almost the same, except

the process of the fuel combustion that occurs in both engines. In SI engines, the burning of

fuel occurs by the spark generated by the spark plug located in the cylinder head. The fuel

is compressed to high pressures and its combustion takes place at a constant volume. In CI

engines the burning of the fuel occurs due to compression of the fuel to excessively high

pressures which does not require any spark to initiate the ignition of fuel. In this case the

combustion of fuel occurs at constant pressure. Both SI and CI engines can work either on

two-stroke or four stroke cycle. Both the cycles have been described below: 

In the four-stroke engine the cycle of operations of the engine are completed in four

strokes of the piston inside the cylinder. The four strokes of the 4-stroke engine are: suction

of fuel, compression of fuel, expansion or power stroke, and exhaust stroke. In 4-stroke

engines the power is produced when piston performs expansion stroke. During four strokes of

the engine two revolutions of the engine's crankshaft are produced.

In case of the 2-stroke, the suction and compression strokes occur at the same time.

Similarly, the expansion and exhaust strokes occur at the same time. Power is produced

during the expansion stroke. When two strokes of the piston are completed, one revolution of

the engine's crankshaft is produced.

In 4-stroke engines the engine burns fuel once for two rotations of the wheel, while in

2-stroke engine the fuel is burnt once for one rotation of the wheel. Hence the efficiency of 4-

stroke engines is greater than the 2-stroke engines. However, the power produced by the 2-

stroke engines is more than the 4-stroke engines.

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Contents

Page No.

Certificate

Abstract

Introduction…………………………………………………………………………6

1.1 Energy Conversion…………………………………………………………….6

1.1.1 Definition of ‘Engine’……………………………………………………6

1.1.2 Definition of ‘Heat Engine.........................................................................6

1.1.3 Classification and Some Basic Details of Heat Engines…………………7

1.2 Basic Engine Components and Nomenclature……………………………...8

1.2.1 Engine Components………………………………………………………8

1.2.2 Electrical Components……………………………………………….…..16

2.1 Working Principle of Engine……………………………………………...…20

2.1.1 Four-Stroke Spark-Ignition Engine………………………………………20

2.1.2 Four-Stroke Compression-Ignition Engine………………………………22

2.1.3 Two-Stroke Spark-Ignition Engine………………………………………25

2.1.4 Two-Stroke Diesel Engine……………………………………………….27

3.1 Difference between Two & Four Stroke Cycle Petrol Engines……………28

3.2 Comparison of Two & Four Stroke Cycle Diesel Engines…………………29

3.3 Comparison of CI & SI Engine……………………………………………...30

References……………………………………………………………………………32

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Figure Contents

Page No

1 Classification of Heat Engine………………………………………………………7

2 Lay Out of Engine………………………………………………………………….8

3Cylinder……………………………………………………………………………..9

4 Piston and Piston Ring……………………………………………………………..10

5 Crankshaft………………………………………………………………………….11

6 Connecting Rod………………………………………………………………….…12

7 Cylinder Head……………………………………………………………………...12

8 Camshaft…………………………………………………………………………...13

9 Valves and Valves Spring………………………………………………………….14

10 Rocker Arm……………………………………………………………………….14

11 Crank Case………………………………………………………………………..15

12 Oil Pump and Sump………………………………………………………………15

13 Fuel Pump………………………………………………………………………...16

14 Alternator…………………………………………………………………………17

15 Starter Motor……………………………………………………………………...17

16 Spark Plug………………………………………………………………………...18

17 Electronic Fuel Injector…………………………………………………………...18

18 Ignition Coil………………………………………………………………………19

19 Four-Stroke-Cycle Petrol Engine…………………………………………………21

20 Four-Stroke-Cycle Diesel Engine………………………………………………....23

21 Two-Stroke-Cycle Petrol Engine………………………………………………....25

22 Two-Stroke-Cycle Diesel Engine………………………………………………....27

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INTRODUCTION

1.1 ENERGY CONVERSION

The distinctive features of our civilization today, one that makes it different from all others, is the wide use of mechanical power. At one time, the primary source of power for the work of peace or war was chiefly man’s muscles. Later, animals were trained to help and afterwards the wind and the running stream were harnessed. But, the great step was taken in this direction when man learned the art of energy conversion from one form to another. The machine which does this job of energy conversion is called an engine.

1.1.1 Definition of ‘Engine’

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’.

1.1.2 Definition of ‘Heat Engine’

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 engine can be broadly classified into two categories:

i. Internal Combustion Engine (IC Engine)

ii. External Combustion Engine (EC Engine)

In an internal combustion engine the products of combustion are directly the motive fluid. Petrol, gas, and diesel engines, wankel engine, and open cycle gas turbines are examples of internal combustion engines. Jet engines and rockets are also internal combustion engines. In an external combustion engine the products of combustion of air and fuel transfer heat to a second fluid which is the working fluid of the cycle, as in the case of a steam engine or a steam turbine plant where the heat of combustion is employed to generate steam which is used in a piston engine or a turbine. Another example of an external combustion engine is a closed cycle gas turbine plant in which heat of combustion in an external furnace is transferred to gas, usually air, which is used in a gas turbine. Stirling engine is also an external combustion engine.

The main advantages of internal combustion engines over external combustion engines like steam plants are greater mechanical simplicity, lower ratio of weight and bulk to output due to absence of auxiliary apparatus like boiler and condenser and, hence lower first cost (except in the case of very large units), higher overall efficiency, and lesser requirement of water for dissipation of energy through cooling system.

The advantages of external combustion plants are : use of cheaper fuels including solid fuels, and high starting torque (internal combustion engines are not self-starting). The steam turbine plant, which is the most important external combustion engine, is mainly used for

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large electric power generation, whereas internal combustion engines are mainly used for transport vehicles – automobiles, locomotives, aircrafts, etc. External combustion engines are less suitable for transport vehicles because of bulk and weight, and difficulty of transporting the working fluid.

The revolution in the life style of the people caused by transport vehicles, the extensive use of which became possible due to tremendous development of internal combustion engines, is the unique event of the twentieth century. In the United States, today, the total kW installed in the automobiles is much larger than that installed in electric generating stations.

1.1.3 Classification and Some Basic Details of Heat Engines

Engines whether Internal Combustion or External Combustion are of two types:

i. Rotary engines

ii. Reciprocating engines

A detailed classification of heat engines is given in the Fig-1 Of the various types of heat engines, the most widely used ones arc 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 hi 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.Classification of Heat Engine

Another advantage of the reciprocating interna1 combustion engine over the other two types is that all its components work at 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. 

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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 to 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 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 factors the reciprocating internal combustion engines have been found suitable for use in automobiles, motorcycles and scooters, power boats, ships, slow speed aircraft, locomotives and power units of relatively small output.

1.2 BASIC ENGINE COMPONENTS AND NOMENCLATURE

Even through reciprocating internal combustion engines look quite simple, they are highly complex machines. There are hundreds of components which have to perform there functions effectively to produce output power. There are two types of engines, viz., spark-ignition (SI) and compression-ignition (CI) engine. Let us now go through some of the important engine components and the nomenclature associated with an engine.

1.2.1 Engine Components

A cross section of a single cylinder spark-ignition engine with overhead valves is shown in figure. The major components of the engine and their functions are briefly described below.

Fig 2.Lay out of Engine

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1.  CYLINDER

The engine cylinder is the part or space where fuel is admitted and reciprocating motion of the piston is obtained by burning it. The engine cylinder is characterized by its ‘bore’ and ‘stroke’.

Fig 3.Cylinder

Bore represents its inner diameter and Stroke is the effective length along which the piston reciprocates i.e. the distance travelled by the piston. Two terms related to stroke are ‘Top Dead Center’ (TDC) and ‘Bottom Dead Center’ (BDC). Top Dead Center is the uppermost point of the stroke while the Bottom Dead Center is the lowermost point of the stroke. The velocities of the piston at TDC and BDC are zero.

The part of the engine where the cylinder is located is called the engine block or cylinder block. Cylinders are generally lined with liners or sleeves of some other harder material or coated with some wear resistant material like Nikasil. Liners can be easily replaced when worn out. Cylinder blocks are also provided with hollow spaces around and in between the individual cylinders that are known as jackets in case of liquid cooled engines. The coolant is circulated in these jackets which enables effective heat dissipation.

Engine displacement is determined by the cylinder/cylinders. The volume swept by the piston in one stroke i.e. the cross sectional area times the stroke, is called the Swept Volume and it is measured in cubic centimetres (cm3). This is the engine displacement in case of a single cylinder engine. In case of multi cylinder engines, the engine displacement is the swept volume multiplied by the number of cylinders. The volume enclosed by the cylinder head and the piston at TDC is called the Clearance Volume. The sum of swept volume and the clearance volume is equal to the total volume of the cylinder.

The engines are classified according to the dimensions of the cylinders and their orientation. The bore by stroke ratio classifies the engines as under square, over square or square accordingly as the ratio is less than one, greater than one or equal to one.

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Materials used:

It is made of steel alloy/cast iron or aluminium by casting process. The reason for using aluminium is its light weight and good heat dissipation capacity.

2.  PISTON

The piston is the cylindrical part which moves up and down in the cylinder and enables compression and expansion of the charge during the combustion cycle. The diameter of the piston is slightly less than the bore of the cylinder to avoid direct wear of the cylindrical piston surface. Three rings known as piston rings are fitted in the circular recesses machined on the piston surface.

Fig 4.Piston and Piston Ring

These rings are in direct contact with the cylinder liner thus preventing piston wear. The top two rings are known as compression rings. Compression rings are chamfered on the outer periphery. They prevent the fresh charge or waste gases inside the combustion chamber from going into the crankcase, a process known as ‘blowby’. The lowermost or third ring is called the oil ring. Its purpose is to ensure proper oil distribution along the cylinder walls and also prevent the leakage of oil into the combustion chamber.

Materials used:

The top two rings are made of cast iron or aluminium alloy which have high wear resistance. The oil ring is made from aluminium.

3. CRANKSHAFT

Crankshaft is a part of the engine which has projections bent and offset from the shaft axis. These projections are called crank throws or crankpins. This design converts the sliding motion obtained from the piston into rotary motion via a connecting rod. Crankshaft is placed below the cylinder block in a casing called the crankcase. In multi cylinder engines one crankpin per cylinder is provided to attach the piston by the connecting rod.

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The crankpin journal bearing is called the big end and has plain or sliding bearings. Crankshafts have some counterbalance weights which are either bolted to the crank body or form an integral part and are called crank balance. Crank balance is provided to counter the torsional vibrations experienced by the crankshaft due to the reciprocating unbalance of the piston which arises due to the jerks from the combustion process. Crankshafts may be manufactured in parts or as a single piece. The single piece design is preferred as it gives superior strength, better fiber flow and good stress bearing capabilities.

Fig 5.Crankshaft

Materials used:

Crankshafts are made from steel by roll forging process or from ductile steel through casting. The single piece crankshafts are made from carbon steels followed by heat treatment. Vanadium micro-alloyed steels are also used frequently because they give high strengths without heat treatment and the low alloy content makes them cheaper than the high alloy steels.

4. CONNECTING ROD

The connecting rod is the link connecting the piston to the crankshaft. It converts the linear motion of the piston into rotary motion of the crank. One end of the connecting rod is attached to the piston through a piston pin/gudgeon pin/wrist pin and is called the small end while the other end is attached to the crankpin journal through bolts holding the upper and lower bearing caps and is called the big end. The bearing is in the form of two half shells which is held in place around the crank journal by the big end of the connecting rod. Both the ends of the connecting rod are not rigidly fixed but are hinged so that they can rotate through an angle. Thus both its ends are in continuous motion and under tremendous stress from the pressure from the piston. The connecting rods are the most sensitive parts and are most prone to failure and hence they are manufactured with high degree of precision.

Materials used:

They are generally made from forged steel but are also made from aluminium alloy for light weight and high impact absorbing ability.

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Fig 6.Connecting Rod

5. CYLINDER HEAD

The cylinder head is the part which sits on top of the cylinder block and houses the valves, rocker arms and the ignition elements. It is bolted to the cylinder block with the head gasket in between. In overhead camshaft engines, the camshaft is present in the head and there is no pushrod arrangement for valve mechanism. The inlet and exhaust ports are also machined within the head to which the inlet and exhaust manifold are then attached. Cylinder heads

Fig 7.Cylinder Head

also form part of the combustion chamber that is machined into it on the underside. Holes and channels are made for bolting and for flow of coolant. Inline engines have a single cylinder head for all the cylinders while V-type and horizontally opposed have a separate head for each bank of cylinders.

Materials used:

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They are made from cast iron or aluminium alloy. Aluminium is light weight and conducts the heat away more quickly than cast iron and hence is the preferred choice.

6. CAMSHAFT

Camshaft is a shaft to which cams are fitted or are machined. The function of the camshaft is to operate the valves directly by sitting over them, or indirectly through a mechanism (rocker arm, pushrod). Camshaft rotation decides the valve timing and is of critical importance. The opening and closing of valves is governed by camshaft which is coupled to the crankshaft either directly through a reduction gear or indirectly through a pulley and a timing belt. Engines in which the camshaft is coupled to the crank by a gear require a pushrod and tappet mechanism along with rocker arms. The gear on the crank has half the teeth than on the camshaft gear. This causes the camshaft to run at half the RPM of the crank. In engines

Fig 8.Cam Shaft

where the timing belt and pulley are used, the camshaft is placed inside the head and there is no need of a pushrod. Instead latches are used which rotate the rocker arm and operate the valves. Such a design is called Overhead Camshaft (OHC) design. Some engines use a single camshaft to operate both the inlet and exhaust valves and are called Single Overhead Camshaft (SOHC) while others utilize a separate camshaft for operating the two types of valves which are arranged in two separate rows and are called Double or Dual Overhead Camshaft (DOHC) engines. OHC design reduces manufacturing cost and is less prone to failure. The lobes of the cam are tapered slightly so that the valves lifters rotate slightly with each depression and wear uniformly. There is considerable sliding friction between cam and follower surface and so they are surface hardened.

Materials used:

Chilled iron castings are the most common material used for fabricating camshaft as chilling of iron castings gives them greater wear resistance and surface hardness. Billet steel is also used for making high quality camshafts.

7. VALVES

Valves used in the IC engines are called poppet valves. They have a long thin circular rod known as the valve stem at the end of which is a flat circular disk called the valve head. The valve head has a tapered section connecting to the rod which forms the valve seat. The valve slides in a valve guide and sit in the valve seat when closed which is machined in the

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head. This sliding motion is enabled by the camshaft and associated linkages and the valves are kept closed or returned to their seats when not in use by valve springs. The valves are responsible for intake of fresh charge and exhaust of waste gases. The valve which admits fresh charge is called intake/inlet valve and the one which allows exhaust gases to go out is called exhaust/outlet valve.

Fig 9.Valves and Valves spring

Materials used:

The valves are made from steel alloys and they may be filled with sodium to increase the heat transfer capacity

8. ROCKER ARM

The rocker arm transmits the rotary motion of the cam or camshaft through a latch/ tappet and converts it into a linear motion of the valve stem which depresses the valve head.

Rocker arm rocks or oscillates about a fixed pivot rod (rocker shaft) in the cylinder head.

Fig 10.Rocker Arm

Materials used:

The rocker arms are made from steel stampings for light and medium duty engines whereas most of the heavy duty diesel engines use cast iron and forged carbon steel rocker arms for greater strength and stiffness.

9. CRANKCASE

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The metallic case or housing in which the crankshaft is placed is called the crankcase. The crankcase is located below the cylinder block. The crankcase also has the main bearing in which the crank rotates. The main bearing is a plain or sliding bearing with proper oil supply in it. The four cylinder inline petrol engines have three main bearings, one on each end and one in the middle while the diesel counterparts have five main bearings one on each end and one between each cylinder. The crankcase encloses the crankshaft and connecting rod assembly

Fig 11.Crank Case

and protects it from dust, dirt and other foreign materials. The crankcase is filled with air and oil and is sealed off from the fuel air mixture and exhaust gases in the combustion chamber by the piston rings.

Materials used:

The crankcase is made from the same materials as the cylinder block i.e. aluminium or cast iron.

10. OIL PUMP AND SUMP

The oil pump pumps the oil to various parts of the engine for effective lubrication, cooling and cleaning. The oil pump in the engine is a gear type pump which is driven by the crankshaft gear. The oil is pressurized to the passages machined in various components, which then lubricates and cools them. They pressure relief valves to maintain the required pressure in the passages and mainly in the crankshaft journal bearing.

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Fig 12.Oil Pump and Sump

The oil is stored in an oil chamber known as the oil sump. The oil is lifted by the oil pump from the sump through a wire mesh strainer which retains debris and dirt. The oil then passes through an oil filter and an oil cooler before being distributed to the engine parts. The oil after doing its job returns through controlled leaks from pistons, rings, valves, camshaft and finally to the crankcase from where it is drained back to the oil sump.

11. FUEL PUMP

The fuel (diesel) in a CI engine is not mixed with air unlike SI engines, but is sprayed into the combustion chamber through a nozzle for ignition. The fuel pump pressurizes the fuel to such a pressure that it atomizes and when it is sprayed in the cylinder, it gets ignited when it

Fig 13.Fuel Pump

comes in contact with the air. The fuel pump consist of a spring loaded piston valve in a cylinder. When fuel from the fuel filter is introduced in the fuel pump, the spring loaded piston applies pressure on it. The pressure is transmitted through the pressure lines and finally to the injector which has a nozzle opening in the cylinder. Thus the fuel gets atomized and ignited.

1.2.2 Electrical Components

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1. ALTERNATOR

The alternator is an AC generator which charges the car’s battery and powers its electrical system when the engine is running. The alternator is an electromechanical device which converts the mechanical energy into electrical energy. It consists of a rotating magnet known as the rotor turning within a stationary set of conductors which have windings on them and called the stator. The rotation of the rotor changes the magnetic flux around it and induces an e.m.f. in the stator windings in the form of AC voltage. The construction of the alternator is robust enough to drive it by a pulley smaller than the crankshaft pulley via a belt so as to turn it faster than the engine. The alternator uses a set of rectifiers or diodes to convert AC to DC.

Fig 14.Alternator

2. STARTER MOTOR

The starter motor is a small high torque motor for cranking the engine. For the engine to be started, the piston must move downwards with the inlet valve open to suck in fresh air/fuel. To accomplish this, the starter motor is connected to a key operated switch (a relay) and a starting battery. When the key is turned, the switch turns on, the circuit is completed and it pushes out the pinion on the motor shaft which goes into mesh with the flywheel gear. This turns the crankshaft and moves the piston down via the connecting rod. As the piston moves down sucking in fresh charge, the ignition element or the injection element burn the fuel and the engine gets started. At this point, the starter motor pinion goes out of mesh of the flywheel.

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Fig 15.Starter Motor

3. SPARK PLUG

The spark plug is an electrical device which ignites the fuel in the combustion chamber at the end of the compression stroke. The fuel gets ignited and as a result expands and pushes the piston down so as to obtain the power stroke. The spark plug is fitted into the cylinder head on the underside of the combustion chamber through threads on the plug body. The spark plug has two electrodes: one central electrode which is connected to the ignition coil or magneto through a highly insulated high tension wire; the other electrode is at the base of the plug and is grounded. There is a small gap between the two electrodes generally between 0.9-1.8 mm. When high voltage current from the ignition coil/magneto is supplied the air between the gap gets ionized and a spark is generated which is sufficient to ignite the fuel. The electrode gap is of critical importance for proper sparking at all speeds. The spark plug requires a voltage in the range of 12,000 - 25,000 V to fire properly. The spark plug has a terminal, an insulator and its tip, metal jacket and the seals in the body structure. The terminal is connected to the ignition coil, the insulator (made of porcelain) provides insulation and mechanical support, the metal jacket conducts heat away from the plug body and to the cylinder head, and the seals properly seal the recess in the cylinder head where the spark plug is fitted.

Fig 16.Spark Plug

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4. ELECTRONIC FUEL INJECTOR

The injector does the job of mixing the fuel and air at such a high pressure that it gets ignited. The electronic fuel injection electronically controls the injection process so that always the required amount of fuel is injected into the cylinder before firing. This is controlled by a hardware which is programmed to meter the fuel accurately and optimize the air/fuel ratio at all speeds of the engine. It is assisted by sensors on the crankshaft or camshaft to monitor the engine rotational speed, mass flow sensors, oxygen sensors and throttle position sensors to give the hardware timely feedback. It forms a closed-loop feedback system which helps in proper fuel distribution in all the cylinders and leads to dependable starting, improved engine performance and lesser maintenance.

Fig 17.Electronic Fuel Injector

5. IGNITION COIL

The ignition coil generates the voltage to be supplied to the spark plug. It has two coils, one primary and another secondary. The primary is connected to the battery and the secondary to the capacitor and a distributor and is grounded. The turns in the primary coil are thick and few in number while those in the secondary coil are thin and large in number. The current produced in the primary coil induces a current in the secondary current by mutual induction which is then stored in the capacitor and is supplied to the distributor which distributes the current among the spark plugs.

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Fig 18.Ignition Coil

2.1 WORKING PRINCIPLE OF ENGINE

If an engine is to work successfully then it has to follow a cycle of operations in a sequential manner. The sequence is quite rigid and cannot be changed. In the following sections the working principle of both SI and CI engines is described. Even though both engines have much in common there are certain fundamental differences.

The credit of Inventing the spark-ignition engine goes to Nicolaus A. Otto (1876) whereas compression-ignition engine was invented by Rudolf Diesel (1892). Therefore, they are often referred to as Otto engine and Diesel engine.

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2.1.1 Four-Stroke Spark-Ignition Engine

Gasoline or petrol engines are also known as spark-ignition (S.I.) engines. Petrol engines take in a flammable mixture of air and petrol which is ignited by a timed spark when the charge is compressed. The first four stroke spark-ignition (S.I.) engine was built in 1876 by Nicolaus August Otto, a self-taught German engineer at the Gas-motoreufabrik Deutz factory near Cologne, for many years the largest manufacturer of internal-combustion engines in the world. It was one of Otto's associates - Gottlieb Daimler - who later developed an engine to run on petrol which was described in patent number 4315 of 1885. He also pioneered its application to the motor vehicle (Fig. 19).

Four stroke Spark-ignition (S.I) engines require four piston strokes to complete one cycle: an air-and-fuel intake stroke moving outward from the cylinder head, an inward movement towards the cylinder head compressing the charge, an outward power stroke, and an inward exhaust stroke.

Induction stroke (Fig. 19(a)) The inlet valve is opened and the exhaust valve is closed. The piston descends, moving away from the cylinder head (Fig. 19(a)). The speed of the piston moving along the cylinder creates a pressure reduction or depression which reaches a maximum of about 0.3 bar below atmospheric pressure at one-third from the beginning of the stroke. The depression actually generated will depend on the speed and load experienced by the engine, but a typical average value might be 0.12 bar below atmospheric pressure. This depression induces (sucks in) a fresh charge of air and atomized petrol in proportions ranging from 10 to 17 parts of air to one part of petrol by weight.

An engine which induces fresh charge by means of a depression in the cylinder is said to be 'normally aspirated' or 'naturally aspirated'.

Compression stroke (Fig. 19(b)) Both the inlet and the exhaust valves are closed. The piston begins to ascend towards the cylinder head (Fig. 19(b)). The induced air-and-petrol charge is progressively compressed to something of the order of one-eighth to one-tenth of the cylinder's original volume at the piston's innermost position. This compression squeezes the air and atomized-petrol molecules closer together and not only increases the charge pressure in the cylinder but also raises the temperature. Typical maximum cylinder compression pressures will range between 8 and 14 bar with the throttle open and the engine running under load.

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Fig 19.Four-stroke-cycle petrol engine

Power stroke (Fig. 19(c)) Both the inlet and the exhaust valves are closed and, just before the piston approaches the top of its stroke during compression, a spark-plug ignites the dense combustible charge (Fig. 19(c)). By the time the piston reaches the innermost point of

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its stroke, the charge mixture begins to burn, generates heat, and rapidly raises the pressure in the cylinder until the gas forces exceed the resisting load. The burning gases then expand and so change the piston's direction of motion and push it to its outermost position. The cylinder pressure then drops from a peak value of about 60 bar under full load down to maybe 4 bar near the outermost movement of the piston.

Exhaust stroke (Fig. 19(d)) At the end of the power stroke the inlet valve remains closed but the exhaust valve is opened. The piston changes its direction of motion and now moves from the outermost to the innermost position (Fig. 19(d)). Most of the burnt gases will be expelled by the existing pressure energy of the gas, but the returning piston will push the last of the spent gases out of the cylinder through the exhaust-valve port and to the atmosphere. During the exhaust stroke, the gas pressure in the cylinder will fall from the exhaust-valve opening pressure (which may vary from 2 to 5 bar, depending on the engine speed and the throttle-opening position) to atmospheric pressure or even less as the piston nears the innermost position towards the cylinder head.

Cycle of events in a four-cylinder engine (Figs. 19(e)-(g)) Fig. 19(e) illustrates how the cycle of events - induction, compression, power, and exhaust - is phased in a four-cylinder engine. The relationship between cylinder pressure and piston stroke position over the four strokes is clearly shown in Figs. 19(f) and (g) and, by following the arrows, it can be seen that a figures of eight is repeatedly being traced.

2.1.2 Four-Stroke Compression-Ignition Engine

Compression-ignition (C.I) engines burn fuel oil which is injected into the combustion chamber when the air charge is fully compressed. Burning occurs when the compression temperature of the air is high enough to spontaneously ignite the finely atomized liquid fuel. In other words, burning is initiated by the self-generated heat of compression (Fig. 20). Compression-ignition (C.I) engines are also referred to as 'oil engines', due to the class of fuel burnt, or as 'diesel engines' after Rudolf Diesel, one of the many inventors and pioneers of the early C.I. engine. Note: in the United Kingdom fuel oil is known as 'DERV', which is the abbreviation of 'diesel-engine road vehicle'.

Just like the four-stroke-cycle petrol engine, the Compression-ignition (C.I.) engine completes one cycle of events in two crankshaft revolutions or four piston strokes. The four phases of these strokes are (i) induction of fresh air, (ii) compression and heating of this air, (iii) injection of fuel and its burning and expansion, and (iv) expulsion of the products of combustion.

Induction stroke (Fig. 20(a)) With the inlet valve open and the exhaust valve closed, the piston moves away from the cylinder head (Fig. 20(a)).

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Fig 20.Four-stroke-cycle diesel engine

The outward movement of the piston will establish a depression in the cylinder, its magnitude depending on the ratio of the cross-sectional areas of the cylinder and the inlet port and on the speed at which the piston is moving. The pressure difference established

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between the inside and outside of the cylinder will induce air at atmospheric pressure to enter and fill up the cylinder. Unlike the petrol engine, which requires a charge of air-and-petrol mixture to be drawn past a throttle valve, in the diesel-engine inlet system no restriction is necessary and only pure air is induced into the cylinder. A maximum depression of maybe 0.15 bar below atmospheric pressure will occur at about one-third of the distance along the piston's outward stroke, while the overall average pressure in the cylinder might be 0.1 bar or even less.

Compression stroke (Fig. 20(b)) With both the inlet and the exhaust valves closed, the piston moves towards the cylinder head (Fig. 20(b)).

The air enclosed in the cylinder will be compressed into a much smaller space of anything from 1/12 to 1/24 of its original volume. A typical ratio of maximum to minimum air-charge volume in the cylinder would be 16:1, but this largely depends on engine size and designed speed range.

During the compression stroke, the air charge initially at atmospheric pressure and temperature is reduced in volume until the cylinder pressure is raised to between 30 and 50 bar. This compression of the air generates heat which will increase the charge temperature to at least 600 °C under normal running conditions.

Power stroke (Fig. 20(c)) With both the inlet and the exhaust valves closed and the piston almost at the end of the compression stroke (Fig. 20(c)), diesel fuel oil is injected into the dense and heated air as a high-pressure spray of fine particles. Provided that they are properly atomized and distributed throughout the air charge, the heat of compression will then quickly vaporize and ignite the tiny droplets of liquid fuel. Within a very short time, the piston will have reached its innermost position and extensive burning then releases heat energy which is rapidly converted into pressure energy. Expansion then follows, pushing the piston away from the cylinder head, and the linear thrust acting on the piston end of the connecting-rod will then be changed to rotary movement of the crankshaft.

Exhaust stroke When the burning of the charge is near completion and the piston has reached the outermost position, the exhaust valve is opened. The piston then reverses its direction of motion and moves towards the cylinder head (Fig. 20(d)).

The sudden opening of the exhaust valve towards the end of the power stroke will release the still burning products of combustion to the atmosphere. The pressure energy of the gases at this point will accelerate their expulsion from the cylinder, and only towards the end of the piston's return stroke will the piston actually catch up with the tail-end of the outgoing gases. Fig. 20(e) illustrates the sequence of the four operating strokes as applied to a four-cylinder engine, and the combined operating events expressed in terms of cylinder pressure and piston displacement are shown in Figs. 20(f) and (g).

2.1.3 Two Stroke Spark-Ignition Engine

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The first successful design of a three-port two-stroke Spark-ignition (S.I) engine was patented in 1889 by Joseph Day & Son of Bath. This employed the underside of the piston in conjunction with a closed crank-case to form a scavenge pump ('scavenging' being the pushing-out of exhaust gas by the induction of fresh charge) (Fig. 21).

Fig 21.Two-stroke-cycle petrol engine

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The two stroke spark-ignition (S.I) engine completes the cycle of events - induction, compression, power, and exhaust - in one revolution of the crankshaft or two complete piston strokes.

Crankcase-to-cylinder mixture transfer (Fig. 21(a)) The piston moves down the cylinder and initially uncovers the exhaust port (E), releasing the burnt exhaust gases to the atmosphere. Simultaneously the downward movement of the underside of the piston compresses the previously filled mixture of air and atomized petrol in the crankcase (Fig. 21(a)). Further outward movement of the piston will uncover the transfer port (T), and the compressed mixture in the crankcase will then be transferred to the combustion-chamber side of the cylinder. The situation in the cylinder will then be such that the fresh charge entering the cylinder will push out any remaining burnt products of combustion - this process is generally referred to as cross-flow scavenging.

Cylinder compression and crankcase induction (Fig. 21(b)) The crankshaft rotates, moving the piston in the direction of the cylinder head. Initially the piston seals off the transfer port, and then a short time later the exhaust port will be completely closed. Further inward movement of the piston will compress the mixture of air and atomized petrol to about one-seventh to one-eighth of its original volume (Fig. 21(b)).

At the same time as the fresh charge is being compressed between the combustion chamber and the piston head, the inward movement of the piston increases the total volume in the crank-case so that a depression is created in this space. About half-way up the cylinder stroke, the lower part of the piston skirt will uncover the inlet port (I), and a fresh mixture of air and petrol prepared by the carburetor will be inducted into the crank-case chamber (Fig. 21(b)).

Cylinder combustion and crankcase compression (Fig. 21(c)) Just before the piston reaches the top of its stroke, a spark-plug situated in the centre of the cylinder head will be timed to spark and ignite the dense mixture. The burning rate of the charge will rapidly raise the gas pressure to a maximum of about 50 bar under full load. The burning mixture then expands, forcing the piston back along its stroke with a corresponding reduction in cylinder pressure (Fig. 21(c)).

Considering the condition underneath the piston in the crankcase, with the piston initially at the top of its stroke, fresh mixture will have entered the crankcase through the inlet port. As the piston moves down its stroke, the piston skirt will cover the inlet port, and any further downward movement will compress the mixture in the crankcase in preparation for the next charge transfer into the cylinder and combustion-chamber space (Fig. 21(c)).

The combined cycle of events adapted to a three-cylinder engine is shown in Fig. 21(d). Figs. 21(e) and (f) show the complete cycle in terms of opening and closing events and cylinder volume and pressure changes respectively.

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2.1.4 Two Stroke Diesel Engine

The pump scavenge two stroke diesel engine designed by Sir Dugald Clerk in 1879 was the first successful two-stroke engine; thus the two-stroke-cycle engine is sometimes called the Clerk engine. Uniflow scavenging took place – fresh charge entering the combustion chamber above the piston while the exhaust outflow occurred through ports uncovered by the piston at its outermost position.

Fig 22.Two-stroke-cycle diesel engine

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Low- and medium-speed two-stroke marine diesels engines still use this system, but high-speed two-stroke diesel engines reverse the scavenging flow by blowing fresh charge through the bottom inlet ports, sweeping up through the cylinder and out of the exhaust ports in the cylinder head (Fig. 22(a)).

With the two-stroke diesel engine, intake and exhaust phases take place during part of the compression and power stroke respectively, so that a cycle of operation is completed in one crankshaft revolution or two piston strokes. Since there are no separate intake and exhaust strokes, a blower is necessary to pump air into the cylinder for expelling the exhaust gases and to supply the cylinder with fresh air for combustion.

Scavenging (induction and exhaust) phase (Fig. 22(a)) The piston moves away from the cylinder head and, when it is about half-way down its stroke, the exhaust valves open. This allows the burnt gases to escape into the atmosphere. Near the end of the power stroke, a horizontal row of inlet air ports is uncovered by the piston lands (Fig. 22(a)). These ports admit pressurized air from the blower into the cylinder. The space above the piston is immediately filled with air, which now blows up the cylinder towards the exhaust valves in the cylinder head. The last remaining exhaust gases will thus be forced out of the cylinder into the exhaust system. This process of fresh air coming into the cylinder and pushing out unwanted burnt gas is known as scavenging.

Compression phase (Fig. 22(b)) Towards the end of the power stroke, the inlet ports will be uncovered. The piston then reaches its outermost position and reverses its direction of motion. The piston now moves upwards so that the piston seals and closes the inlet air ports, and just a little later the exhaust valves close. Any further upward movement will now compress the trapped air (Fig. 22(b)). This air charge is now reduced to about 1/15 to 1/18 of its original volume as the piston reaches the innermost position. This change in volume corresponds to a maximum cylinder pressure of about 30-40 bar. Power phase (Fig. 2.1.4(c)) Shortly before the piston reaches the innermost position to the cylinder head on its upward compression stroke, highly pressurized liquid fuel is sprayed into the dense intensely heated air charge (Fig. 22(c)). Within a very short period of time, the injected fuel droplets will vaporize and ignite, and rapid burning will be established by the time the piston is at the top of its stroke. The heat liberated from the charge will be converted mainly into gas-pressure energy which will expand the gas and so do useful work in driving the piston outwards.

An overall view of the various phases of operation in a two-stroke-cycle three-cylinder diesel engine is shown in Figs. 22(d), and Figs. 22(e) and (f) show the cycle of events in one crankshaft revolution expressed in terms of piston displacement and cylinder pressure.

3.1 Difference Between Two & Four Stroke Cycle Petrol Engines

The differences between two- and four-stroke-cycle petrol engines regarding the effectiveness of both engine cycles are given below:

a) The two-stroke engine completes one cycle of events for every revolution of the crankshaft, compared with the two revolutions required for the four-stroke engine cycle.

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b) Theoretically, the two-stroke engine should develop twice the power compared to a four-stroke engine of the same cylinder capacity.

c) In practice, the two-stroke engine's expelling of the exhaust gases and filling of the cylinder with fresh mixture brought in through the crankcase is far less effective than having separate exhaust and induction strokes. Thus the mean effective cylinder pressures in two-stroke units are far lower than in equivalent four-stroke engines.

d) With a power stroke every revolution instead of every second revolution, the two-stroke engine will run smoother than the four-stroke power unit for the same size of flywheel.

e) Unlike the four-stroke engine, the two-stroke engine does not have the luxury of separate exhaust and induction strokes to cool both the cylinder and the piston between power strokes. There is therefore a tendency for the piston and small-end to overheat under heavy driving conditions.

f) Due to its inferior scavenging process, the two-stroke engine can suffer from the following:

i) Inadequate transfer of fresh mixture into the cylinder,

ii) Excessively large amounts of residual exhaust gas remaining in the cylinder,

iii) Direct expulsion of fresh charge through the exhaust port. These undesirable conditions may occur under different speed and load situations, which greatly influences both power and fuel consumption.

g) Far less maintenance is expected with the two-stroke engine compared with the four-stroke engine, but there can be a problem with the products of combustion carburizing at the inlet, transfer, and exhaust ports.

h) Lubrication of the two-stroke engine is achieved by mixing small quantities of oil with petrol in proportions anywhere between 1:16 and 1:24 so that, when crankcase induction takes place, the various rotating and reciprocating components will be lubricated by a petrol-mixture mist. Clearly a continuous proportion of oil will be burnt in the cylinder and expelled into the atmosphere to add to unwanted exhaust emission.

i) There are fewer working parts in a two-stroke engine than in a four-stroke engine, so two-stroke engines are generally cheaper to manufacture.

3.2 Comparison of Two & Four Stroke Cycle Diesel Engines

A brief but critical comparison of two and four stroke diesel engine is made below:

a) Theoretically, almost twice the power can be developed with a two-stroke engine compared with a four-stroke engine.

b) A comparison between a typical 12 liter four-stroke engine and a 7 liter two-stroke engine having the same speed range would show that they would develop similar torque and

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power ratings. The ratio of engine capacities for equivalent performance for these four-stroke and two-stroke engines would be 1.7:1.

c) In a four-stroke engine, the same parts generate power and empty and fill the cylinders. With the two-stroke engine, the emptying and filling can be carried out by light rotary components.

d) With a two-stroke engine, 40–50% more air consumption is necessary for the same power output; therefore the air-pumping work done will be proportionally greater.

e) About 10–20% of the upward stroke of a two-stroke engine must be sacrificed to emptying and filling the cylinder.

f) The time available for emptying and filling a cylinder is considerably less in a two-stroke-cycle engine – something like 33% of the completed cycle as compared to 50% in a four-stroke engine. Therefore more power will be needed to force a greater mass of air into the cylinder in a shorter time.

g) Compared with a two-stroke engine, more power is needed by the piston for emptying and filling the cylinder in a four-stroke engine, due to pumping and friction losses at low speeds. At higher engine speeds the situation is reversed, and the two-stroke’s Rootes blower will consume proportionally more engine power – this could be up to 15% of the developed power at maximum speed.

h) With reduced engine load for a given speed, a two-stroke engine blower will consume proportionally more of the power developed by the engine.

i) A two-stroke engine runs smoother and relatively quietly, due to the absence of reversals of loading on bearings as compared with a four-stroke engine.

3.3 Comparison of CI & SI Engine

Comparison of S.I. and C.I. engines is made from various aspects is made below:

Fuel economy The chief comparison to be made between the two types of engine is how effectively each engine can convert the liquid fuel into work energy. Different engines are compared by their thermal efficiencies. Thermal efficiency is the ratio of the useful work produced to the total energy supplied. Petrol engines can have thermal efficiencies ranging between 20% and 30%. The corresponding diesel engines generally have improved efficiencies, between 30% and 40%. Both sets of efficiency values are considerably influenced by the chosen compression-ratio and design.

Power and torque The petrol engine is usually designed with a shorter stroke and operates over a much larger crankshaft-speed range than the diesel engine. This enables more power to be developed towards the upper speed range in the petrol engine, which is necessary for high road speeds; however, a long-stroke diesel engine has improved pulling torque over a relatively narrow speed range, this being essential for the haulage of heavy commercial vehicles.

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At the time of writing, there was a trend to incorporate diesel engines into cars. This new generation of engines has different design parameters and therefore does not conform to the above observations.

Reliability Due to their particular process of combustion, diesel engines are built sturdier, tend to run cooler, and have only half the speed range of most petrol engines. These factors make the diesel engine more reliable and considerably extend engine life relative to the petrol engine.

Pollution Diesel engines tend to become noisy and to vibrate on their mountings as the operating load is reduced. The combustion process is quieter in the petrol engine and it runs smoother than the diesel engine. There is no noisy injection equipment used on the petrol engine, unlike that necessary on the diesel engine. The products of combustion coming out of the exhaust system are more noticeable with diesel engines, particularly if any of the injection equipment components are out of tune. It is questionable which are the more harmful: the relatively invisible exhaust gases from the petrol engine, which include nitrogen dioxide, or the visible smoky diesel exhaust gases.

Safety Unlike petrol, diesel fuels are not flammable at normal operating temperature, so they are not a handling hazard and fire risks due to accidents are minimized.

Cost Due to their heavy construction and injection equipment, diesel engines are more expensive than petrol engines.

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ReferencesV Ganesan

M.L. Mathur & R.P. Sharma

www.newworldencyclopedia.org/entry/Internal_combustion_engine

www.freeinfosociety.com/media/pdf/4438.pdf

www.first-hand.info/Fourstrokecycle.html

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