Internal Combustion Engines

Internal Combustion Engines
Faculty - Er. Ashis Saxena

Index Unit 1 Unit 2 Unit 3 Unit 4 Unit 5 Introduction to I.C Engines
Fuels Unit 2 SI Engines Unit 3 CI Engines Unit 4 Engine Cooling Lubrication Supercharging Testing and Performance Unit 5 Compressors

Unit - 2 Chapter – 2 SI Engines

Carburetion What if carburetion process is absent?
In an engine, carburetion is the process of preparation of a proper ratio of oxygen with a gaseous form of a fossil fuel, like natural gas or gasoline, so it can combust. Internal combustion engines run by igniting fuel that has been sprayed into a fine vapor and mixed with air. This mixture, called an emulsion, will burn with the right amount of energy to fuel the engine. Carburetion usually involves all these stages, from vaporizing the gasoline to letting in the air and finally moving the mixture to where it can be combusted. Carburetion is responsible for allowing an engine to perform at an optimal level whether it is starting, running at full throttle, or idling. What if carburetion process is absent? If there is too much fuel or too little oxygen, the engine runs "rich" and wastes fuel, produces smoke, creates too much heat, or ruins parts of the engine. If there is too little fuel or too much air, the engine runs "lean" and might sputter, stop, or cause engine damage.

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: Engine displacement. Maximum revolution per minute (rpm) of engine. Carburetor air flow capacity. Volumetric efficiency of engine. There is a direct relationship between an engine’s air flow and it’s 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.

Air Fuel Mixtures An engine is generally operated at different loads and speeds. For this proper air-fuel mixture should be supplied to the engine cylinder. Fuel and air are mixed to form three different types of mixtures. Chemically correct mixture: Chemically correct or stoichiometric mixture is one in which there is just enough air for complete combustion of the fuel. Complete combustion means all carbon in the fuel is converted to CO2 and all hydrogen to H2O. For example, to burn one kg of octane (C8H8) completely kg of air is required. Hence chemically correct A/F ratio for octane is 15.12:1; usually approximated to 15:1. Rich mixture: A mixture which contains less air than the stoichiometric requirements is called a rich mixture (example, A/F ratio of 12:1, 10:1 etc.). Lean mixture: A mixture which contains more air than the stoichiometric requirements is called a lean mixture (example, A/F ratio of 17:1, 20:1 etc.). There is however, a limited range of A/F ratios (approximately 8:1 (rich) to 18.5:1 (lean)), in which combustion in an SI engine will occur. Outside this range, the ratio is either too rich or too lean to sustain flame propagation.

Air Fuel Mixtures

Air Fuel Mixtures (for different throttle operations)
For successful operation of the engine, the carburetor has to provide mixtures which follow the general shape of the curve ABCD (single cylinder) A'B'C'D' (multi-cylinder) in Fig. The three general ranges of throttle operation are: Idling : (mixture must be enriched) Cruising (mixture must be leaned) High Power (mixture must be enriched)

Air Fuel Ratios (power versus economy)

Idling Idling is running a vehicle's engine when the vehicle is not in motion. This commonly occurs when drivers are stopped at a red light, waiting in a fast food drive-thru lane, or waiting while parked outside a business or residence. When an engine idles, the engine runs without any loads except the engine accessories.

Should I shut off the engine when I‘m Idling my vehicle
If you're in a drive-through restaurant/business line or waiting for someone and you'll be parked and sitting for 10 seconds or longer... turn off your car's engine. Why?? For every two minutes a car is idling, it uses about the same amount of fuel it takes to go about one mile. But you're not going anywhere. Idling gets ZERO km per liter.

Myths associated with idling
Myth #1: The engine should be warmed up before driving. Reality: Idling is not an effective way to warm up your vehicle, even in cold weather. The best way to do this is to drive the vehicle. With today's modern engines, you need no more than 30 seconds of idling on winter days before driving away. Myth #2: Idling is good for your engine. Reality: Excessive idling can actually damage your engine components, including cylinders, spark plugs, and exhaust systems. Fuel is only partially combusted when idling because an engine does not operate at its peak temperature. This leads to the build up of fuel residues on cylinder walls that can damage engine components and increase fuel consumption.

Myths associated with idling
Myth #3: Shutting off and restarting your vehicle is hard on the engine and uses more gas than if you leave it running. Reality: Frequent restarting has little impact on engine components like the battery and the starter motor. Component wear caused by restarting the engine is estimated to add $10 per year to the cost of driving, money that will likely be recovered several times over in fuel savings from reduced idling. The bottom line is that more than ten seconds of idling uses more fuel than restarting the engine.

Air Fuel Mixtures (for different throttle operations) Idling range
An idling engine is one which operates at no load and with nearly closed throttle. Under idling conditions, the engine requires a rich mixture. This is due to the existing pressure conditions within the combustion chamber and the intake manifold which cause exhaust gas dilution of the fresh charge. Since, the clearance volume is constant, the mass of exhaust gas in the cylinder at the end exhaust stroke tends to remain constant throughout the idling range. The amount of fresh charge brought, in during idling, however, is much less than that during full throttle operation, due to very small opening of the throttle. This results in a much larger proportion of exhaust gas being mixed with the fresh charge under idling conditions. Further, with nearly closed throttle the pressure in the intake manifold is considerably below atmospheric due to restriction to the air flow. When the intake valve opens, the pressure differential between the combustion chamber and the intake manifold results in initial backward How of exhaust gases into the intake manifold. As the piston proceeds down on the intake stroke, these exhaust gases are drawn back into the cylinder, along with the fresh charge. As a result, the final mixture of fuel and air in the combustion chamber is diluted more by exhaust gas & the contact of fuel and air particles is obstructed resulting in poor combustion & in loss of power. Hence more fuel is required by richening the air-fuel mixture which improve the probability of contact between fuel and air particles and thus improves combustion. As the throttle is gradually opened from A to B, the pressure differential between the inlet manifold and the cylinder becomes smaller and the exhaust gas dilution of the fresh charge diminishes.

Air Fuel Mixtures (for different throttle operations) Idling range
Cruising Range: In the cruising range from B to C the exhaust gas dilution problem is relatively insignificant. The primary interest lies in obtaining the maximum fuel economy. Consequently, in this range, it is desirable that the carburetor provides the engine with the best economy mixture. Power Range: During peak power operation the engine requires a richer mixture, as indicated by the line CD for the following reasons: To provide best power: Since high power is desired, it is logical to transfer the economy settings of the cruising range to that mixture which will produce the maximum power, or a setting in the vicinity of the best power mixture, usually in the range of 12:1. To prevent overheating of exhaust valve and the area near it: At high power, the increased mass of gas at higher temperatures passing through the cylinder results in the necessity of transferring greater quantities of heat away from critical areas such as those around the exhaust valve. Enriching the mixture reduces the flame temperature and the cylinder temperature. This reduces the cooling problem and also reduces the tendency to damage exhaust valves at high power.

Principle of carburetion
Both air & gasoline are drawn through the carburetor and into the engine cylinders by the suction created by the downward movement of the piston. 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 fuel discharged from a tube. This tube has a fine orifice called carburetor jet which is exposed to the air path. 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.

Principle of carburetion Venturi tube operation
Venturi tube has a narrower path at the centre so that the flow area through which the air must pass 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 suction is proportionately increased.

Carburetor - Construction & Working
A simple carburetor mainly consists of a float chamber, fuel discharge nozzle & a metering orifice, a venturi, a throttle valve and a choke. The float & needle valve system maintains 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 & 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. During suction stroke air is drawn through the venturi. 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.

Choke Valve Fuel discharge nozzle Venturi Throttle Valve

Carburetor - Construction & Working
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 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 which 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.

Combustion in SI engine
The combustion is defined as a rapid chemical reaction between the hydrogen and carbon with oxygen in the air and liberates energy in the form of heat. The conditions necessary for combustion are the presence of combustible mixture and some means of initiating the process. The process of combustion in engines generally takes place either in a homogeneous or a heterogeneous fuel vapor-air mixture depending on the type of engine. In an SI engine Fuel and air are homogeneously mixed together in the intake system, inducted through the intake valve into the cylinder where it mixes with residual gases and is then compressed. Under normal operating conditions, combustion is initiated towards the end of the compression stroke at the spark plug by an electric discharge. A turbulent flame develops following the ignition and propagates through this premixed charge of fuel and air, and also the residual gas in the clearance volume until it reaches the combustion chamber walls.

Stages of Combustion in SI engine
A typical pressure-crank angle diagram, during the compression (a  b), combustion (b  c) and expansion (c  d) in an ideal four stroke spark-ignition engine is shown in Fig. In an ideal engine the entire pressure rise during combustion takes place at constant volume i.e., at TDC. However, in an actual engine it does not happen.

The pressure variation due to combustion in a practical engine is shown below. In this figure, A is the point of passage of spark (say 20° bTDC), B is the point at which the beginning of pressure rise can be detected (say 8° bTDC) and C the attainment of peak pressure. Thus AB represents the first stage, BC the second stage and CD the third stage.

1. Ignition lag stage 2. Flame propagation stage 3. After burning stage

Ignition Lag Sir Ricardo, known as the father of engine research, describes the combustion process in a SI engine as consisting of three stages: There is a certain time interval between instant of spark and instant where there is a noticeable rise in pressure due to combustion. This time lag is called IGNITION LAG. Ignition lag is the time interval in the process of chemical reaction during which molecules get heated up to self ignition temperature, get ignited and produce a self propagating nucleus of flame.

Ignition Lag The period of ignition lag is shown by path AB.
Ignition lag is very small and lies between to seconds. An ignition lag of seconds corresponds to 35 degree crank rotation when the engine is running at 3000 RPM. This is a chemical process depending upon the nature of fuel, temperature and pressure, proportions of exhaust gas and rate of oxidation or burning.

Flame propagation stage:
The second stage (BC) is a physical one and it is concerned with spread of the flame throughout the combustion chamber. The starting point of the second stage is where the first measurable rise of pressure is seen on the indicator diagram i.e., the point where the line of combustion departs from the compression line (point B). This can be seen from the deviation from the motoring curve.

Flame propagation stage
During the second stage the flame propagates practically at a constant velocity. Heat transfer to the cylinder wall is low, because only a small part of the burning mixture comes in contact with the cylinder wall during this period. The rate of heat-release depends largely on the turbulence intensity and also on the reaction rate which is dependent on the mixture composition. The rate of pressure rise is proportional to the rate of heat release because during this stage, the combustion chamber volume remains practically constant (since piston is near the top dead center).

After burning The starting point of the third stage is usually taken as the instant at which the maximum pressure is reached on the indicator diagram (point C). The flame velocity decreases during this stage. The rate of combustion becomes low due to lower flame velocity and reduced flame front surface. Since the expansion stroke starts before this stage of combustion, with the piston moving away from the top dead center, there can be no pressure rise during this stage.

Factors influencing the flame speed
The study of factors which affect the velocity of flame propagation is important since the flame velocity influences the rate of pressure rise in the cylinder and it is related to certain types of abnormal combustion that occur in spark-ignition engines. Turbulence: The flame speed is quite low in non-turbulent mixtures and increases with increasing turbulence. This is mainly due to the additional physical intermingling of the burning and unburned particles at the flame front which expedites reaction by increasing the rate of contact. The turbulence in the incoming mixture is generated during the admission of fuel air mixture through comparatively narrow sections of the intake pipe, valve openings etc., in the suction stroke. The increase of flame speed due to turbulence reduces the combustion duration and hence minimizes the tendency of abnormal combustion. However, excessive turbulence may extinguish the flame resulting in rough and noisy operation of the engine.

Fuel-Air Ratio: The fuel-air ratio has a very significant influence on the flame speed. The highest flame velocities (minimum time for complete combustion) are obtained with somewhat richer mixture which shows the effect of mixture strength on the rate of burning as indicated by the time taken for complete burning in a given engine. When the mixture is made leaner or richer the flame speed decreases. Less thermal energy is released in the case of Iean mixtures resulting in lower flame temperature. Very rich mixtures lead to incomplete combustion which results again in the release of less thermal energy. Temperature and Pressure: Flame speed increases with an increase in intake temperature and pressure. A higher initial pressure and temperature may help to form a better homogeneous air-vapor mixture which helps in increasing the flame speed. This is possible because of an overall inert in the density of the charge.

Compression Ratio: A higher compression ratio increases the pressure and temperature of the working mixture which reduce the initial phase 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. Increased compression ratio reduces the clearance volume and therefore increases the density of the cylinder gases during burning. This increases the peak pressure and temperature and the total combustion duration is reduced. Thus engines having higher compression ratios have higher flame speeds.

Engine Output: The cycle pressure increases when the engine output is increased. With the increased throttle opening the cylinder gets filled to a higher density. This results in increased flame speed. When the output is decreased by throttling, the initial and final compression pressures decrease and the dilution of the working mixture increases. The smooth development of self propagating nucleus of flame becomes unsteady and difficult. The main disadvantages of SI engines are the poor combustion at low loads and the necessity of mixture enrichment which causes wastage of fuel and discharge of unburnt hydrocarbon and the products of incomplete combustion like carbon monoxide etc. in the atmosphere.

Engine Speed: The flame speed increases almost linearly with engine speed since the increase in engine speed increases the turbulence inside the cylinder. The time required for the flame to traverse the combustion space would be halved, if the engine speed is doubled. Double the engine speed and hence half the original time would give the same number of crank degrees for flame propagation. The crank angle required for the flame propagation during the entire phase of combustion, will remain nearly constant at all speeds. Engine Size: The size of the engine does not have much effect on the rate of flame propagation. In large engines the time required for complete combustion is more because the flame has to travel a longer distance. This requires increased crank angle duration during the combustion. This is one of the reasons why large sized engines are designed to operate at low speeds.

Abnormal Combustion In normal combustion, the flame initiated by the spark travels across the combustion chamber in a fairly uniform manner. Under certain operating conditions the combustion deviates from its normal course leading to loss of performance and possible damage to the engine. This type of combustion can be termed as an abnormal combustion or knocking combustion. The consequences of this abnormal combustion process are the loss of power, recurring pre-ignition and mechanical damage to the engine.

Phenomenon of knock in SI engines
In a spark-ignition engine combustion which is initiated between the spark plug electrodes spreads across the combustible mixture. A definite flame front which separates the fresh mixture from the products of combustion travels from the spark plug to the other end of the combustion chamber. Heat release due to combustion increases the temperature and consequently the pressure, of the burned part of the mixture above those of the unburned mixture. In order to effect pressure equalization the burned part of the mixture will expand, and compress the unburned mixture adiabatically thereby increasing its pressure and temperature. This process continues as the flame front advances through the mixture and the temperature and pressure of the unburned mixture are increased further. If the temperature of the unburnt mixture exceeds the self-ignition temperature of the fuel and remains at or above this temperature during the period of preflame reactions (ignition lag), spontaneous ignition or auto ignition occurs at various pin-point locations. This phenomenon is called knocking. The process of auto ignition leads towards engine knock.

This phenomenon of knock may be explained by referring to Fig. (a) which shows the cross-section of the combustion chamber with flame advancing from the spark plug location A without knock whereas Fig. (c) shows the combustion process with knock. Normal Combustion In the normal combustion the flame travels across the combustion chamber from A towards D. The advancing flame front compresses the end charge BB'D farthest from the spark plug thus raising its temperature. The temperature is also increased due to heat transfer from the hot advancing flame-front. Also some preflame oxidation may take place in the end charge leading to further increase in temperature. In spite of these factors if the temperature of the end charge had not reached its self-ignition temperature, the charge would not auto ignite and the flame will advance further and consume the charge BB‘D. This is the normal combustion process which is illustrated by means of the pressure-time diagram, Fig. (b).

Abnormal Combustion However, if the end charge BB'D reaches its auto ignition temperature and remains for some length of time equal to the time of preflame reactions the charge will auto ignite, leading to knocking combustion. In Fig. (c), it is assumed that when flame has reached the position BB', the charge ahead of it has reached critical auto ignition temperature. During the preflame reaction period if the flame front could move from BB' to only CC’ then the charge ahead of CC' would auto ignite.

Because of the auto-ignition, another flame front starts traveling in the opposite direction to the main flame front. When the two flame fronts collide, a severe pressure pulse is generated. The gas in the chamber is subjected to compression and rarefaction along the pressure pulse until pressure equilibrium is restored. This disturbance can force the walls of the combustion chambers to vibrate at the same frequency as the gas. Gas vibration frequency in automobile engines is of the order of 5000 cps. The pressure-time trace of such a situation is shown in (d). It is to be noted that the onset of knocking is very much dependent on the properties of fuel. It is clear from the above description that if the unburned charge does not reach its auto-ignition temperature there will be no knocking. Further, if the initial phase i.e., ignition lag period, is longer than the time required for the flame front to burn through the unburned charge, there will be no knocking. But, if the critical temperature is reached and maintained, and the ignition lag is shorter than the time it takes for the flame front to burn through the unburned charge then the end charge will detonate. Hence, in order to avoid or inhibit detonation, a high auto-ignition temperature and a long ignition lag are the desirable qualities for SI engine fuels.

In summary, when autoignition occurs, two different types of vibration may be produced. In one case a large amount of mixture may autoignite giving rise to a very rapid increase in pressure throughout the combustion chamber and there will be a direct blow on the engine structure. The human ear can detect the resulting thudding sound and consequent noise from free vibrations of the engine parts. In the other case, large pressure differences may exist in the combustion chamber and the resulting gas vibrations can force the walls of the chamber to vibrate at the same frequency as the gas. An audible sound may be evident. The impact of knock on the engine components and structure can cause engine failure and in addition the noise from engine vibration is always objectionable. The pressure differences in the combustion chamber cause the gas to vibrate and scrub the chamber walls causing increased loss of heat to the coolant.

Effect of engine variables on knock (Density Factors)
Any factor which reduces the density of the charge tends to reduce knocking by providing lower energy release. Compression Ratio: Compression ratio of an engine is an important factor which determines both the pressure and temperature at the beginning of the combustion process. Increase in compression ratio increases the pressure and temperature of the gases at the end of the compression stroke. This decreases the ignition lag of the end gas and thereby increasing the tendency for knocking. The overall increase in the density of the charge due to higher compression ratio increases the preflame reactions in the end charge thereby increasing the knocking tendency of the engine. The increase in the knocking tendency of the engine with increasing compression ratio is the main reason for limiting the compression ratio to a lower value. Mass of Inducted Charge: A reduction in the mass of the inducted charge into the cylinder of an engine by throttling or by reducing the amount of supercharging reduces both temperature and density of the charge at the time of ignition. This decreases the tendency of knocking.

Inlet Temperature of the Mixture: Increase in the inlet temperature of the mixture makes the compression temperature higher thereby, increasing the tendency of knocking. Further, volumetric efficiency will be lowered. Hence a lower inlet temperature is always preferable to reduce knocking. It is important that the temperature should not be so low as to cause starting and vaporization problems in the engine. Temperature of the Combustion Chamber Walls: Temperature of the combustion chamber walls play a predominant role in knocking. In order to prevent knocking the hot spots in the combustion chamber should be avoided. Since, the spark plug and exhaust valve are two hottest parts in the combustion chamber, the end gas should not be compressed against them.

Retarding the Spark Timing: By retarding the spark timing from the optimized timing, i.e., having the spark closer to TDC, the peak pressures are reached farther down on the power stroke and are thus of lower magnitude. This might reduce the knocking. However, this will affect the brake torque and power output of the engine. Power Output of the Engine: A decrease in the output of the engine decreases the temperature of the cylinder and the combustion chamber walls and also the pressure of the charge thereby lowering mixture and end gas temperatures. This reduces the tendency to knock.

Effect of engine variables on knock (Time Factors)
Increasing the flame speed or increasing the duration of the ignition lag or reducing the time of exposure of the unburned mixture to auto-ignition condition will tend to reduce knocking. Turbulence: Turbulence depends on the design of the combustion chamber and on engine speed. Increasing turbulence increases the flame speed and reduces the time available for the end charge to attain auto-ignition conditions thereby decreasing the tendency to knock. Engine Speed: An increase in engine speed increases the turbulence of the mixture considerably resulting in increased flame speed, and reduces the time available for preflame reactions. Hence knocking tendency is reduced at higher speeds. Flame Travel Distance: The knocking tendency is reduced by shortening the time required for the flame front to traverse the combustion chamber. Engine size (combustion chamber size), and spark plug position are the three important factors governing the flame travel distance.

Effect of engine variables on knock (Time Factors)
Engine Size: The flame requires a longer time to travel across the combustion chamber of a larger engine. Therefore, a larger engine has a greater tendency for knocking than a smaller engine since there is more time for the end gas to auto-ignite. Hence, an SI engine is generally limited to size of about 150 mm bore. Combustion Chamber Shape: Generally, the more compact the combustion chamber is, the shorter is the flame travel and the combustion time and hence better antiknock characteristics. Therefore, the combustion chambers are made as spherical as possible to minimize the length of the flame travel for a given volume. If the turbulence in the combustion chamber is high, the combustion rate is high and consequently combustion time and knocking tendency are reduced. Hence, the combustion chamber is shaped in such a way as to promote turbulence. Location of Spark Plug: In order to have a minimum flame travel the spark plug is centrally located in the combustion chamber, resulting in minimum knocking tendency. The flame travel can also be reduced by using two or more spark plugs in case of large engines.

Effect of engine variables on knock (Composition Factors)
Once the basic design of the engine is finalized, the fuel-air ratio and properties of the fuel, particularly the octane rating, play a crucial role in controlling the knock. Fuel-Air Ratio: The flame speeds are affected by fuel-air ratio. Also the flame temperature and reaction time are different for different fuel-air ratios. Maximum flame temperature is obtained when Ø ≡ 1.1 to I.2 whereas Ø = 1 gives minimum reaction time for auto-ignition. Figure below shows the variation of knock limited compression ratio with respect to equivalence ratio for iso-octane. The maximum tendency to knock takes place for the fuel-air ratio which gives minimum reaction time.

Effect of engine variables on knock (Composition Factors)
Octane Value of the Fuel: A higher self-ignition temperature of the fuel and a low preflame reactivity would reduce the tendency of knocking. In general paraffin series of hydrocarbon have the maximum and aromatic series the minimum tendency to knock & the naphthene series comes in between the two. Usually, compounds with more compact molecular structure are less prone to knock. In aliphatic hydrocarbons, unsaturated compounds show lesser knocking tendency than saturated hydrocarbons, the exception being ethylene, acetylene and propylene.

Combustion Chamber Design
The design of the combustion chamber for an SI engine has an important influence on the engine performance and its knocking tendencies. The design involves the shape of the combustion chamber, the location of spark plug and the location of inlet and exhaust valves. Combustion chambers must be designed carefully, keeping in mind the following general objectives. Smooth Engine Operation The aim of engine design is to have a smooth operation and a good economy. These can be achieved by the following: Moderate rate of pressure rise: The rate of pressure rise can be regulated such that the greatest force is applied to the piston as closely after TDC on the power stroke as possible, with a gradual decrease in the force on the piston during the power stroke. The forces must be applied to the piston smoothly, thus limiting the rate of pressure rise as well as the position of the peak pressure with respect to TDC.

Reducing the Possibility of Knocking: Reduction in the possibility of knocking, in an engine can be achieved by, Reducing the distance of the flame travel by centrally locating the spark plug and also by avoiding pockets of stagnant charge. Satisfactory cooling of the spark plug and of exhaust valve area which are the source of hot spots in the majority of the combustion chambers. Reducing the temperature of the last portion of the charge, through application of a high surface to volume ratio in that part where the last portion of the charge burns. Heat transfer to the combustion chamber walls can be increased by using high surface to volume ratio thereby reducing the temperature.

High Power Output and Thermal Efficiency The main objective of the design and development of an engine is to obtain high power as well as high thermal efficiency. This can be achieved by considering the following factors: A high degree of turbulence is needed to achieve a high flame front velocity. Turbulence is induced by inlet flow configuration or squish. Squish can be induced in spark-ignition engines by having a bowl in piston or with a dome shaped cylinder head. Squish is the rapid radial movement of the gas trapped in between the piston and the cylinder head into the bowl or the dome. High volumetric efficiency, i.e., more charge during the suction stroke, results in an increased power output. This can be achieved by providing ample clearance around the valve heads, large diameter valves and straight passages with minimum pressure drop. Any design of the combustion chamber that improves its antiknock characteristics permits the use of a higher compression ratio resulting in increased output and efficiency. A compact combustion chamber reduces heat loss during combustion and increases the thermal efficiency.

Types of Combustion chambers
Different types of combustion chambers have been developed over a period of time. T-Head Type: This was first introduced by Ford Motor Corporation in This design has following disadvantages. Requires two cam shafts (for actuating the in-let valve and exhaust valve separately) by two cams mounted on the two cam shafts. Very prone to detonation. There was violent detonation even at a compression ratio of 4. This is because the average octane number in 1908 was about

L-Head Type L-Head Type:
This was first introduced by Ford motor in and was quite popular for some time. A modification of the T-head type of combustion chamber is the L-head type which provides the two valves on the same side of the cylinder and the valves are operated by a single camshaft. This design has an advantage both from manufacturing and maintenance point of view.

L-Head Type

L-Head Type Advantages
Valve mechanism is simple and easy to lubricate. Detachable head easy to remove for cleaning and decarburizing without disturbing either the valve gear or main pipe work. Valves of larger sizes can be provided. Disadvantages Lack of turbulence as the air had to take two right angle turns to enter the cylinder and in doing so much initial velocity is lost. Extremely prone to detonation due to large flame length and slow combustion due to lack of turbulence. More surface-to-volume ratio and therefore more heat loss. Extremely sensitive to ignition timing due to slow combustion process. Valve size restricted. Thermal failure in cylinder block also. In I-head engine the thermal failure is confined to cylinder head only

Ricardo’s Turbulent Head-Side Valve Combustion Chamber
Ricardo developed this head in 1919. His main objective was to obtain fast flame speed and reduce knock in L design. In Ricardo’s design the main body of combustion chamber was concentrated over the valves, leaving slightly restricted passage communicating with cylinder.

Advantages Additional turbulence during compression stroke is possible as gases are forced back through the passage. By varying throat area of passage designed degree of additional turbulence is possible. This design ensures a more homogeneous mixture by scoring away the layer of stagnant gas clinging to chamber wall. Both the above factors increase the flame speed and thus the performance. Deign make engine relatively insensitive to timing of spark due to fast combustion Higher engine speed is possible due to increased turbulence Ricardo’s design reduced the tendency to knock by shortening length of effective flame travel by bringing that portion of head which lay over the further side of piston into as close a contact as possible with piston crown. This design reduces length of flame travel by placing the spark plug in the centre of effective combustion space.

Disadvantages With compression ratio of 6, normal speed of burning increases and turbulent head tends to become over turbulent and rate of pressure rise becomes too rapid leads to rough running and high heat losses. To overcome the above problem, Ricardo decreased the areas of passage at the expense of reducing the clearance volume and restricting the size of valves. This reduced breathing capacity of engine, therefore these types of chambers are not suitable for engine with high compression ratio.

Over head valve or I head combustion chamber
The disappearance of the side valve or L-head design was inevitable at high compression ratio of 8 : 1 because of the lack of space in the combustion chamber to accommodate the valves. Diesel engines, with high compression ratios, invariably used overhead valve design. Since 1950 or so mostly overhead valve combustion chambers are used.

This type of combustion chamber has both the inlet valve and the exhaust valve located in the cylinder head. An overhead engine is superior to side valve engine at high compression ratios.

Advantages Lower pumping losses and higher volumetric efficiency from better breathing of the engine from larger valves or valve lifts and more direct passageways. Less distance for the flame to travel and therefore greater freedom from knock, or in other words, lower octane requirements. Less force on the head bolts and therefore less possibility of leakage (of compression gases or jacket water). The projected area of a side valve combustion chamber is inevitably greater than that of an overhead valve chamber. Removal of the hot exhaust valve from the block to the head, thus confining heat failures to the head. Absence of exhaust valve from block also results in more uniform cooling of cylinder and piston. Lower surface-volume ratio and, therefore, less heat loss and less air pollution. Easier to cast and hence lower casting cost.

F- Head combustion chamber
In such a combustion chamber one valve is in head and other in the block. This design is a compromise between L-head and I-head combustion chambers. One of the most F-head engines (wedge type) is the one used by the Rover Company for several years. Another successful design of this type of chamber is that used in Willeys jeeps.

Advantages High volumetric efficiency Maximum compression ratio for fuel of given octane rating High thermal efficiency It can operate on leaner air-fuel ratios without misfiring. Disadvantages This design is the complex mechanism for operation of valves and expensive special shaped piston.

Ignition In SI engines, as compression ratio is lower, and the self-ignition temperature of gasoline is higher, for igniting the mixture for the initiation of combustion an ignition system is a must. The electrical discharge produced between the two electrodes of a spark plug by the ignition system starts the combustion process in a spark-ignition engine. This takes place close to the end of the compression stroke. The high temperature plasma kernel created by the spark, develops into a self sustaining and propagating flame front. In this thin reaction sheet certain exothermic chemical reactions occur. The function of the ignition system is to initiate this flame propagation process. It must be noted that the spark is to be produced in a repeatable manner viz., cycle-by-cycle, over the range of load and speed of the engine at the appropriate moment in the engine cycle.

Ignition By implication, ignition is merely a prerequisite for combustion. Therefore, the study of ignition is a must to understand the phenomenon of combustion so that a criterion may be established to decide whether ignition has occurred.

Ignition System In principle, a conventional ignition system should provide sufficiently large voltage across the spark plug electrodes to effect the spark discharge. Further, it should supply the required energy for the spark to ignite the combustible mixture adjacent to the plug electrodes under all operating conditions. As air is a poor conductor of electricity an air gap in an electric circuit acts as a high resistance. But when a high voltage is applied across the electrodes of a spark plug it produces a spark across the gap. When such a spark is produced to ignite a homogeneous air-fuel mixture in the combustion chamber of an engine it is called the spark-ignition system. The ignition systems are classified depending upon how the primary energy for operating the circuit is made available as: Battery ignition systems Magneto ignition systems

Requirements of an Ignition System
It should provide a good spark between the electrodes of the plugs at the correct timing. It should function efficiently over the entire range of engine speed. It should be light, effective and reliable in service. It should be compact and easy to maintain. It should be cheap and convenient to handle. The interference from the high voltage source should not affect the functioning of the radio and television receivers inside an automobile.

Battery Ignition System
Most of the modern spark-ignition engines use battery ignition system. In this system, the energy required for producing spark is obtained from a 6 or 12 volt battery. The construction of a battery ignition system is extremely varied. It depends on the type of ignition energy storage as well as on the ignition performance which is required by the particular engine. The reason for this is that an ignition system is not an autonomous machine, that is, it does not operate completely by itself, but instead it is but one part of the internal combustion engine, the heart of the engine. It is therefore extremely important that the ignition system be matched sufficiently well to its engine. Passenger cars, light trucks, some motorcycles and large stationary engines are fitted with battery ignition systems.

Battery Ignition System
The essential components of the system are-: Battery Ignition switch Ballast resistor Ignition coil Contact breaker Capacitor Distributor Spark plug

1. Battery, 2. Ignition and starting switch, 3. Ignition Coil, 4
1. Battery, 2. Ignition and starting switch, 3. Ignition Coil, 4. Ignition distributor, 5. Ignition capacitor, 6. Contact breaker, 7. Spark plugs, and Rv ballast resistor for boosting the starting voltage.

Parts of a Battery Ignition System
To provide electrical energy for ignition, a storage battery is used. It is charged by a dynamo driven by the engine. Owing to the electro-chemical reactions, it is able to convert the chemical energy into electrical energy. It must be mechanically strong to withstand the strains to which it is constantly subjected to. Two types of batteries are used for spark-ignition engines, the lead acid battery and the alkaline battery. The former is used in light-duty commercial vehicles and the later on heavy duty commercial vehicles. Ignition Switch Battery is connected to the primary winding of the ignition coil through an ignition switch and ballast resistor. With the help of the ignition switch the Ignition system can be turned on or off.

Parts of a Battery Ignition System
Ballast Resistor A ballast resistor is provided in series with the primary winding to regulate the primary current. The object of this is to prevent injury to the spark coil by overheating if the engine should be operated for a long time at low speed, or should be stalled with the breaker in the closed position. This coil is made of iron wire, and iron has the property that its electrical resistance increases very rapidly if a certain temperature is exceeded. The coil is therefore made of wire of such size that if the primary current flows nearly continuously, the ballast coil reaches a temperature above that where this rapid increase in resistance occurs. This additional resistance in the primary circuit holds the primary current down to a safe value. For starting from cold this resistor is by-passed to allow more current to flow in the primary circuit.

Ignition Coil Ignition coil is the source of ignition energy in the conventional ignition system. The purpose of the ignition coil is to step up the 6 or 12 volts of the battery to a high voltage sufficient to induce an electric spark across the electrodes of the spark plug. The ignition coil consists of a magnetic core of soft iron wire or sheet and two insulated conducting coils, called primary and the secondary winding. The secondary coil consists of about 21,000 turns of gauge enameled copper wire sufficiently insulated to withstand the high voltage. The primary winding, located outside the secondary coil is generally formed of turns of 20 gauge wire to produce a resistance of about 1.15 . More heat is generated in the primary than in the secondary and with the primary coil wound over the secondary coil, it is easier to dissipate the heat. Current flows from the battery through the primary winding of the coil.

Ignition Coil

Contact Breaker A contact breaker is an electrical device generally used in the ignition system of an internal combustion engine. Contact breakers are sometimes referred to as "points" and are typically used to temporarily interrupt the electrical current passing through primary winding of an ignition coil. When the points are closed the current flows and when they are open, the circuit is broken and the flow of current stops. An eight cylinder engine running at 3000 rpm requires sparks per minute, i.e. 200 sparks per second. This device is often utilized in combination with an electrical capacitor and is usually located in the distributor component of an engine. Contact breakers generally require frequent readjustments to perform properly, and their use has declined in recent years. Most modern internal combustion engines utilize an electronic ignition system that does not contain a contact breaker.

Distributor The distributor handles several jobs. Its first job is to distribute the ignition surges to the individual spark plugs in the correct sequence and at the correct instants of time. Depending on whether a particular engine has 4, 6 or 8 cylinders there are 4, 6 or 8 ignition pulses (surges) generated for every rotation of the distributor shaft. This is done by the cap and rotor. The coil is connected to the rotor, which spins inside the cap. The rotor spins past a series of contacts, one contact per cylinder. As the tip of the rotor passes each contact, a high-voltage pulse comes from the coil. The pulse arcs across the small gap between the rotor and the contact (they don't actually touch) and then continues down the spark-plug wire to the spark plug on the appropriate cylinder.

Distributor The contact breaker and the spark advance mechanism are combined with the distributor in a single unit because the absolute necessity that the distributor operate in synchronism with the crankshaft. This section does the job of breaking the current to the coil. The ground side of the coil is connected to the breaker points. A cam in the center of the distributor pushes a lever connected to one of the points. Whenever the cam pushes the lever, it opens the points. This causes the coil to suddenly lose its ground, generating a high-voltage pulse. Spark timing is so critical to an engine's performance that most cars don't use points. Instead, they use a sensor that tells the engine control unit (ECU) the exact position of the pistons. The engine computer then controls a transistor that opens and closes the current to the coil.

Distributor

Spark Plug: Construction
The spark plug provides the two electrodes with a proper gap across which the high potential discharges to generate a spark and ignite the combustible mixture within the combustion chamber. A spark plug essentially consists of a steel shell, an insulator and two electrodes. The central electrode to which the high tension supply from the ignition coil is connected, is well insulated with porcelain or other ceramic materials. The other electrode is welded to the steel shell of the plug and thereby is automatically grounded when the plug is installed on the cylinder head of the engine. The electrodes are usually made of high nickel alloy to withstand the severe erosion and corrosion to which they are subjected in use.

Spark Plug: Working Spark Plug forces electricity to arc across a gap.
The electricity must be at a very high voltage in order to travel across the gap and create a good spark. Voltage at the spark plug can be anywhere from 40,000 to 100,000 volts. The spark plug must have an insulated passageway for this high voltage to travel down to the electrode, where it can jump the gap and, from there, be conducted into the engine block and grounded. The plug also has to withstand the extreme heat and pressure inside the cylinder, and must be designed so that deposits from fuel additives do not build up on the plug.

IGNITION SYSTEM – Spark Plug
Cap Connector Ceramic Body Hexagon Outer Casing Copper Sealing Gasket Gap Securing Thread Outer Electrode IGNITION SYSTEM – Spark Plug

IGNITION SYSTEM – Spark Plug
Cap Connector Ceramic Body Seal Hexagon Centre Electrode Outer Casing Change Spark Plugs at specified times Copper Sealing Gasket Securing Thread Make sure the correct Spark Plug is fitted These surfaces must be kept clean Outer Electrode IGNITION SYSTEM – Spark Plug

Why do Spark Plugs have ceramic body?
Spark plugs use a ceramic insert to isolate the high voltage at the electrode, ensuring that the spark happens at the tip of the electrode and not anywhere else on the plug; This insert does double-duty by helping to burn off deposits. Ceramic is a fairly poor heat conductor, so the material gets quite hot during operation. This heat helps to burn off deposits from the electrode.

Hot Plugs & Cold Plugs Hot plug is designed with a ceramic insert that has a smaller contact area with the metal part of the plug. This reduces the heat transfer from the ceramic, making it run hotter and thus burn away more deposits. Cold plugs are designed with more contact area, so they run cooler.

Battery Ignition System - Working
The source of the ignition energy in the battery ignition system is the ignition coil. This coil stores the energy in its magnetic field and delivers it at the instant of ignition (firing point) in the form of a surge of high voltage current (ignition pulse) through the high tension ignition cables to the correct spark plug. When the ignition switch is closed, the primary winding of the coil is connected to the positive terminal post of the storage battery. If the primary circuit is closed through the breaker contacts, a current flows, the so called primary current. This current, flowing through the primary coil produces a magnetic field in the core. A cam driven by the engine shaft, is arranged to open the breaker points whenever an ignition discharge is required. When the breaker points open, the current which had been flowing through the points now flows into the condenser, which is connected across the points. As the condenser becomes charged, the primary current falls and the magnetic field collapses. The collapse of the field induces a voltage in the primary winding, which charges the condenser to a volt much higher than battery voltage. The condenser then discharges into the battery, reversing the direction of both the primary current and the magnetic field. The rapid collapse and reversal of the magnetic field in the core induce a very high voltage in the secondary winding of the ignition coil. The high secondary voltage is led to the proper spark plug by means of a rotating switch called the distributor, which is located in the secondary or high tension circuit the ignition system.

Magneto Ignition System
Magneto is a special type of ignition system with its own electric generator to provide the necessary energy for the system. It is mounted on the engine and replaces all the components of the coil ignition system except the spark plug. A magneto when rotated by the engine is capable of producing a very high voltage and does not need a battery as a source of external energy. The working principle of the magneto ignition system is exactly the same as that of the coil ignition system. With the help of a cam, the primary circuit flux is changed and a high voltage is produced in the secondary circuit. Since the cranking speed at start is low the current generated by the magneto is quite small. As the engine speed increases the flow of current also increases. Thus, with magneto there is always a starting difficulty and sometimes a separate battery is needed for starting. The magneto is best at high speeds and therefore is widely used for sports and racing cars, aircraft engines etc. In comparison, the battery ignition system is more expensive but highly reliable. Because of the poor starting characteristics of the magneto system invariably the battery ignition system is preferred to the magneto system in automobile engines. However, in two wheelers magneto ignition system is favored due to light weight and less maintenance.

IGNITION SYSTEM IGNITION SYSTEM
Magneto Operation Secondary Windings Coil Primary Windings Soft Iron Core Engine Driven Rotor and Magnets IGNITION SYSTEM IGNITION SYSTEM

IGNITION SYSTEM – Faraday Law
Principal - S N S N S N S N S N S N S N S N Coil Windings Volt Meter Each time the magnetic field ‘washes’ through wires An electrical current is produced Called an EMF – Electro Motive Force IGNITION SYSTEM – Faraday Law

IGNITION SYSTEM IGNITION SYSTEM
Magneto Operation EMF produced in the coil windings The magnetic field passes through soft iron core IGNITION SYSTEM IGNITION SYSTEM

IGNITION SYSTEM No EMF produced in the coil windings
No magnetic field in soft iron core IGNITION SYSTEM

IGNITION SYSTEM EMF produced in the coil windings again
The magnetic field passes through soft iron core again IGNITION SYSTEM

IGNITION SYSTEM – Magneto System
Magneto Unit Rotor Arm Spark Generation Condenser Power Generation Coil Contact Breaker Distribution Magneto Ignition Switch IGNITION SYSTEM – Magneto System

Firing Order The order in which various cylinders of a multi-cylinder engine fire is called the firing order. There are three factors which must be considered before deciding the optimum firing order of an engine. Engine Vibrations Engine Cooling Development of Back Pressure

Firing Order – Engine Vibrations
Consider Cylinder -1 fires first. Pressure, p, generated in cylinder -1 will give rise to force equal to pA[b/(a+b)] & pA[a/(a+b)] on the two bearings A & B respectively. Load on bearing A would be much more than bearing B. If the next cylinder fired is cylinder -2, this imbalance in load on the two bearings would aggregate the problem of balancing of the crankshaft vibrations. Firing cylinder -3 after Cylinder -1 the load may be more or less evenly distributed.

Firing Order – Engine Cooling
On firing Cylinder -1 its temperature increases. If next cylinder fired is no. 2 , the portion of the engine between the cylinder no. 1 & 2 gets overheated. If then the 3rd cylinder is fired the overheating is shifted between cylinder no. 2 & 4. The task of cooling system becomes very difficult because it is then, required to cool more at one place than at other places and this imposes great strain on the cooling system. If the third cylinder is fired after the first the overheating problem can be controlled to a greater extent.

Firing Order – Back Pressure Development
Development of Back Pressure After firing the first cylinder, exhaust gases flow out to the exhaust pipe. If the next cylinder fired is cylinder no. 2, we find that before the gases exhausted by the first cylinder go out of the exhaust pipe the gases exhausted from the second cylinder try to overtake them. This require exhaust pipe to be bigger. Otherwise the back pressure in it would increase and the possibility of back flow would arise. It instead of firing cylinder no 2 cylinder no 3 is fired then by the time the gases exhausted by the cylinder 3 come into the exhaust pipe, the gases from cylinder 1 would have sufficient time to travel the distance between cylinder 1 and cylinder 3 and thus the development of a high back pressure is avoided.

Firing Order – 4 & 6 cylinder engines
For a four cylinder engine the possible firing orders are: or For a six cylinder engine the possible firing orders are: or or or

Ignition Timing How early or late the spark plug fires in relation to the position of the engine piston. Ignition timing must change with the changes in engine speed, load, and temperature.

Ignition Timing Timing Advance occurs when the plug fires sooner on compression stroke (High engine speed) Timing Retard occurs when plug fires later on compression stroke (Lower engine speed) BASE TIMING Timing without vacuum or computer control.

Methods of controlling Timing
Distributor Centrifugal Advance Controlled by engine speed. Consists of two weights and two springs. At high speeds the weights fly out(held by the springs), rotating the cam, hence advancing the timing.

Vacuum Advance Controlled by engine intake manifold vacuum and engine load. The vacuum diaphragm rotates the pickup coil against the direction of distributor shaft rotation.

Electronic Advance Sensors input influences the ignition timing. Crank shaft Position Sensor (RPM) Cam Position Sensor (tells which cylinder is on compression stroke) Manifold Absolute Pressure (MAP) (engine vacuum and load)

Electronic Advance Sensors input influences the ignition timing. Intake Air Temperature Sensor Knock Sensor (Retards timing when pinging or knocking is sensed) Throttle Position Sensor(TPS) Engine coolant Temperature

Internal Combustion Engines

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