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Both air and gasoline are drawn through the carburettorand into the engine cylinders by the suction created by the downward movement of the piston. In the carburettor, air passing into the combustion chamber picks up fuel discharged from a tube. This tube has a fine orifice called carburettor jet which 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 venturiand on the area of the outlet of the tube. In order that the fuel drawn from the nozzle maybe 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 carburettor 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 as shown in figure to minimise the throttling losses. The end of the fuel jet is located at the venturi or throat of the carburettor. The spray of gasoline from the nozzle and the air entering through the venturi tube aremixed together in this region and 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 the rate of evaporation of fuel. There are three general types of carburettor depending on the direction of flow of air. The first is the updraught type shown in fig in which the air enters at the bottom and leaves at the top so that the direction of its floor is upwards. The disadvantage of the up-draught carburettor is that it must lift the sprayed fuel droplet by air friction. Hence it must be designed for the relatively small mixing tube and throat so that even at low engine speed, the air velocity is sufficient to lift and carry the fuel particles along. Otherwise, the fuel droplets tend to separate out providing only a lean mixture to engine. On the other hand, the mixing tube is finite and small, then it cannot supply mixture to the engine at a sufficiently rapid rate at high speeds. In order to overcome this drawback, the downdraught carburettor fig. is adopted. It is placed at a level higher than the inlet manifold and in which the air and mixture generally follow a downward course. Here, the fuel does not have to be lifted by air friction as in the updraughtcarburettors but move into the cylinders by gravity even if the air velocity is low.Hence, the mixing tube and throat can be made large which makes high engine speeds and high specific outputs possible. A crossdraughtcarburettor consists of a horizontal mixing tube with a float chamber on one side of it, fig. By using a cross draught carburettor in engines, one right angled turn in the inlet passage is eliminated and the resistance to flow is reduced. In the constant chokecarburettor, the air and fuel flow areas are always maintained to be constant. But the pressure difference or depression which causes the flow of fuel and air are being varied as per the demand on the engine. Solex and Zenith carburettors belong to this class. In the constant vacuum carburettor, air and fuel flow areas are being varied as per the demand on the engine, while the vacuum is maintained to be always same. The SU and Carter carburettor belong to this class. Multiple venturi system uses double or triple fig. venturi. The boost venturi is located concentrically within the main venturi. The discharge edge of the boost venturi is located at the throat of the main venturi. The boost venturi is positioned upstream of the throat of the larger main venturi. Only a fraction of the total air flows through the boost venturi. A single barrel carburettor has only one barrel whereas a dualcarburettor has two barrels. Each of these two barrels in a dual carburettor contains a fuel jet, a venturi tube, an idling system, a choke and a throttle. The float chamber and the accelerating pump are common to both the barrels. Carburettor with 2 outlets connected totwo intake manifolds are known as two barrel or two throat carburettors. Such a unit is basically 1 with 2carburettors.
The figure shows the details of a simple carburettor. The simple carburettor mainly consists of a float chamber, fuel discharge nozzle and a metering orifice, a venturi, a throttle valve and a choke. The float and needle valve system maintain a constant fuel 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 wall 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 venturi. During suction, stroke air is drawn through the venturi. As the air passes through the venturi, the velocity increases reaching a maximum at the venturithroat. 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 carburettor 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. A simple carburettor of the type described above suffers from a fundamental drawback in that it provides the required A/F ratio only at 1 throttle position. At the otherthrottle position, the mixture is either leaner or richer depending on whether the throttle is opened less or more.
Modern gasoline injection systems useengine sensors, a computer and solenoid operated fuel injectors to meter and inject the right amount of fuel into the engine cylinders. This system called electronic fuel injection (EFI) use electrical and electronic devices to monitor and control engine operation. An electronic control unit (ECU) on the computer receives electrical signals in the form of current or voltage from various sensors. It then uses the stored data to operate the injectors, ignition system and other engine related devices. As a result, less unburnt fuel leaves the engine as emissions and the vehicles give better mileage. The fuel injector in an EFIis nothing but a fuel valve. When it is not energized, spring pressure makes the injector to remain closed and the fuel will enter the engine. When the computer sends the signal through the injector coil, the magnetic field attracts the injector armature. Fuel then spurtsinto the intake manifold. The injector pulse width is an indication of the period for which each injector is energized and kept open. The computer decides and controls the injector pulse width based on the signals received from the various sensors. Under full load, the computer will sense a wide-openthrottle, high intake manifold pressure and high inlet air flow. The ECU will then increase the injection pulse width to enrich the mixture which will enable the engine to produce high power. Under low load and idling conditions, the ECU will shorter the pulse width by which the injectors are kept in the closed position over a longer period of time. Because of this, the air fuel mixture will become leaner and will result in better fuel economy. Electronic fuel injection system has a cold start injector too. This is an extra injector that sprays fuel into the centre of the engine intake manifold, when the engine is cold. It serves the same purpose as the carburet orchoke. The cold start injector ensures easy engine start-up in very cold weather. Improvement in the volumetric efficiency. Atomization of fuel is independent of cranking speed and therefore starting will be easier. Better atomization and vaporization will make the engine less knock prone. Formation of ice on the throttle plate is eliminated. Distribution of fuel being independent of vaporization, less volatile fuel can be used. Variation of air fuel ratio is almost negligible even when the vehicle takes different positions like turning, moving or gradients, uneven roads etc. Position of the injection unit is not so critical and thereby the height of the engine (and hood) can be less. High maintenance cost. Difficulty in servicing and Possibility of malfunction of some sensors.
The combustion process in SI engine as consisting of three stages: The pressure variation due to combustion in a practical engine is shown in fig. 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 and BC second stage and CD third stage. The first stage(A-B) is referred to as the ignition lag or preparation phase in which growth and development of a self-propagating nucleus of flame takes place. This is a chemical process depending upon both temperature and pressure, the nature of the fuel and the proportion of the exhaust residual gas. Further, it also depends upon the relationship between the temperature and rate of reaction. The second stage (B-C) is a physical one and it is concerned with the spread of flame throughout the combustion. The starting point of the second stage is where the first measurable rise of pressure is seen on the indicator diagram. 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 starting point of the third stage is usually taken as the instant at which the maximum pressure is reached on the indicator diagrams (point C). The flame’s 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 centre, there can be no pressure rise during this stage.
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 may be termed as an abnormal combustion or knocking combustion.
In an S.I. engine, once the spark is supplied at the spark plug and Flame nucleus is formed, the flame travels across the combustion chamber in an orderly manner. The pressure keeps on increasing continuously and the uniformly throughout the combustion chamber. The peak pressures are normally attained when the flame reaches far side of the combustion chamber. The flame originating at point A travels across the combustion chamber up to point E. The end part of the gas is represented by DED. This end gas has experienced rapid compression during the compression stroke and later on by advancing flame front. Therefore, the pressure temperature and density of end part of the gas is high. If the combustion process is normal, the pressure of end gas will almost be equal to peak pressure when the flame front is about to reach to this end gas. If the ignition lag or ignition delay of the unburnt end gas is consumed before the flame front reaches it, the end part of the charge will auto ignite because it is at much higher temperature than its self-ignition temperature. This auto ignition of end gas is completed in almost negligible time and causes a violent pressure rise due to extremely high rate of Liberation of chemical energy and almost at constant volume. This rise in pressure of end gas is almost three to four times the anticipated peak pressure with normal combustion. This large pressure differential caused by the auto ignition of end gas results into severe pressure waves travelling across the combustion chamber at very fast speed. The pressure wave is reflected back and forth several times by the cylinder walls and sets the engine parts vibrating, giving rise to a pinging or ringing sound and the detonation is said to occur. It has been mentioned earlier that about 10% rich mixtures have the minimum delay period and the flame speeds are high. But it is observed that the effect of slightly rich mixtures on delay period is more dominant compared to flame speeds due to which the detonation tendency increases. Similar engines of various sizes have the delay period nearly the same. However, in case of larger sized engines, the flame has to travel longer distance of combustion space compared to smaller sized engines. Therefore, the larger engines have more tendency to detonate compared to smaller engines. In general, more the compact combustion chamber, shorter will be flame travel and combustion time, hence it will give better antiknock characteristics. Also, if the combustion chamber design is such that it promotes turbulence, then the flame speed will increase which would reduce the tendency to detonate. In case The Spark plug is located centrally in the combustion chamber, it reduces the length of flame travel and reduces the tendency to detonate. The flame travel can be reduced by using two spark plugs. The fuel with lower self-ignition temperature or with its greater pre flame reactions will have more tendency to detonate. Fuel of paraffin series have maximum tendency to detonate and of aromatic series have minimum tendency to detonate. Naphthalene series fuels come in between the two.
Resistance of knocking is an extremely important characteristic of fuel for Spark ignition engines. These fuels differ widely in their ability to resist knock depending on their chemical composition. A satisfactory rating method for comparing the antiknock qualities of the various fuels has been established. In addition to the chemical characteristics of hydrocarbons in the fuel, other operating parameters such as fuel air ratio, ignition timing, dilution, engine speed, shape of the combustion chamber ambient conditions, compression ratio, etc affect the tendency to knock in the engine cylinder. Therefore, in order to determine the knock resistance characteristic of the fuel, the engine and its operation variables must be fixed at Standard values. According to a standard practice, the antiknock valve of SI engine fuel is determined by comparing its antiknock property with a mixture of two reference fuels. Iso-octane 
and normal heptane 
. Iso- Octane chemically being a very good antiknock fuel arbitrarily assigned a rating of 100 octane number. Normal heptane 
on the other hand has very poor antiknock qualities and is given a rating of zero Octane number. The Octane number fuel is defined as the percentage, volume of the isooctane in a mixture of isooctane and normal heptane which exactly matches the knocking on the fuel in a standard engine under a set of standard operating conditions. The addition of certain compound (e.g., Tetraethyl lead) to iso-octane produces fuels of greater antiknock quality. The antiknock effectiveness of the tetraethyl lead for same quantity of lead added decreases as the total content of the lead in fuel increases. Furthermore, each Octane number at the higher range of the Octane scale will produce greater antiknock effect compared to the same unit at the lower end of the scale. Because of this nonlinear variation, a new scale was derived which expresses the approximate relative engine performance and the units of this scale are known as the performance numbers, PN. Octane numbers, ON above 100 can be computed by ON (>100) =100 + 
Where A is TEL in ml/gal of fuel or from the performance number, PN. Octane Number = 100 + 
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 plugs and the location inlet and exhaust valves. The important requirements of an SI engine combustion chamber are to provide high power output with minimum Octane requirement, high thermal efficiency and smooth engine operation. The T-head combustion chambers fig. were used in early stage of engine development. Since the distance across the combustion chamber is very long, knocking tendency is high in this type of engines. This configuration provides two values on either side of the cylinder requiring two camshafts. From the manufacturing point of view, providing 2 camshafts is a disadvantage. 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. Figures (b) and (c) show two types of this side valve engines. In these types, it is easy to lubricate the valve mechanism. With the detachable head, it may be noted that the cylinder head can be removed without disturbing valve gear etc. In figure (b), the airflow has to take two right angle turns to enter the cylinder. This causes a loss of velocity head and the loss in turbulence level resulting in a slow combustion process. The main objectives of the Ricardo's turbulent head design fig. (c) are to obtain fast flame speed and reduced knock. The main body of the combustion chamber is concentrated over the valves leaving a slightly restricted passage communicating with the cylinder, thereby creating additional turbulence during the compression stroke. The I-head type is also called the overhead valve combustion chamber in which both the valves are located on the cylinder head. The overhead valve engine fig. is superior to a side valve or a L head engine at high compression ratios. Less surface to volume ratio and therefore less heat loss. Less flame travel length and has greater freedom from knock. Higher volumetric efficiency from larger valves or valve lifts. The F-head type of valve arrangement is a compromise between L-head and I-head types. Combustion Chambers in which one valve is in the cylinder head and the other in the cylinder block are known as F head combustion Chambers fig. Modern F-head engines have exhaust valve in the head and inlet valve in the cylinder block. The main disadvantage of this type is that the inlet valve and the exhaust valve are separately activated by two cams mounted on two camshafts driven by the crankshaft through gears.
Now the plunger is pushed up by the cam movement and both the parts are closed. On further movements of the plunger, the fuel above it is compressed which lifts the delivery valve and the fuel through it goes to the injector. The plunger rises up still further, and at a particular moment, the helical groove connects the spill port, through the rectangular groove to the fuel in the upper part of the plunger. Consequently, there is a sudden pressure drop due to which the delivery valve falls back on its seat under the spring force. The pressure in the delivery pipe also drops. Thus, the discharged from the nozzle of the injector is cut off suddenly. The cycle is repeated again and again. During each stroke of the plunger, the duration of the delivery is more or less according to as the spill port is made to communicate earlier or later, the high-pressure fuel in the upper part of the barrel. This depends upon the position of the helical groove which can be changed by rotating the plunger by the rack. The rack is connected to the accelerator. It meshes with a geared quadrant. The motion of the rack rotates the gear quadrant which ultimately rotates the plunger. The driver simply operates the accelerator which controls the fuel supply to the engine cylinder.
A fuel injector is used to inject the fuel in the cylinder in atomized form and in proper quantity. Fuel injector is the main component of fuel injection. It is a spray delivery device. Pintle Bosch disc injector Lucas disc injector Ball pintle bosch disc ball Lucas disc
After the injection of the fuel in combustion chamber, fuel does not ignite immediately it takes a definite period from the time of injection to actual burning this delay is known as the ignition delay period. Ignition delay further can be divided into two part: In this phase the combustion of pre-ignited fuel occurs which was raised to self-ignition temperature after vaporization. The rate of heat release is maximum in this period. The period of pre-mixed combustion depends on the duration of the delay period (the longer the delay higher the pressure rises due to higher heat release). This phase is called rapid phase because the temperature and the pressure are so high that fuel injected droplets burn as soon as they are injected and any further pressure rise can be controlled by injection rate. It is assumed to be end when maximum temperature is reached inside the combustion chamber. Compression ratio Engine speed Desired outputs Injection timing Fuel properties Intake temperature and pressure
Fuel Injection takes definite interval. Injection & Combustion take place simultaneously. Initial droplets while undergoing ignition delay, additional droplets are being injected. If ignition delay of fuel is short, initial few droplets will commence actual burning. As a result, mass rate of mixture burned produce rate of press. As ignition delay is longer, the actual burning of first few droplets is delayed and accumulation of greater quantity of fuel take place. When actual burning take place under such conditions the rate of pressure rise increases. If ignition delay is quite large and the actual burning is substantially delayed and the accumulation of fuel is high when actual burning take place the rate of pressure rise is almost instantaneous the knocking begins. Knocking is characterized by extreme pressure differentials and violent gas vibrations evidenced by audible knock. In CI engines knocking occurs in the beginning of combustion. In order to decrease knocking the actual burning should start as early as possible after the fuel injection begins. It has been found that provided the rate of press. rise does not exceed 3 bar per ºCA combustion is smooth. Between 3 to 4 bar there is tendency to knock, above this rate of pressure rise the diesel knock will be prominent. Normally audible knock is always present in CI engines, when it becomes sever and cause heavy vibrations in the engine, it is said to be knocking.
In the Compression Ignition engines, the antiknock characteristics will depend on the chemical composition an also on the design and operating conditions of the engine. The knock rating is done by comparing the fuel under prescribed conditions of operations in a special engine with the reference fuels. The Cetane number is defined as the percentage by the volume of the normal cetane and the alpha methylnaphthalene. Since the Ignition delay is the main character that can control the auto-ignition in the compression ignition engines. So, this character can be helpful to conclude the knock character in the CI engines. Which means the Ignition delay is directly related to the knock character.
Direct Injection (DI) Type – These also called open combustion chambers. The entire combustion space lies in the main cylinder and the fuel is injected into this volume. Indirect Injection (IDI) Type – In this type of combustion chambers, combustion space is divided into two parts, one in the cylinder head and other in the main cylinder. Fuel is generally injected in the part which lies in cylinder head. In shallow depth chamber, depth of the cavity provided in the piston in small. Due to this, the squish is negligible. It is usually adopted for large engines running at low speeds. This design also gives small squish. However, its depth to diameter radius can be varied to obtain desired type of squish for better performance. This design is a modification of cylindrical chamber in the form of truncated cone. Swirl is produced by masking the valve up to 180 deg. of circumference. Squish produced is better than the previous two designs. This design produces a powerful squish along with air movement. The masking needed on the valves is small, and thus, it makes better utilization of oxygen. Swirl Chamber is provided in the cylinder head. A throat which is tangent to this chamber connects it with the main cylinder. During compression, air flows into the swirl chamber through the throat. Tangential direction of air motion generates rotary motion inside the swirl chamber. Fuel in injected into the chamber, and the combustion products travel back to the main cylinder at much higher velocity. This type of chamber is used where fuel quality cannot be controlled, where reliability is more important than fuel economy. It consists of a pre-ignition chamber (40% of total combustion space) connected with the main cylinder through small no. of holes. During compression, piston forces the air into pre-ignition chamber. Fuel is then injected into the chamber due to which combustion takes place. The resulting pressure rise forces the combustion products at higher velocities through small holes into the main cylinder, where bulk of combustion takes place. The clearance volume is divided into two parts – one in the main cylinder and rest in energy cell. Energy cell is divided into two parts – major and minor, which are separated from each other and from the cylinder by narrow orifices. Pressure in the energy cell is low, therefore, air is forced at high velocity into the energy cell on compression. After the injection of fuel, combustion takes place inside energy cell and hot burning gases blow out through small holes into the main chamber. These hot gases provide swirling movement in main chamber, which results in complete combustion.
Unit – 3
SI and CI Engines
Theory of Carburetion:
Types of carburettors
Updraught carburettor
Downdraught carburettor
Cross draught carburettor
Constant choke carburettor
Constant vacuum carburettor
Multiple venturicarburettor
Multi jet carburettor
Multi barrel venturi carburettor
The simple carburettor
Merits of EFI System
Demerits of EFI system
Stages of combustion in SI Engines:
The consequences of this abnormal combustion process are the loss of power, recurring preignition and mechanical damage to the engine.
Normal combustion Abnormal combustion
Phenomenon of Detonation in S.I. engine
Effect of engine variables on detonation in SI engines
Intake Temperature:
Increased intake temperature reduces the delay period, therefore increases the detonation tendency. However, it should be noted that the increased temperature also increases the flame speed, thereby reducing the detonation tendency.
But the effect of increased temperature has more pronounced effect on delay period compared to flame speeds due to which the destination tendency is increased with increase in intake temperature.
Intake pressure:
Increased intake pressure increases the density of charge and reduces the delay period but increases the flame speed. The overall effect is to increase the detonation tendency.
Compression ratio:
Increased compression ratio increases both the pressure and temperature and reduces the delay period, hence the tendency to detonation increases.
Ignition advance:
Advancing the Spark timing increases the peak pressures of the cycle and thus reduces the delay period of end part of the gas in the combustion chamber, hence tendency to detonate increases.
Coolant temperature:
Raising the coolant temperature will increase the cylinder wall temperature and reduce the heat transfer rate between gas and cylinder walls.
Increased temperature of the gases would reduce the delay period and increase the detonation tendency.
Engine load:
Higher loads on the engine increases the heating of the engine and reduces the delay period.
Therefore, the increased loads increase the detonation tendency of the engine.
Engine speed:
Increase in engine speed increases the turbulence in the combustion chamber, thereby increasing the flame speed while the effect on the delay period is negligible.
Due to this, the increased speed of the engine reduces the detonation tendency.
Air-fuel ratio:
Engine size:
Combustion chamber design:
Location of spark plug
Type of fuel
T- head type-
L-head type
Fig b Fig c
I-head type or overhead valve:
Some of the important characteristics of this type of valves arrangement are-
F-head Type:
Fuel-Injection System is vital to the working and performance of CI engine. This system serves the purpose of initiating and controlling the combustion to meet the demand requirements.
Fuel is injected into combustion chamber towards the end of compression. It is atomized as it enters under high velocity and the droplets get vaporized to form a fuel-air mixture. Due to continued heat transfer from hot air to fuel, the fuel reaches to its self-ignition temperature to ignite spontaneously initiating combustion. Depending upon the demand requirements the fuel injection system continues to deliver the fuel during initial part of combustion.
The plunger is driven by a cam and tappet mechanism at the bottom. The plunger reciprocates in the barrel. There are as many plungers as the number of cylinders in the engine. The plunger has a rectangular vertical groove.
The delivery valve is lifted off its seat under the pressure of the fuel against the spring. The fuel from the delivery valve goes to the injector. When the plunger is at the bottom of its stroke, the supply port and spill are uncovered, the fuel from a low-pressure pump after filtration is forced into the barrel.
Nozzle:
Nozzle is that part of injector through which the liquid fuel is spread into the combustion chamber
There are essentially four different types of nozzles
Pressure variation along with crank angle in CI engine Stages of combustion in CI engine:
1. Ignition delay period
2. Pre-mixed burning
3. Mixing controlled combustion
4. Tail region / After burning
Ignition delay period:
a) Physical Delay: Since the time of the injection to the beginning of chemical reactions is called physical delay. In this period fuel is atomized, vaporized and mixed with air and raised to self-ignition temperature.
b) Chemical Delay: In the chemical delay, reactions start slowly and then accelerate until inflammation takes place. It depends various factors like temperature, property of the fuel.
Pre-mixed burning:
Mixing controlled combustion:
Tail region / After burning:
The combustion does not stop with the completion of injection process because unburned and partially burnt fuel particles start burning as soon as they come in contact with oxygen. This burning may continue in expansion stroke up to 70 to 80% of crank travel from TDC.
Factors affecting delay period in CI engine:
There are different types of combustion chamber used in CI Engine are as follows:
DIRECT INJECTION COMBUSTION CHAMBERS: -
Shallow Depth Chamber:
Hemispherical Chamber:
Cylindrical Chamber:
Toroidal Chamber:
INDIRECT INJECTION COMBUSTION CHAMBERS: -
Swirl Chamber:
Pre-ignition Chamber:
Air-Cell Chamber:
Reference:
1) V. Ganesan: Internal Combustion Engines, Tata McGraw-Hill
2) M.L. Mathur and R.P. Sharma: A course in Internal combustion engines, Dhanpat Rai
3) H.N. Gupta, Fundamentals of Internal Combustion Engines, PHI Learning Pvt. Ltd.
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