ADVANCED IC ENGINE NOTESME2041 ADVANCED I.C. ENGINES L T P C

3 0 0 3

OBJECTIVES:

To update the knowledge in engine exhaust emission control and alternate fuels

To enable the students to understand the recent developments in IC Engines

UNIT I SPARK IGNITION ENGINES 9

Air-fuel ratio requirements, Design of carburetor fuel jet size and venture size, Stages

of combustion-normal and abnormal combustion, Factors affecting knock, Combustion

chambers, Introduction to thermodynamic analysis of SI Engine combustion process.

UNIT II COMPRESSION IGNITION ENGINES 9

Stages of combustion-normal and abnormal combustion Factors affecting knock,

Direct and Indirect injection systems, Combustion chambers, Turbo charging,

Introduction to Thermodynamic Analysis of CI Engine Combustion process.

UNIT III ENGINE EXHAUST EMISSION CONTROL 9

Formation of NOX , HC/CO mechanism , Smoke and Particulate emissions, Green

House Effect , Methods of controlling emissions , Three way catalytic converter and

Particulate Trap, Emission (HC,CO, NO and NOX , ) measuring equipments, Smoke and

Particulate measurement, Indian Driving Cycles and emission norms

UNIT IV ALTERNATE FUELS 9

Alcohols , Vegetable oils and bio-diesel, Bio-gas, Natural Gas , Liquefied Petroleum

Gas ,Hydrogen , Properties , Suitability, Engine Modifications, Performance ,

Combustion and Emission Characteristics of SI and CI Engines using these alternate

fuels.

UNIT V RECENT TRENDS 9

Homogeneous Charge Compression Ignition Engine, Lean Burn Engine, Stratified

Charge Engine, Surface Ignition Engine, Four Valve and Overhead cam Engines,

Electronic Engine Management, Common Rail Direct Injection Diesel Engine, Gasoline

Direct Injection Engine, Data Acquisition System pressure pick up, charge amplifier PC

for Combustion and Heat release analysis in Engines.

TOTAL: 45 PERIODS

TEXT BOOK:

1. Heinz Heisler, Advanced Engine Technology, SAE International Publications,

USA,1998

2. Ganesan V.. Internal Combustion Engines , Third Edition, Tata Mcgraw-Hill ,2007

REFERENCES:

1. John B Heywood, Internal Combustion Engine Fundamentals, Tata McGraw-Hill

1988

2. Patterson D.J. and Henein N.A,Emissions from combustion engines and their

control, Ann Arbor Science publishers Inc, USA, 1978

3. Gupta H.N, Fundamentals of Internal Combustion Engines ,Prentice Hall of India,

2006

4. Ultrich Adler , Automotive Electric / Electronic Systems, Published by Robert Bosh

GmbH,1995

UNIT I

SPARK IGNITION ENGINES

Air-fuel Requirement in SI Engines

The spark-ignition automobile engines run on a mixture of gasoline and air. The amount of mixture the engine can take in depends upon following major factors:(i) Engine displacement.(ii) Maximum revolution per minute (rpm) of engine.(iii) Volumetric efficiency of engine.There is a direct relationship between an engines air flow and its fuel requirement. This relationship is called the air-fuel ratio.

Air-fuel Ratios

The air-fuel ratio is the proportions by weight of air and gasoline mixed by the carburetor as required for combustion by the engine. This ratio is extremely important for an engine because there are limits to how rich (with more fuel) or how lean (with less fuel) it can be, and still remain fully combustible for efficient firing. The mixtures with which the engine can operate range from 8:1 to 18.5:1 i.e. from 8 kg of air/kg of fuel to 18.5 kg of air/kg of fuel. Richer or leaner air-fuel ratio limit causes the engine to misfire, or simply refuse to run at all.

Stoichiometric Air-Fuel Ratio

The ideal mixture or ratio at which all the fuels blend with all of the oxygen in the air and be completely burned is called the stoichiometric ratio, a chemically perfect combination. In theory, an air fuel ratio of about 14.7:1 i.e. 14.7 kg of air/kg of gasoline produce this ratio, but the exact ratio at which perfect mixture and complete combustion take place depends on the molecular structure of gasoline, which can vary somewhat.

Engine Air-fuel Ratios

An automobile SI engine, as indicated above, works with the air-fuel mixture ranging from 8:1 to 18.5:1. But the ideal ratio would be one that provides both the maximum power and the best economy, while producing the least emissions. But such a ratio does not exist because the fuel requirements of an engine vary widely depending upon temperature, load, and speed conditions. The best fuel economy is obtained with a 15:1 to 16:1 ratio, while maximum power output is achieved with a 12.5:1 to 13.5:1 ratio. A rich mixture in the order of 11:1 is required for idle heavy load, and high-speed conditions. A lean mixture is required for normal cruising and light load conditions. Figure 9.36 represents the characteristic curves showing the effect of mixture ratio on efficiency and fuel consumption.

Fig. 9.36. Effect of air-fuel ratio on efficiency and fuel consumption.

Practically for complete combustion, through mixing of the fuel in excess air (to a limited extent above that of the ideal condition) is needed. Lean mixtures are used to obtain best economy through minimum fuel consumption whereas rich mixtures used to suppress combustion knock and to obtain maximum power from the engine. However, improper distribution of mixture to each cylinder and imperfect/incomplete vaporization of fuel in air necessitates the use of rich mixture to obtain maximum power output. A rich mixture is also required to overcome the effect of dilution of incoming mixture due to entrapped exhaust gases in the cylinder and of air leakage because of the high vacuum in the manifold, under idling or no-load condition. Maximum power is desired at full load while best economy is expected at part throttle conditions. Thus required air fuel ratios result from maximum economy to maximum power. The carburetor must be able to vary the air-fuel ratio quickly to provide the best possible mixture for the engines requirements at a given moment.The best air-fuel ratio for one engine may not be the best ratio for another, even when the two engines are of the same size and design. To accurately determine the best mixture, the engine should be run on a dynamometer to measure speed, load and power requirements for all types of driving conditions.With a slightly rich mixture, the combustion flame travels faster and conversely with a slightly weak mixture, the flame travel becomes slower. If a very rich mixture is used then some neat petrol enters cylinder, washes away lubricant from cylinder walls and gets past piston to contaminate engine oil. A very sooty deposit occurs in the combustion chamber. On the other hand, if an engine runs on an excessively weak mixture, then overheating particularly of such parts as valves, pistons and spark plugs occurs. This causes detonation and pre-ignition together or separately.The approximate proportions of air to petrol (by weight) suitable for the different operating conditions are indicated below:

Starting9 :1

Idling12 : 1

Acceleration12 : 1

Economy16: 1

Full power12 : 1

It makes no difference if an engine is carburetted or fuel injected, the engine still needs the same air-fuel mixture ratios.

Carburetion

Introduction

Spark-ignition engines normally use volatile liquid fuels. Preparation of fuel-air mixture is done outside the engine cylinder and formation of a homogeneous mixture is normally not completed in the inlet manifold. Fuel droplets, which remain in suspension, continue to evaporate and mix with air even during suction and compression processes. The process of mixture preparation is extremely important for spark-ignition engines. The purpose of carburetion is to provide a combustible mixture of fuel and air in the required quantity and quality for efficient operation of the engine under all conditions.

Definition of Carburetion

The process of formation of a combustible fuel-air mixture by mixing the proper amount of fuel with air before admission to engine cylinder is called carburetion and the device which does this job is called a carburetor.

Requirements of an automotive carburetor

The spark ignition engines fitted to automotive vehicles have to operate under variable speed and load conditions. These engines present the most difficult and stringent requirements to the carburetors. They are as follows:-

1. Ease of starting the engine, particularly under low ambient conditions.

2. Ability to give full power quickly after starting the engine.

3. Equally good and smooth engine operation at various loads.

4. Good and quick acceleration of the engine.

5. Developing sufficient power at high engine speeds.

6. Simple and compact in construction.

7. Good fuel economy.

8. Absence of racing of the engine under idling conditions.

9. Ensuring full torque at low speeds.

Factors Affecting Carburetion

Of the various factors, the process of carburetion is influenced by

i. The engine speed

ii. The vaporization characteristics of the fuel

iii. The temperature of the incoming air and

iv. The design of the carburetor

Principle of Carburetion

Both air and gasoline are drawn through the carburetor and into the engine cylinders by the suction created by the downward movement of the piston. This suction is due to an increase in the volume of the cylinder and a consequent decrease in the gas pressure in this chamber. It is the difference in pressure between the atmosphere and cylinder that causes the air to flow into the chamber. In the carburetor, air passing into the combustion chamber picks up discharged from a tube. This tube has a fine orifice called carburetor jet that is exposed to the air path. The rate at which fuel is discharged into the air depends on the pressure difference or pressure head between the float chamber and the throat of the venturi and on the area of the outlet of the tube. In order that the fuel drawn from the nozzle may be thoroughly atomized, the suction effect must be strong and the nozzle outlet comparatively small. In order to produce a strong suction, the pipe in the carburetor carrying air to the engine is made to have a restriction. At this restriction called throat due to increase in velocity of flow, a suction effect is created. The restriction is made in the form of a venturi to minimize throttling losses. The end of the fuel jet is located at the venturi or throat of the carburetor. The geometry of venturi tube is as shown in Fig.16.6. It has a narrower path at the center so that the flow area through which the air must pass is considerably reduced. As the same amount of air must pass through every point in the tube, its velocity will be greatest at the narrowest point. The smaller the area, the greater will be the velocity of the air, and thereby the suction is proportionately increased

As mentioned earlier, the opening of the fuel discharge jet is usually loped where the suction is maximum. Normally, this is just below the narrowest section of the venturi tube. The spray of gasoline from the nozzle and the air entering through the venturi tube are mixed together in this region and a combustible mixture is formed which passes through the intake manifold into the cylinders. Most of the fuel gets atomized and simultaneously a small part will be vaporized. Increased air velocity at the throat of the venturi helps he rate of evaporation of fuel. The difficulty of obtaining a mixture of sufficiently high fuel vapour-air ratio for efficient starting of the engine and for uniform fuel-air ratio indifferent cylinders (in case of multi cylinder engine) cannot be fully met by the increased air velocity alone at the venturi throat.

The Simple Carburetor

Carburetors are highly complex. Let us first understand the working principle bf a simple or elementary carburetor that provides an air fuel mixture for cruising or normal range at a single speed. Later, other mechanisms to provide for the various special requirements like starting, idling, variable load and speed operation and acceleration will be included. Figure 3. shows the details of a simple carburetor.

Figure: 3 The Simple Carburetor

The simple carburetor mainly consists of a float chamber, fuel discharge nozzle and a metering orifice, a venturi, a throttle valve and a choke. The float and a needle valve system maintain a constant level of gasoline in the float chamber. If the amount of fuel in the float chamber falls below the designed level, the float goes down, thereby opening the fuel supply valve and admitting fuel. When the designed level has been reached, the float closes the fuel supply valve thus stopping additional fuel flow from the supply system. Float chamber is vented either to the atmosphere or to the upstream side of the venturi.During suction stroke air is drawn through the venturi.

As already described, venturi is a tube of decreasing cross-section with a minimum area at the throat, Venturi tube is also known as the choke tube and is so shaped that it offers minimum resistance to the air flow. As the air passes through the venturi the velocity increases reaching a maximum at the venturi throat. Correspondingly, the pressure decreases reaching a minimum. From the float chamber, the fuel is fed to a discharge jet, the tip of which is located in the throat of the venturi. Because of the differential pressure between the float chamber and the throat of the venturi, known as carburetor depression, fuel is discharged into the air stream. The fuel discharge is affected by the size of the discharge jet and it is chosen to give the required air-fuel ratio. The pressure at the throat at the fully open throttle condition lies between 4 to 5 cm of Hg, below atmospheric and seldom exceeds8 cm Hg below atmospheric. To avoid overflow of fuel through the jet, the level of the liquid in the float chamber is maintained at a level slightly below the tip of the discharge jet. This is called the tip of the nozzle. The difference in the height between the top of the nozzle and the float chamber level is marked h in Fig.3.

The gasoline engine is quantity governed, which means that when power output is to be varied at a particular speed, the amount of charge delivered to the cylinder is varied. This is achieved by means of a throttle valve usually of the butterfly type that is situated after the venturi tube. As the throttle is closed less air flows through the venturi tube and less is the quantity of air-fuel mixture delivered to the cylinder and hence power output is reduced. As the throttle is opened, more air flows through the choke tube resulting in increased quantity of mixture being delivered to the engine. This increases the engine power output. A simple carburetor of the type described above suffers from a fundamental drawback in that it provides the required A/F ratio only at one throttle position. At the other throttle positions the mixture is either leaner or richer depending on whether the throttle is opened less or more. As the throttle opening is varied, the air flow varies and creates a certain pressure differential between the float chamber and the venturi throat. The same pressure differential regulates the flow of fuel through the nozzle. Therefore, the velocity of flow of air II and fuel vary in a similar manner. At the same time, the density I of air decrease as the pressure at the venturi throat decrease with increasing air flow whereas that of the fuel remains unchanged. This results in a simple carburetor producing a progressively rich mixture with increasing throttle opening.

The Choke and the Throttle

When the vehicle is kept stationary for a long period during cool winter seasons, may be overnight, starting becomes more difficult. As already explained, at low cranking speeds and intake temperatures a very rich mixture is required to initiate combustion. Some times air-fuel ratio as rich as 9:1 is required. The main reason is that very large fraction of the fuel may remain as liquid suspended in air even in the cylinder. For initiating combustion, fuel-vapour and air in the form of mixture at a ratio that can sustain combustion is required. It may be noted that at very low temperature vapour fraction of the fuel is also very small and this forms combustible mixture to initiate combustion. Hence, a very rich mixture must be supplied. The most popular method of providing such mixture is by the use of choke valve. This is simple butterfly valve located between the entrance to the carburetor and the venturi throat as shown in Fig.3.

When the choke is partly closed, large pressure drop occurs at the venturi throat that would normally result from the quantity of air passing through the venturi throat. The very large depression at the throat inducts large amount of fuel from the main nozzle and provides a very rich mixture so that the ratio of the evaporated fuel to air in the cylinder is within the combustible limits. Sometimes, the choke valves are spring loaded to ensure that large carburetor depression and excessive choking does not persist after the engine has started, and reached a desired speed. This choke can be made to operate automatically by means of a thermostat so that the choke is closed when engine is cold and goes out of operation when engine warms up after starting. The speed and the output of an engine is controlled by the use of the throttle valve, which is located on the downstream side of the venturi.

The more the throttle is closed the greater is the obstruction to the flow of the mixture placed in the passage and the less is the quantity of mixture delivered to .the cylinders. The decreased quantity of mixture gives a less powerful impulse to the pistons and the output of the engine is reduced accordingly. As the throttle is opened, the output of the engine increases. Opening the throttle usually increases the speed of the engine. But this is not always the case as the load on the engine is also a factor. For example, opening the throttle when the motor vehicle is starting to climb a hill may or may not increase the vehicle speed, depending upon the steepness of the hill and the extent of throttle opening. In short, the throttle is simply a means to regulate the output of the engine by varying the quantity of charge going into the cylinder.

Stages of Combustion in SI Engine

In a spark-ignition engine a sufficiently homogeneous mixture of vaporized fuel, air and residual gases is ignited by a single intense and high temperature spark between the spark plug electrodes (at the moment of discharge the temperature of electrodes exceeds 10,000C), leaving behind a thin thread of flame. From this thin thread combustion spreads to the envelop of mixture immediately surrounding it at a rate which depends primarily upon the temperature of the flame front itself and to a secondary degree, upon both the temperature and the density of the surrounding envelope. In this manner there grows up, gradually at first, a small hollow nucleus of flame, much in the manner of a soap bubble. If the contents of the cylinder were at rest, this flame bubble would expand with steadily increasing speed until extended throughout the whole mass. In the actual engine cylinder, however, the mixture is not at rest. It is, in fact, in a highly turbulent condition the turbulence breaks the filament of flame into a ragged front, thus presenting a far greater surface area from which heat is radiated; hence its advance is speeded up enormously. The rate at which the flame front travels is dependent primarily on the degree of turbulence, but its general direction of/movement, that of radiating outward from the ignition point, is not much affected. According to Ricardo the combustion can be imagined as if developing in two stages, one the growth and development of a semi propagating nucleus of flame called ignition lag or preparation phase, and the other, the spread of the flame throughout the combustion chamber [see Fig. 9].

Figure: 9. Stages of combustion in SI engineThe former is a chemical process depending upon the nature of the fuel, upon temperature and pressure, the proportion of the exhaust gas, and also upon the temperature coefficient of the fuel, that is, the relationship between temperature and rate of acceleration of oxidation or burning. The second stage is a mechanical one pure and simple. The two stages are not entirely distinct, since the nature and velocity of combustion change gradually. The starting point of the second stage is where first measurable rise of pressure can be seen on the indicator diagram, i.e., the point where the line of combustion departs from the compression line. In Fig. 14.2(b), A shows the point of passage of spark - (say 28 before TDC), B the point at which the first rise of pressure can be detected (say, 8before TDC) and C the attainment of peak pressure. Thus AB represents the first stage (about 20 crank angle rotation) and BC the second stage. Although the point C makes the completion of the flame travel, it does not follow that at this point the whole of the heat of the fuel has been liberated, for even after the passage of the flame, some further chemical adjustments due to re-association, etc., and what is generally referred to as after burning, will to a greater or less degree continue throughout the expansion stroke. The first stage AB, by analogy with diesel engines is called ignition lag, which label is wrong in principle. In spark ignition there is practically no ignition lag and a nucleus of combustion arises instantaneously near the spark plug electrodes. But during the initial period flame front spreads very slowly and the fraction of burnt mixture is small so that an increase of pressure cannot be detected on the indicator diagram. The increase of pressure maybe just one per cent of maximum combustion pressure corresponding to burning of about 1.5per cent of the working mixture, and the volume occupied by the combustion products may be about 5 per cent of the combustion chamber space.

The stage II is the main stage of combustion. The end of second stage is taken as the moment at which maximum pressure is reached in the indicator diagram (see Fig. 9). However, combustion does not terminate at this point and after burning continues for a rather long time near the walls and behind the turbulent flame front. The combustion rate in the stage III reduces, due to surface of the flame front becoming smaller and reduction in turbulence. About 10 per cent or more of heat is evolved in the after-burning stage and hence the temperature of the gases continues to increase to point D in Fig.9. However, the pressure reduces because the decrease in pressure due to expansion of gases and transfer of heat to walls is more than the increase in pressure due to combustion.

Effect of Engine Variables on Flame Propagation

A study of the variables which affect the flame propagation velocity is important because the flame velocity influences the rate of pressure rise in the cylinder, and has bearing or certain types of abnormal combustion.

There are several factors which affect the flame speed, the most important being fuel-air ratio and turbulence. 1. Fuel-air ratio: The composition of the working mixture influences the rate of combustion and the amount of heat evolved. With hydrocarbon fuels the maximum flame velocities occur when mixture strength is 110% of stoichiometric (i.e., about 10% richer than stoichiometric). When the mixture is made leaner or is enriched and still more, the velocity of flame diminishes. Lean mixtures release less thermal energy resulting in lower flame temperature and flame speed. Very rich mixtures have incomplete combustion (some carbon only burns to CO and not to CO2) that results in production of less thermal energy and hence flame speed is again low.

2. Compression Ratio: A higher compression ratio increases the pressure and temperature of the working mixture and decreases the concentration of residual gases. These favorable conditions reduce the ignition lag of combustion and hence less ignition advance is needed. High pressures and temperatures of the compressed mixture also speed up the second phase of combustion. Total ignition angle is reduced. Maximum pressure and indicated mean effective pressure are increased.. Lastly, use of a higher compression ratio increases the surface to volume ratio of the combustion chamber, thereby increasing the part of the mixture which after-burns in the third phase. The increase in compression ratio results in increase in temperature that increases the tendency of the engine to detonate.

3. Intake temperature and pressure: Increase in intake temperature and pressure increases the flame speed.

4. Engine load: With increase in engine load the cycle pressures increase. Hence the flame speed increases. In SI engines with decrease in load, throttling reduces power of an engine. Due to throttling the initial and final compression pressures decrease and the dilution of the working mixture due to residual gases increases. This makes the smooth development of self propagating nucleus of flame difficult and unsteady and prolongs the ignition lag. The difficulty can be overcome to a certain extent by enriching the mixture at low loads (0.8 to 0.9of stoichiometric) but still it is difficult to avoid after-burning during a substantial part of expansion stroke. In fact, poor combustion at low loads and the necessity of mixture enrichment are among the main disadvantages of spark ignition engines which cause wastage of fuel and discharges of a large amount of products of incomplete combustion like carbon monoxide and other poisonous substances.

5. Turbulence: Turbulence plays a very vital role in combustion phenomenon. The flame speed is very low in non-turbulent mixtures. A turbulent motion of the mixture intensifies the processes of heat transfer and mixing of the burned and unburned portions in the flame front (diffusion). These two factors cause the velocity of turbulent flame to increase practically in proportion to the turbulence velocity. The turbulence of the mixture is due to admission of fuel-air mixture through comparatively narrow sections of the intake pipe, valves, etc. in the suction stroke. The turbulence can be increased at the end of the compression by suitable design of combustion chamber that involves the geometry of cylinder head and piston crown. The degree of turbulence increases directly with the piston speed. If there is no turbulence the time occupied by each explosion would be so great as to make the high speed internal combustion engines impracticable. Insufficient turbulence lowers the efficiency due to incomplete combustion of the fuel. However, excessive turbulence is also undesirable.

6. Engine Speed: The higher the engine speed, the greater the turbulence inside the cylinder. For this reason the flame speed increases almost linearly with engine speed. Thus if the engine speed is doubled the time required, in milliseconds, for the flame to traverse the combustion space would be halved. Double the original speed arid hence half the original time would give the same number of crank degrees for flame propagation. The crank angle required for the flame propagation, which is the main phase of combustion, will remain almost constant at all speeds. This is an important characteristic of spark ignition engines. However, the increase in engine speed would lead to ignition advance due to the first phase of combustion. This can be illustrated with a numerical example. Consider a petrol engine running at 1500rpm. Let us say for the first stage of combustion the ignition lag, the time required in terms of crank angle, is 8 of crank rotation, and for the second stage, the propagation of flame through the combustion space, 12oofcrank rotation is required. Thus the total ignition period is 20of crank rotation. Now if the engine speed is doubled from 1500 to 3000 rpm, the time required for the second stage will again be 12 of crank rotation (due to doubling of turbulence intensity time in milliseconds is halved and in terms of crank angle remains constant), but for the first stage time in milliseconds is constant and hence in terms of crank angle it will be doubled, i.e., it would be 16.This would make the total ignition period of 16 + 12 = 28 crank rotation at 3000rpm compared to 8 + 12= 20 at .1500 rpm. From this it follows that with increase in engine speed ignition must be advanced. This is done in practice by automatic ignition advance mechanism.

7. Engine size: Engines of similar design generally run at the same piston speed. This is achieved by smaller engines having larger rpm and larger engines having smaller rpm. Due to the same piston speed, the inlet velocity, the degree of turbulence, and flame speed are nearly same in similar engines regardless of the size. However, in small engines the flame travel is small and in large engines large. Therefore, if the engine size is doubled the time required (in milliseconds) for propagation of flame through combustion space will also be doubled. But with lower rpm of larger engines the time for flame propagation in terms of crank angle would be nearly same as in smaller engines. In other words the number of crank degrees required for flame travel will be about the same irrespective of engine size, provided the engines are similar.

Rate of Pressure RiseThe rate of pressure rise is a very important aspect of flame development from engine design and operation point of view. It considerably influences the maximum cylinder pressure, the power produced and the smooth running of the engine. The rate or pressure rise depends on the mass rate of combustion of the mixture in the cylinder. Fig. 10 shows pressure-crank angle diagrams for three different combustion rates. One is for a high, the second for the usual and the third for a low rate of combustion

Figure: 10. Relationship b/w pressure and crank angle for different rates of combustion

It is clear from the figure that with lower rates of combustion longer time is required for combustion that necessitates the initiation of burning at an earlier point on the compression stroke. With higher rates of burning the time required for combustion is smaller and the rate of pressure rise is higher. Also, the peak pressure produced is close to TDC, which is desirable because it produces greater force acting through a large portion of the power stroke. But peak pressure and hence peak temperature too close to TDC gives a long time for rapid heat loss from the cylinder. The higher rate of pressure rise causes rough running of the engine because of vibrations and jerks produced in crankshaft. If the rate of pressure rise is very high it results in abnormal combustion called detonation. In practice the engine is so designed that approximately one-half of the pressure rise takes place as the piston reaches TDC. This results in peak pressure and temperature 10 to 15 after TDC. In this way very small portion of the expansion stroke is-lost and the gain is smooth engine operation and saving an appreciable period of time during which loss of heat is rapid. In the old engines with low compression ratios of 5 to 6 a rate of pressure rise of 2 bar per crank degree used to be thought as optimum. Today with higher compression ratios of the order of 8 to 9, a rate of pressure rise of 3 to 4 bar per crank degree may be employed if the engine mountings are sufficiently stiff and efficient.

Gasoline CombustionVaporization of the hydrocarbons in gasoline and start of decomposition take place at temperatures below 593 K, which exist in the combustion chamber before the initiation of ignition. The products of combustion are mostly gases containing a large quantity of heat. The heat energy increases the gas pressure in the combustion chamber to produce the force on the engine piston, required to operate the engine. The liquid gasoline must be converted to a vapour to burn in an engine. In carburetted engines vaporization of the gasoline must be done in one-third of a second at idle speeds and in one-thirtieth of a second at normal operating speeds. In fuel injected engines this must occur much faster. The carburetor during the process of mixing liquid fuel and air supports the vaporization process by breaking the liquid gasoline into sudsy foam that rapidly mixes with the air. The molecules of fuel and the molecules of oxygen in the air must combine in correct numbers. At sea level the air being dense a relatively small quantity is required for a given amount of gasoline. The air becomes less dense at high altitudes and at high atmospheric temperatures due to which the same volume of air contains a smaller number of oxygen molecules causing the air-fuel mixture to become richer in fuel. This causes problem on some emission controlled engines requiring leaner carburetor settings on automobiles used in the mountains than those used at sea level. Since automobiles are frequently operated in both mountains and at sea level, carburetors are provided with altitude compensation devices to prevent over-rich mixtures at high elevations.

the charge is trapped in the combustion chamber, the molecules of oxygen in the air come into close contact with the hydrocarbon molecules of the gasoline. This causes rapid burning. A litre of gasoline if completely burned produces nearly a litre of water as well as sulphur dioxide in an amount dependent on the sulphur content in the gasoline. As the water is in a vapour form at normal operating temperatures it leaves the cylinder as a part of exhaust gas. When the engine is first started in cold weather condensed water vapour is visible in the exhaust. Condensed moisture with sulphur dioxide produces the acidic water, which is corrosive. During low temperature operating conditions such as suburban driving when the engine is cold, much of the moisture is condensed inside the engine. The combination of corrosion and wear under these conditions is the major reason for excessive wear of the top ring area of the cylinder wall.Normal Combustion

In a SI engine a homogeneous air-fuel mixture within the combustible range sustains the progress of a definite flame front across the combustion chamber, and combustion takes place in any location where fuel particle exists. In a CI engine, on the other hand, the air-fuel ratios in the various part of the chamber very widely, so no definite flame front is evident, and hence combustion occurs in many locations within the chamber.A spark plug ignites the charge in the combustion chamber near the end of the compression stroke. The spark, produced across the spark plug electrodes at the correct time, must have sufficient energy to raise the gas temperature between the electrodes at a point so that the charge burning becomes self-sustaining. From this point, a flame front moves smoothly across the combustion. The flame front movement during normal combustion is illustrated in Fig. 8.6. Burning of charge takes place during approximately fifty degrees of crankshaft rotation due to which maximum force is exerted on the crankshaft. Actual combustion is much more complex and the combustion gases pass through many phases during the combustion process. For better understanding, the combustion is divided into two phases i.e. pre-flame reactions, and combustion.

As the gases are compressed and the temperature rises, pre-flame chemical reactions take place in the compressed charge thereby changing the character of the charge. These pre-flame reactions prepare the charge for burning.As ignition takes place, depending upon combustion chamber turbulence the flame front moves out in a modified spherical fashion. The heat energy released behind the flame front increases combustion chamber pressure and temperature. Due to higher combustion chamber pressure and temperature the pre-flame reactions are increased in a portion of the charge, called the end gases, which remain ahead of the flame front. Pre-flame reactions increase more rapidly at higher engine compression ratios. If pre-flame reactions become too rapid, abnormal combustion takes place.

Abnormal CombustionAbnormal combustion may be divided into two main types i.e. knock or detonation and surface ignition. Each of these types causes loss of power and excessive temperature. Continued operation under either type of abnormal combustion gives rise to physical damage of the engine.Detonation.Engine knock or detonation is the out come of rapid pre-flame reactions within the highly stressed end gases. Due to the too rapid reactions spontaneous ignition of the end gases takes place as shown in Fig. 8.7. This causes very rapid combustion within the end gases, accompanied by high-frequency pressure waves. These waves hit the combustion chamber walls; as a result vibration noise sets which is called knock or detonation.

Detonation is affected by(i) compression ratio,(ii) the temperature and pressure at the end of compression, (Hi) the temperature of combustion chamber wall,(iv) engine speed,(v) fuel mixture strength,(vi) combustion chamber shape,(vii) the type of fuel,(viii) ignition timing,(ix) position of spark plug, and(x) position of exhaust valve.

Fig. 8.6. Flame front movement during normal combustion.

Fig. 8.7. Flame front movement during detonation.

The tendency of an engine to knock with a given fuel can be suppressed by lowering either combustion pressure or temperature, or both ; or by reducing the time the end gases are subjected to high pressures and temperatures. Also, using a fuel, which is less susceptible to rapid pre-flame reactions, reduces the tendency to knock. Octane rating is a measure of the anti-knock properties of a fuel. A fuel, which has high anti-knock characteristics, has a high octane rating.

Compression ratio has predominant effects on compression pressure. With the increase of compression pressure the output power of an engine increases. This is due to the higher combustion pressures, which are produced. High combustion pressures, however, increase the knock tendency. Fuels with high antiknock properties are used in higher-compression ratio engines to run engine knock-free while developing increased power. Lower compression ratios are used in low-emission engines so that they can run knock-free on low-octane unleaded gasoline.

Combustion chamber design also affects knock tendency. If combustion chambers end gases are in a squash or quench area, the engine has low knocking tendencies. This happens, as theend gases are thin and close to a cool metal surface. Cooling the gases reduces and slows the end gas pre-flame reactions, thereby decreasing the engine knock tendency. This quenching of end gases is the main reason for a rotating combustion chamber engine to run knock-free on low octane gasoline.Combustion chamber turbulence, as illustrated in Fig. 8.8, also helps to reduce knocking tendency by mixing cool and hot gases, thus preventing a concentration of static hot end gases where rapid pre-flame reactions can take place.

Fig. 8.8. End gases cooled in the quench area.

Fig. 8.9. Flame front movement during pre-ignition.

The detonation can be reduced by(a) decreasing the combustion pressure and temperature,(b) reducing the time the end gases are subjected to high pressures and temperatures,(c) the use of fuel with a high octane number,(d) proper design of combustion chamber where end gases are in a squash or quench area, and(e) increasing combustion chamber turbulence.Surface Ignition.

Surface ignition or secondary ignition, an abnormal combustion, starts at any source of ignition other than the spark plug. This is illustrated in Fig. 8.9. As surface ignition produces a secondary ignition source, its effect is to complete the combustion process sooner than normal, thereby developing maximum pressure at a wrong time in the engine cycle producing less power.

One potential source of secondary ignition is a hot spot, such as a spark plug electrode, a protruding gasket, a sharp valve edge, etc. These items can become extremely hot during engine operation forming a second source of ignition. These sources rarely occur in modern engine designs provided the engines are properly maintained. Another source of secondary ignition is combustion chamber deposits, which result from the type of fuel and oil used in the engine as well as from the type of operation of the engine. A deposit ignition source may be a hot loose deposit flake capable of igniting one charge before it is exhaustedfrom the engine with the spent exhaust gases. This is called wild ping. Sometimes, the flake remains attached to the combustion chamber wall. Under this situation, it ignites successive charges until the deposit is consumed or the engine operating conditions are changed.When surface ignition occurs before firing of the spark plug, it is called pre-ignition. It may be audible or inaudible. It may be a wild ping or it may be a continuous runaway surface ignition. If it occurs after the ignition is turned off, it is called run-on or dieseling. Another phenomenon resulting from pre-ignition is engine rumble. Rumble is a low-frequency vibration of the lower part of the engine that occurs when the maximum pressure is reached earlier than normal in the cycle. Rumble has been almost eliminated from modern engines.The knock-resistant fuels and antiknock additives generally tend to increase combustion chamber deposits thereby increasing the tendency to cause surface ignition. Fuel manufacturers therefore, use additional additives in the gasoline to reduce the deposit ignition tendency resulting from the antiknock additives deposits. Abnormal combustion seldom occurs in modern mass-produced automotive engines provided the recommended grade of fuel and motor oil is used and the engine is maintained and adjusted correctly. Some problems may exist in engines that are used exclusively for low-speed, short-trip driving. Abnormal combustion frequently occurs in engines modified for maximum performance and also some in emission controlled engines.

Pre-ignition

Ignition of air fuel mixture by some hot spot which exists within the combustion chamber, before the occurrence of spark is called pre-ignition.

In a spark ignition engine, the spark that jumps across the terminals of the spark plug initiates combustion. Similarly if there is any other hot source in the combustion chamber it will heat up the air fuel mixture surrounding it. Then preflame reaction will certainly be accelerated by this hot spot. The hot spot may activate. The charge in its immediate vicinity and produce a flame. The flame may then propagate from this point before the occurrence of spark. Pre-ignition combustion can be seen in fig.

As indicated under surface ignition, carbon deposit from fuel or oil, an over heated spark plug center electrode or the edge of the gasket that protrudes into the combustion chamber can act as a hot spot and cause pre-ignition. An overheated exhaust valve head or edge can cause preigniton. Using unsuitable type spark plug (one that runs too hot or has a long reach) or igniton timing too far retarded or mixture too weak or rich which gives too slow a burning rate may also cause preigniton. The minimum tendency to preignite exists at fuel air ratios usually richer than the chemically correct. Tetra ethyl lead which is added to a fuel to increase its antiknock characteristics also reduces the tendency to preignite.

The amount of charge that burns instantaneously due to preigniton depends upon the surface area of the hot spot and the temperature of the hot spot. When a considerable amount of charge burns, steep pressure rise and pressure pulsation may occur. A knock, metallic sound will be heard.

Different abnormal combustion that may take place in a SI engine

The definitions that follow the spirit of the CRC report 278, SAE special publication are as follows:-Knock The noise associated with auto-ignition of a portion of the mixture ahead of a flame front advancing at normal velocity (whether or not surface ignition is present).

Normal combustion - Combustion initiated by a timed spark, with the flame front moving in a uniform manner at a normal velocity, without auto ignition.

Abnormal combustion Combustion with surface ignition (phosphorous additives to the gasoline are used for control of surface ignition and spark plug fouling).

Spark knock Recurrent knock which can be controlled in intensity (or eliminated) by adjusting the spark advance.

Surface ignition Initiation of a flame front by a hot surface other than the spark.

Pre-ignition Surface ignition occurring before the spark.

Post ignition Surface ignition occurring after the spark.

Wild ping Erratic pings or sharp cracks (probably as the result of early surface ignition from deposit particles)

Rumble A low pitched thud (probably caused by multiple, early, surface ignition raising the pressure greatly with consequent deflection of mechanical parts).

Effects of combustion knock The auto ignitions of the charge, steep pressure rise which sets up pressure wave, vibration of the gas and increased heat transfer to the cylinder walls, piston and other engine components during knocking combustion may result in the following:

1. Reduction in power output and efficiency.

2. Burning of piston crown due to increased temperature or due to blow by of very hot gases past the piston rings from the piston top to the crankcase.

3. The impact of the high pressure wave that is set up might even fracture the piston crown.

4. Burning of cylinder head and valve head.

5. Gumming of piston rings in the piston grooves leading to ring sticking.

6. Loosening of valve seat inserts in the cylinder head.

7. Erosion of piston head may occur at the position of the end mixture. The eroded surface has the appearance of being blasted and not melted.

Operating conditions causing detonation

The following are some of the operating conditions which may cause detonation in an engine.

1. Slow burning lean air fuel mixture supplied by faulty carburetor or fuel injector, fuel pump, blocked fuel filter or fuel line, vacuum leak at higher engine speeds caused by bad positive crankcase ventilation (PCV) valve or exhaust gas recirculation (EGR) valve.

2. Gasoline with low octane or anti clock rating. This is more common with unleaded gasoline.

3. Carbon deposits increasing compression ratio. This is the result of lubricating oil entering the cylinders or poor detergent action of gasoline.

4. Engine operating at above normal temperature due to low coolant level or circulation, water jacket blockage in the head.

5. Ignition timing very much advanced due to improper setting of initial ignition timing, inaccurate distribution or advance curve etc.

6. Bad rings and / or valve seals allowing oil (low octane hydrocarbon) to be burned in the cylinders.

7. Air cleaner clogged, which allows too much hot exhaust gas to remain in the engine cylinder.

8. Excessive turbocharger boost pressure from a bad pressure limiting valve.

Ways and means of knock reductionInvestigations indicate that one or more of the following factors will decrease the possibility of knock in the SI engine.1. Decreasing the compression ratio or reducing the inlet pressure.

2. Decreasing inlet air temperature.

3. Decreasing coolant inlet temperature.

4. Decreasing temperature of the cylinder and combustion chamber walls or part opening of the throttle (decreasing the load).

5. Retarding spark timing.

6. Decreasing the distance of flame travel in order to complete combustion within a shorter period.

7. Increasing the turbulence of the mixture and thus increasing the flame speed.

8. Increasing the engine speed, thus increasing the speed (movement) of the mixture and decreasing the time available for preflame reactions.

9. Increasing octane rating of the fuel.

10. Supplying rich or lean mixtures.

11. Stratifying the mixtures so that the end gas is less reactive.

12. Increasing the humidity of the entering air.

Increase in variableMajor effect onunburned reduce chargeAction to be takento Knocking

Compression ratioIncreases temperature and pressurereduce

Mass of chargeinducedIncrease pressureReduce

Inlet temperatureIncreases temperatureReduce

Chamber wall temperatureIncreases temperatureReduce

Spark advanceIncreases temperature & pressureRetard

A / F ratioIncreases temperature & pressureMake very rich

TurbulenceDecreases time factorIncreases

Engine speedDecreases time factorIncreases

Distance of flame travelIncreases time factorReduce

Types of combustion chambers

Combustion chamber shape depends principally upon the valve arrangement, piston head and combustion chamber contours. Different types of combustion chambers such as T head, L head, F head, L head turbulent, valve in head, valve in head with inclined valves have been tried and used by different engine manufacturers. These can be seen in figure.

The T head design stipulates the use of the lowest compression ratios to prevent knocking with a given fuel. F head design is an improvement over the T head. In this the inlet valves are located in the cylinder head and the exhaust valves are located in the cylinder or vice versa. This improves the volumetric efficiency and also reduces the width of the combustion chambers. T head design stipulates two camshafts one operating the inlet valves and the other operating the exhaust valves. F head and other designs can have a single camshaft operating all the valves. However, F head design presents difficulties in the design of the valve operating mechanism.

Overhead valve designs result in higher volumetric efficiency. These may have a single camshaft located by the side of the cylinder operating the valves through tappets, push rods and rocker arms, or a single camshaft located in the cylinder head and operating the valves through rocker arms or a single camshaft located in the cylinder head and operating the valves directly.

In the turbulent combustion chamber, very small clearance is provided between the piston crown and the cylinder head over a portion of the piston crown surface. This causes squish turbulence in the mixture, better mixing of fuel and air and improves combustion. Further, this narrow space when made to contain the end mixture, knocking is avoided because of better cooling. Even if knock occurs its severity will be lesser. This feature was incorporated in the General Motors Research combustion chamber and this permitted the use of a 12.5:1 compression ratio with 100 ON fuel. This principle was also incorporated in the Ricardo turbulent combustion chamber.

QUESTION BANK OF UNIT I

Part A

1. What is steady running

2. What is Transient operation

3. What is back firing

4. Define idling in an engine

5. What is the effect of inlet and exhaust pressure on mixture requirements

6. What are the factors that influence carburetion

7. What are the essential features of a carburetor

8. What is pre ignition

9. What are the effects of pre ignition

10. What are the effects of knock in SI engines

11. Name some types of combustion chambers in SI engines

12. Write a short notes on T- head combustion chamber

13. What are the various additives used to suppress knock in SI engines

14. Why rich mixture is required for idling

15. What is stoichiometric air fuel ratio

16. What are different air fuel mixtures on which an engine can operate

17. How can the location of the spark plug influence knocking tendency

18. What is delay period and what are the factors that affect the delay period?

19. Write any four factors that affect the process of combustion

20. What is a homogeneous and heterogeneous mixture?

21. What is meant by Carburetion?

22. What are the functions of a carburettor?

23. What is ignition lag

24. What is period of afterburning in SI engines

25. What are the variables that affect ignition lag

26. What is the effect of inlet temp and pressure on ignition lag

27. What is the effect of fuel - air ratio on flame propagation

28. What is the effect of compression ratio on flame propagation

29. Write short notes on effect of turbulence on flame propagation

30. What are the engine variables that effect Knock in SI engines.

31. How spark advance effects knock in SI engines

32. Write any four methods of controlling knock

33. What are the methods of detecting knock

34. What are the basic requirements of a good combustion chamber

35. Write short notes on Side Valve(I-Head) combustion chambers

36. Write short notes on Over head valve combustion chamber

37. What are the basic types of carburettor

38. What are the drawbacks of a simple carburettor

Part B

1. Discuss the design criteria for a S.I engine combustion chamber

2. Explain with figures various types of combustion chambers used in SI engines.

3. Explain the effect of various engine variables on SI engine Knock

4. Explain the phenomenon of knock in SI engines.what are the factors which influence the knock.describe the methods used to supress it.

5. Explain the fuel/air mixture requirements for an engine based on various speeds.

6. With the help of neat sketch explain the working principle of a simple carburettor

7. Explain the following related to SI engines:

i)Pre ignition

ii)Auto ignition

iii)Knock

8. What is Ignition Lag ?Discuss the effect of engine variables on ignition lag

9. Discuss the effect of the following engine variables on flame propagation :

a) Fuel - air ratio

b) Compression ratio

c) engine load

d) turbulence

e) engine speed

10. Discuss the ill effects of detonation

11. Explain the two theories of detonation

12. Explain the phenomenon of pre-ignition? How pre ignition leads to detonation

UNIT II

COMPRESSION IGNITION ENGINES

Four stages of combustion in a CI engine

Herry Ricardo has investigated the combustion in a compression ignition engine and divided the same into the following four stages:

1. Ignition delay or delay period.

2. Uncontrolled combustion.

3. Controlled combustion.

4. After burning.

Fig: Pressure time diagram illustrating in a compression ignition engine.

1. Ignition delay

2. Uncontrolled combustion

3. Controlled combustion

4. After burning.

The details of these stages of combustion are given below:

Pressure Vs crank angle of a CI engine in a simplified from is shown in fig. The curved line ABCG represents compression and expansion of the air charge in the engine cylinder when the engine is being motored, without fuel injection. This curve is mirror symmetry with respect to TDC line. The curve ABCDEFH shows the pressure trace of an actual engine.

Delay period

In an actual engine, fuel injection beings at the point B during the compression stroke. The injected fuel does not ignite immediately. It takes some time to ignite. Ignition sets in at the point C. During the crank travel B to C pressure in the combustion chamber does not rise above the compression curve. The period corresponding to the crank angle B to C is called delay period or ignition delay (about 0.001 seconds).

During ignition delay, the following events take place. The injected spray enters the combustion chamber and slowly (at about 55 m/min) bores hole in the air mass, while the fuel particles are stripped away. Some of these particles are vapourized. Thus, the main body of the spray is surrounded by vapour liquid particle air envelope. In small combustion chambers, the spray body may impinge on the walls. Some of the impinged fuel may bounce off the surface, while the rest may glide on the walls. Vapourization of fuel particles tends to lower the compression pressure and temperature slightly. At the same time, the energy released in the pre-flame reactions tends to raise the pressure. Now in the outer envelope of the spray, ignition nuclei are formed. Mostly, the nuclei are cool flame reactions, on the verge of auto-ignition. By oxidation or cracking reactions, luminescent carbon particles are formed.

Uncontrolled combustion

At the end of the delay period i.e. at the point C, fuel starts burning. At this point, a good amount of fuel would have already entered and got accumulated inside the combustion chamber. This fuel charge is surrounded by hot air. The fuel is finely divided and evaporated. Majority of the fuel burns with an explosion like effect. This instantaneous combustion is called uncontrolled combustion. This combustion causes a rapid pressure rise.

During uncontrolled combustion the following take place. Flame appears at one or more locations and spreads turbulently, with glowing luminosity. Flame of low luminosity marks regions of vapourized fuel and air (premixed flame. Flames of higher luminosity marks regions of liquid droplets and air (diffusion flame). The initial spreading of non luminous and luminous flame arises from auto-ignition and flame propagation. This is the knock reaction with a high rate of energy release and correspondingly high rate of pressure rise.

Combustion during crank travel C to D is called uncontrolled combustion. This is because no control over this combustion is possible by the engine operator. Since this combustion is more or les instantaneous, it is also called rapid combustion.

If more fuel is present in the cylinder at the end of delay period, and undergoes rapid combustion when ignition sets in, the rate of pressure rise and the peak pressure attained will be greater. During this combustion the piston is around TDC, and is almost stand still. Too rapid a pressure rise and severe pressure impulse at this position of the piston will result in combustion noise called Diesel Knock.

The severity of the knock reactions is in proportion to the mass enflamed. The regions of premixed flame are probably hotter (and older) than the regions where liquid droplets are present. As such, the knock reaction may be propagated mainly in the low luminosity state of the flame.

The rate at which the uncontrolled combustion takes place will depend upon the following:

1. The quantity of fuel in the combustion chamber at the point C. This quantity depends upon the rate at which fuel is injected during delay period and the duration of ignition delay.

2. The condition of fuel that has got accumulated in the combustion chamber at the point C.

The rate of combustion during the crank travel C to D and the resulting rate of pressure rise determine the quietness and smoothness of operation of the engine.

Controlled combustion

During controlled combustion, following thing happen. The flame spreads rapidly (but less than 135 m/min), as a turbulent, heterogeneous or diffusion flame with a gradually decreasing rate of energy release. Even in this stage, small auto-ignition regions may be present. The diffusion flame is characterized by its high luminosity. Bright, white carbon flame with a peak temperature of 2500o C is noticed. In this stage, radiation plays a significant part in engine heat transfer.

During the period D to E, combustion is gradual. Further by controlling the rate of fuel injection, complete control is possible over the rate of burning. Therefore, the rate o pressure rise is controllable. Hence, this stage of combustion is called Gradual combustion or Controlled combustion.

The period corresponding to the crank travel D to E is called the period of controlled combustion.

The rate of burning during the period of controlled combustion depends on the following:

1. Rate of fuel injection during the period of controlled combustion.

2. The fineness of atomization of the injected fuel.

3. The uniformity of distribution of the injected fuel in the combustion chamber.

4. Amount and distribution of the oxygen left in the combustion space for reaction of the injected fuel.

At the point E, injection of fuel ends, the period of controlled combustion ends at this point. When the load on the engine is greater, the period of controlled combustion is also greater.

During controlled combustion, the pressure in the cylinder may increase or remain constant or decrease. Usually during this period, the combustion is more or less at constant pressure (on a PV diagram) because the downward movement of the piston (i.e. increase in volume) compensates for the effect of heat release and the consequent pressure rise.

After burning

At the last stage, i.e. between E and F the fuel that is left in the combustion space when the fuel injection stops is burnt. This stage of combustion is called After burning (burning on the expansion stroke). In the indicator diagram, after burning will not be visible. This is because the downward movement of the piston causes the pressure to drop inspired of the heat that is released by the burning of the last portion of the charge.

Increasing excess air, or air motion will shorten after burning i.e. reduce the quantity of fuel that may undergo after burning).

THE PHENOMENON OF KNOCK IN CI ENGINES

In CI engines the injection process takes place over a definite interval of time. Consequently, as the first few droplets to be injected are passing through the ignition delay period, additional droplets are being injected into the chamber. If the ignition delay of the fuel being injected is short, the first few droplets will commence the actual burning phase in a relatively short time after injection and a relatively small amount of fuel will be accumulated in the chamber when actual burning commences.

Effect of Variables on the Delay Period

Increases in variableEffect on Delay PeriodReason

Cetane number of fuel Reduces Reduces the self-ignition temperature

Injection pressure ReducesReduces physical delay due to greater surface volume ration

Injection timing advance ReducesReduced pressures and temperatures when the injection begins

Compression ration ReducesIncreases air temperature and pressure and reduces auto-ignition temperature

Intake temperature ReducesIncreases air temperature

Jacket water temperature Reduces Increases wall and hence air temperature

Fuel temperature Reduces Increases chemical reaction due to better vaporization

Intake pressure (Supercharging)Reduces Increases density and also reduces auto-ignition temperature

Speed Increases in terms of crank angle. Reduce in terms of milliseconds Reduces loss of heat

Load (Fuel air ratio)Decreases Increase the operating temperature

Engine size Decrease in terms of crank angle. Little effect in terms of milliseconds Larger engines operator normally at low speeds

Type of combustion chamber Lower for engines with pre-combustion Due to compactness of the chamber.

CHARACTERISTICS TENDING TO REDUCE DETONATION OR KNOCK

S.NoCharacteristicsSI EnginesCI Engines

1.Ignition temperature of fuel HighLow

2.Ignition delay LongShort

3.Compression ratio LowHigh

4.Inlet temperature LowHigh

5.Inlet pressure LowHigh

6.Combustion wall temperature LowHigh

7.Speed, rpm HighLow

8.Cylinder size SmallLarge

Factors influencing diesel knock

The diesel combustion process which includes ignition delay, premixed burning due to delay period and diffusion burning and injector needle lift and pressure variation with respect to crank angle can be seen in fig. The premixed burning is responsible for diesel knock.

The following are the factors which influence ignition delay and thereby contribute to knock:

The different engine factors that control diesel knock can be seen in fig.

Fig: Diesel combustion and injector needle lift

Higher inlet air pressure, air temperature and compression ratio reduce knock. Supercharging reduces knock. Increased humidity increases knock.

Combustion chamber design and associated air motion influence heat losses from the compressed air. Tendency to knock will be lesser, with less heat losses. A combustion chamber with a minimum surface to volume ratio and with lesser intensity of air motion is desirable.

Knocking tendency is lesser in engines where compressed air injects the fuel into the combustion space. In the case of mechanical injection of fuel, finer the atomization of fuel, lesser is the tendency to knock.

A fuel with a long preflame reactions (i.e. self ignition possible only at a higher temperature) will result in the injection of a considerable amount of fuel before the initial part ignities. This in turn results in a large amount or number of parts of the mixture to ignite at the same time and produce knock. Thus, a good CI engine fuel should have a short ignition delay and low self ignition temperature, if knock is to be avoided.

Fig: Factors influencing combustion knock in the CI engine.

Ignition delay of fuels is generally measured in terms of cetane number. Fuels of higher cetane number have shorter ignition delay and thus will have a lesser tendency to knock.

The ignition delay of CI engine fuels may be decreased by the addition of small amounts of certain compounds (called ignition accelerators or improves). These compounds are ethyl nitrate and amyl thionitrate. These compounds affect the combustion process by speeding the molecular interactions.

Direct injection engines These engines have a single, open combustion chamber into which the entire quantity of fuel is injected directed directly. An open combustion chamber is one in which the combustion space incorporates no restrictions that are sufficiently small to cause large differences in pressure between different parts of the chamber during the combustion process.

Indirect injection engines In these engines the combustion space is divided into two parts and the fuel is injected into the auxiliary chamber which is connected to the main chamber via a nozzle or one or more number of orifices. The main chamber is situated above the piston. The restrictions or throat are so small to cause considerable pressure differences between them during the combustion process.

Combustion chambers for CI engines

Combustion chamber is the space wherein combustion of fuel with air takes place. In IC engines, combustion chamber is the closed space formed by three engine components, namely, cylinder head, top portion of cylinder and piston crown, when the piston is close to TDC, at the end of compression. This space is more or less equal to the clearance volume in an engine.

Functions of the combustion chambers are as follows:

1. Efficient preparation of fuel air charge for combustion. This stipulates (i) an even distribution of the injected fuel throughout the compressed air and (ii) a thorough mixing of the fuel with air to ensure complete combustion, with minimum excess air supply.

2. Efficient and smooth combustion. This stipulates (i) a sufficiently high air temperature to cause ignition of fuel, (ii) a small ignition lag or delay period, (iii) a moderate rate of pressure rise during uncontrolled combustion stage, (iv) a controlled, even burning during controlled combustion stage, (v) a minimum of after burning and (vi) minimum heat losses and energy losses to ensure high thermal efficiency.The open combustion chamber may be located either in the cylinder head or in the piston crown as shown in fig.

Fig: Types of open combustion chambers, changing the shapes of the cavity in the piston crown affects piston height and hence engine size.

It may also be partly in the cylinder head and partly in the piston crown. Presence of valves and fuel injector in the cylinder head makes it difficult to locate the combustion chamber in the cylinder head. Hence, it is usually located in the piston crown, either centrally or so. Locating the chamber in the piston crown has an advantage i.e. reduced heat losses from the working fluid.

Open combustion chamber is invariably circular in plan. This helps organized rotational air movement to prevail. The cross section is of different shapes. The chamber shape usually confirms to the shapes of the fuel spray used.

The shape of some of the open combustion chambers used in automotive diesel engines can be seen in fig. How the combustion chamber design affects weight and height of an engine can also be seen n this figure.

The fig. shows the performance results of some direct injection engines having different shapes of open combustion chambers. The torroidal shape seems to give better performance over the operating range.

Fig: Performance of D.I. engines having different open combustion chambers

In the open combustion chamber, air mass is more or less quiescent in nature. As such atomization (i.e. disintegration of the fuel jet into drops of different sizes), distribution of these drops and mixture formation (i.e. mixing of fuel with air) are to be effected by the injection system. Hence, fuel must be injected at high velocity. This means high injection pressure is required. Injector nozzle should also contain more of holes of comparatively small diameter (dor = 0.15 to 0.25 mm)

In open combustion chamber even though there may be many sprays, still the air between the sprays may be utilized fully, due to quiescent nature of the air charge. As such, in this chamber the minimum possible excess air coefficient is amin > 1.5

The fuel injector is usually arranged along the chamber axis for effective distribution of fuel spray. With a centrally located multihole injector nozzle, the design goal is to keep the amount of fuel which may impinge on the piston bowl walls to be a minimum.

In some engines, injector and combustion chamber are located away from the cylinder axis. This arrangement helps to increase the size of the valves, inlet manifold and exhaust manifold.

In the case of open combustion chambers the injection timing, rate of injection, injection pressure, engine speed, size of each fuel orifice and viscosity and ignition quality of the fuel dictate the pressure rise and completeness of combustion.

Pre-combustion chamber

In some CI engines, combustion space is divided into two parts, namely, pre-combustion chamber and main combustion chamber. Pre-combustion chamber is always located in the cylinder head. Main combustion chamber is enclosed between the piston and cylinder head. The two combustion chambers are interconnected by one or more number of orifices.

Pre-combustion chamber is built in various shapes and relative sizes. Pre-combustion chamber volume is about 30 to 40 percent of the total combustion space. The manner in which combustion of fuel in this type of engine is taking place is discussed below:

During compression, part of the air in the cylinder enters the pre-combustion chamber. At the end of compression, the whole of the fuel is injected into the pre-combustion chamber. The hot air ignitives the fuel. Combustion starts in this chamber. Pressure rises in it. Rise of pressure in the pre-combustion chamber forces out the products of combustion, partially burned and unburned fuel and remaining air into the main combustion chamber.

These constituents flow out at high velocity into the main chamber. As such, these constituents mix thoroughly with the air in the main chamber. The orifices connecting the two chambers are so sized, and shaped and located to effect good mixing. Air motion thus created is called combustion induced swirl or combustion turbulence.

In the pre-combustion chamber engine, ignition and combustion starts in the pre-combustion chamber. But the combustion of the entire quantity of the injected fuel will not be completed in this chamber itself. This is because only a smaller quantity of total air sucked in is present in this chamber. About 20 percent of the fuel injected during each cycle, burns in the pre-combustion chamber and the reminder burns in the main combustion chamber.

Merits and demerits of pre-combustion chamber- the merits and demerits of the ecombustion chamber engines are as follows:

1. There is better mixing of air and fuel due to combustion induced swirl. Air movement is one of turbulence in character. As such, lower fuel injection pressure (60 to 100 kscm) can be used. Lower injection pressure eliminates dripping of fuel from the injector tip. Lower injection pressure necessitates the use of fairly large injection orifices to deliver with carbon particles. Such injectors, therefore, require less frequent maintenance. Because of larger orifices and lower injection pressures, higher viscosity fuels can be used.

2. Brake mean effective pressures are much lower in these engines.

3. Only a fraction of the fuel is burnt in the pre-combustion chamber. Thus combustion process proceeds at a slower rate. As such, rate of pressure rise and peak pressure (seldom exceeds 90 kscm) will be lower. The engine will be very smooth running. The cycle becomes almost a constant pressure cycle.

4. During compression , at any instant, pressure and temperature of air in the pre-combustion chamber will lag behind in magnitude compared to those in the main combustion chamber. Throttling effect of orifices is responsible for this. As such, at the start of fuel injection, pressure and temperature of air in the pre-combustion chamber will be lesser. This factor increases ignition delay. Possibility of knock of knock occurring is greater, especially during cold weather and while starting.

5. Heat losses through the orifices are greater during compression. Hence, cold starting is difficult. To effect easy cold starting, electric heater or starting cartridges or higher compression ratios are used. Using higher compression ratio (usually from 16 to 19) results in a relatively heavier engine.

6. Air flows from the main chamber into the pre-combustion chamber during compression. During combustion and expansion, burning gases flow out from the pre-combustion chamber into the main combustion chamber. These fluid flow through the orifices result in higher fluid friction, and energy and heat losses. These aspects reduce power output by about 10 to 15 percent and also reduce thermal efficiency. Specific fuel consumption is more by about 10 to 12percent compared to that of a open combustion chamber.

7. Pre-combustion chamber imprisons the first combustion shock. This prevents high, knocking pressure from being applied on the piston and through the connecting rod to the engine knock on the ening ecomponents, inferior ignition quality fuels can be used.

8. Pre-combustion chambers are suitable and are being used in engines operating at relatively high speed. This becomes possible because of reduced or elimination of the ill effect of knock.

9. Scavenging the pre-combustion chamber is difficult. This causes inefficient combustion.

10. Considerable amount of fuel that is injected burns after the same entering the main combustion chamber. This combustion occurs relatively late in the expansion stroke. This aspect reduces thermal efficiency.

11. Pre-combustion chamber utilizes the energy of initial combustion for creating air movement in the main chamber. Greater will be the air movement if greater amount of fuel is burnt in the pre-combustion chamber. The amount of fuel that is burnt initially does not depend upon the speed. As such this type of combustion chamber is very much suitable for engines meant for constant speed operation.

Swirl combustion chamber

The swirl combustion chamber is also a divided type combustion chamber with certain differences and modifications. Swirl chamber is usually located in the cylinder head. In one case, it is located in the cylinder block itself, by the side of the engine cylinder.

Swirl chamber is spherical or cylindrical in shape. Volume of the swirl chamber is greater than that of the precombusiton chamber. Volume of the chamber over the piston ranges from a minimum to usually not more than half the total clearance volume. A much larger passage called transfer passage or throat connects the swirl chamber with the chamber in the cylinder. This connection passage is tangential to the swirl chamber. The figure shows the location of the swirl combustion chamber either in the cylinder head or in the cylinder block and the air motion created in them.

Fig: Different arrangements of swirl combustion chambers

During compression, air from the cylinder is forced through the throat into the swirl chamber. A tangential velocity of swirl is produced in the swirl chamber. This swirl is called compression swirl.

At the end of compression, fuel is injected into the swirl chamber. Vigorous swirl in the chamber helps the injected fuel and air to mix well. Fuel injector is so located and fuel sprays are so aimed to achieve this goal.

Ignition and combustion of fuel starts in the swirl chamber. Bulk of the injected fuel burns in the swirl chamber itself. This becomes possible because of the presence of the major portion of air in it. Combustion causes pressure rise. This pressure rise forces combustion products and air fuel mixture into the engine cylinder. Piston is also pushed outward on the working stroke. Further mixing of unburned and partially burnt fuel with air occurs and this results in efficient combustion. Hence, a swirl chamber engine uses both compression induced swirl and combustion induced swirl.

M Combustion system

Dr Meurer of MAN, Germany has developed a simple but a peculiar diesel combustion chamber based upon the following three rules:

1. The fuel must be allowed to oxidize slowly and gradually and must be heated only as vapour in the mixed state.

2. The fuel quantity undergoing auto-ignition must be minimized

3. The mixture of fuel vapour and air must be done faster as combustion proceeds and the mixture must never be richer than the stoichiometric ratio.

The M combustions chamber is located in the piston crown as shown in fig. It is open type and is somewhat shallower. It has a recess at the top just below the injector nozzle. The nozzle directs the fuel towards the combustion chamber walls, tangentially. The intake port is inclined. The intake valve has a mask. These create an air swirl about the axis of the cylinder. The direction of the swirl is in the same direction as that of the fuel jet.

The fuel particles injected at the first instance meet high resistance due to the dense hot air in the chamber. Hence, these particles get well dispersed into the hot air. The succeeding particles due to lesser air resistance get deposited on the combustion chamber walls, in the form of a thin film. At full load, the thickness of the fuel film be about 0.150 mm. The fuel dispersed into the air mass is only about 5% of the total fuel injected.

The bottom surface of the combustion chamber is cooled by the lubricating oil that is splashed continuously from the crankcase. The combustion chamber wall temperature is maintained at about 330oC.

The combustion of the 5% of the fuel which gets injected into air mass starts. It undergoes, usual droplet combustion. But the combustion of the fuel sprayed on the cooled combustion chamber walls does not follow immediately. The wall deposited fuel starts evaporating in the absence of the hot air and moves towards the center. The swirling air removes the fuel vapours from the zone evaporation.

Fig: M Combustion chamber

The fuel vapours mix with air and after slow oxidation burns. The combustion of the air fuel vapour mixture is initiated by the red hot carbon particles produced by combustion of air deposited fuel (that act like spark produced in a spark ignition engine). As combustion proceeds, the chamber temperature increases. This in turn increase the rate of vapourization and mixture formation. By this controlled evaporation and slow combustion, the fuel has little or no chance to crack resulting in diesel knock and smoky exhause. Hence, combustion is smooth and efficient in this system. A comparison of the indicator diagrams of the conventional diesel engine and the M combustion chamber diesel can also be seen in fig which will reveal this fact. Rate of pressure rise and peak pressure are lesser in the M combustion chamber engine.

Merits and demerits of M combustion system: The advantages of the M combustion system are as follows:

In the M combustion system, complete and effective burning of the fuel takes place. This controlled burning eliminates diesel knock and free carbon particles in the exhaust.

1. About 5 10 % higher power output is realized.

2. Specific fuel consumption is lesser.

3. Smooth running of the engine even during idling becomes possible which is very rare in normal diesel engines.

4. Much lower smoke density upto three fourths full load and almost identical with that of a conventional diesel engine at full load. Smoke density is the ratio of carbon present in the exhaust to the amount of carbon in the quantity of fuel injected.

5. Lesser contamination of insoluble in the lubricating oil. In the bohr test, in an ordinary diesel engine, the insolubles were about 0.9% and in the M combustion engine, the insolubles where only about 0.25%.

6. M combustion system is more adaptable for multi fuel operation because of the elimination of diesel knock.

Turbo charging

In turbo charging, the supercharger or blower is being driven by a gas turbine which uses the energy in the exhaust gases. In this case, there is no mechanical linkage between the engine and the supercharger. The major parts of a turbocharger are turbine wheel, turbine housing, turbo shaft, compressor wheel, compressor housing and bearing housing.

During engine operation, hot exhaust gases blow out through the exhaust valve opening into the exhaust manifold. The exhaust manifold and the connecting tubing route these gases into the turbine housing. As the gases pass through the turbine housing, they strike on the fins or blades on the turbine wheel. When the engine load is high enough, there is enough gas flow and this makes the turbine wheel to spin rapidly. The turbine wheel is connected to the compressor wheel by the turboshaft. As such, the compressor wheel rotates with the turbine. Compressor wheel rotation sucks air into the compressor housing. Centrifugal force throws the air outward. This causes the air to flow out of the turbocharger and into the engine cylinder under pressure.

In the case of turbocharging, there is a phenomena called turbolag. It refers to the short delay period before the boost or manifold pressure increases. This is due to the time the turbocharger assembly takes the exhaust gases to accelerate the turbine and compressor wheel to speed up.

If the supercharger is driven directly by the engine, part of the power developed by the engine will be used in running the supercharger.

Fig: Comparative heat balance of naturally aspirated and supercharged diesel engines.

If is found that the gain in the power output of an engine due to supercharging will be many time the power required to drive the supercharger. Of course, this is possible only with increased fuel supply to the engine. It is to be noted that at full loads, the compression of the supercharger is not fully utilized. This will result in greater loss. Therefore, the specific fuel consumption of a mechanically driven supercharged engine will be more at part loads when compared to that of a naturally aspirated engine.

In the case of the exhaust gas turbine driven supercharger, the engine is not required to supply any power to run the supercharger turbine. This type of supercharging is called turbo charging. The turbo charging gives about 5% higher thermal efficiency at full load. This increase in efficiency results in reduced fuel consumption compared to that of a naturally aspirated engine for the same power output.

Effects of turbocharging:

The following are the effects of supercharging engines. Some of the points refer to CI engines:

1. Higher power output

2. Mass of charge inducted is greater

3. Better atomization of fuel

4. Better mixing of fuel and air

5. Combustion is more complete and smoother

6. Can use inferior (poor ignition quality) fuels.

7. Scavenging of products is better

8. Improved torque over the whole speed range

9. Quicker acceleration (of vehicle) is possible

10. Reduction in diesel knock tendency and smoother operation

11. Increased detonation tendency in SI engines

12. Improved cold starting

13. Eliminates exhaust smoke

14. Lowers specific fuel consumption, in turbocharging

15. Increased mechanical efficiency

16. Extent of supercharging is limited by durability, reliability and fuel economy

17. Increased thermal stresses

18. Increased turbulence may increase heat losses

19. Increased gas loading

20. Valve overlap period has to be increased to about 60 to 160 degrees of crank angle

21. Necessitates better cooling of pistons and valves.

QUESTION BANK OF UNIT II

Part A

1. What are the stages of combustion in CI engines.

2. What is Ignition delay period

3. What is period of rapid combustion

4. What is contrilled combustion in CI engines.

5. What is period of after burning.

6. What are the factors that affect delay period.

7. What is knock in CI engines.

8. State different types of combustion chambers in CI engines.

9.Write a short notes on Direct injection combustion chambers.

10. What is Pre - Combustion chamber.

11.What are homogeneous and heterogeneous mixtures.

12. What is turbo charging.

13. What are the advantages of turbo charging.

14. What are the dis-advantages of turbo charging.

15. What is ignition delay period in CI engines

16. What is Uncontrolled combustion in CI engines

17. What is controlled combustion in CI engines

18. What is period of afterburning in CI engines

19. What variables affect delay period

20. What is the affect of size of droplet on delay period

21. What is the affect of compression ratio on delay period

22. List various methods to control delay period in CI engine

23. What are the methods of generating air swirl in CI engine

24. What are the advantages of induction swirl

25. What are various cold starting aids in CI engine.

Part B

1. Explain the various stages of Combustion in CI engines.

2. Explain the phenomenon of Knock in CI engines.

3. Explain various types of Combustion chambers used in CI engines with figures.

4. What is meant by delay period. Explain about the types of delay period

5. What are the three methods of generating swirl in CI engine combustion chamber

UNIT III

ENGINE EXHAUST EMISSION CONTROL

POLLUTION:

The mixing of unwanted and undesirable substances into our surroundings that cause undesirable effects on both living and non living things is known as pollution.

AIR POLLUTION:

Air pollution is defined as the addition of unwanted and undesirable things to our atmosphere that have harmful effect upon our planned life.

Major sources of Air pollution:

1. Automotive Engines

2. Electrical power generating stations

3. Industrial and domestic fuel consumption

4. Refuse burning of industrial processing, wastes etc.,

Sources of Pollutants from Gasoline Engine:

There are four possible sources of atmospheric pollution from a petrol engine powered vehicle. They are

1. Fuel Tank