UNIT II.

UNIT II

Prof. Yogesh Sonawane(9975708447)
Content (1) Theory of Carburetion, Types of Carburettors, (No-Numerical on Carburetion) (2) Electronic fuel injection system, Gasoline direct injection (GDI), MPFI System. (3) Combustion in SI engines, stages of combustion, flame propagation, rate of pressure rise, abnormal combustion, Phenomenon of Detonation in S.l. engines, effect of engine variables on detonation. Combustion chambers. (4) Rating of fuels in SI. engines, Additives Prof. Yogesh Sonawane( )

Carburetion The SI engines use usually the volatile fuels e.g. petrol, alcohol, etc. The mixture of fuel and air is prepared outside the engine cylinder and partly evaporated mixture is supplied to the engine. The mixture prepared outside the cylinder is never homogeneous. Droplets of fuel continue to evaporate even during the suction and exhaust strokes. Therefore the process of formation of mixture is very important for SI for the engine to operate efficiently under all operating conditions. Prof. Yogesh Sonawane( )

Carburetion The process of preparation of mixture of atomized fuel and air before entering to the cylinder is called carburetion. The device which supplies the metered spray of fuel mixed with correct amount of air for efficient combustion in cylinder at all operating conditions is called carburettor. Prof. Yogesh Sonawane( )

Induction System (Fuel Feeding) for S.I. Engines
Prof. Yogesh Sonawane( )

Induction System (Fuel Feeding) for S.I. Engines
Induction System is to supply partly vaporized mixture of fuel and air to various cylinders of the engine. It consists of supply of fuel from fuel tank and air from surroundings to carburettor in which the fuel is partially evaporated (atomised). The partly evaporated fuel and air mixture from carburettor is carried through a pipe line, called intake manifolds, to the engine cylinder. Prof. Yogesh Sonawane( )

Factors Affecting Carburetion
(1) Speed: The time available for formation of mixture by the carburettor is greatly affected by the speed of the engine. If an engine runs at 5000 rpm, the time available for the process of carburetion is in the range of to 0.01 seconds. In such a short period, the fuel is required to be atomised, mixed with air, vaporized and to be inducted into the engine cylinder. So, the design of a carburettor becomes important to accomplish the above processes in such a short period, particularly, with regard to design of its venturi. Prof. Yogesh Sonawane( )

(2) Temperature of inlet air: Higher the surrounding air temperature, higher the vaporization of fuel and homogeneity of mixture. However increased temperatures reduce the volumetric efficiency, hence, the power output. (3) Volatility of fuel: Complete vaporization could be achieved by using highly volatile fuels, which are expensive to produce, or by using heat in intake manifolds to promote vaporization. Prof. Yogesh Sonawane( )

However, excessive vaporization of fuel decreases the efficiency and reduces the power output of the engine. (4) Design of intake manifolds: When the multicylinder engines receive a partially vaporized mixture of fuel and air, each cylinder does not receive the same amount of fuel. Therefore, the design of intake manifolds becomes essential to ensure proper distribution of fuel. Prof. Yogesh Sonawane( )

Mixture Requirements:
Petrol fuel used in SI engine is mainly octane (C8 H18) for which the chemically correct or stoichiometric mixture of air-fuel ratio is : 1 by mass approximately. This mixture gives most rapid combustion of fuel, almost the great power and reasonable economy of fuel. Rich mixtures give more power in the ratio of 11: 1 to 15: 1 of air and fuel and or lean mixtures of about 16 to 18 : 1 gives better fuel economy. Rich mixtures having A.F. ratio below 11: 1 and lean mixtures above 20: 1 cannot burnt effectively. Prof. Yogesh Sonawane( )

Mixture Requirements at Variable Loads and Speeds
The function of carburettor is to form a homogeneous mixture of very fine liquid fuel particles and air in desired proportions (A.F. ratio 11 to 18 : 1), under unsteady conditions such as sudden throttle opening and closing, acceleration and deceleration, at various loads on the engine, starting, idling and initiate maximum power. The carburettor must provide a different proportion of fuel and air for various conditions of running of the engine. The various requirements of S I engine are: (a) Maximum power. (b) Minimum specific fuel consumption or maximum economy. (c) Starting, idling and low load running. (d) Acceleration. (e) Part load running range-cruising. Prof. Yogesh Sonawane( )

Fig. Effective throttle opening (%) air-fuel ratio requirements Prof. Yogesh Sonawane( )

(1) The maximum power: would be obtained if all the oxygen present in the cylinder is utilized. Practically the mixture of fuel and vapour is never homogeneous and it further diluted by the residual gases. It is possible that some parts of the oxygen present in the cylinder may not find fuel for burning due to non-homogeneity of mixture though the mixture supplied is chemically correct. It reduces the power developed by the engine. Therefore a little rich mixture of r-l ratio 12.5 to 13.5: 1 (approximately) is necessary to ensure that all the oxygen present is fully utilized and such a mixture will give maximum combustion temperatures and power . (Refer Fig.) Fig. Effect of air-fuel ratio on power output and efficiency at full throttle and constant speed Prof. Yogesh Sonawane( )

(2) Maximum Economy of Fuel: • For the maximum economy of fuel consumption it is necessary that all the fuel present in the cylinder is burnt completely. To ensure the effective burning of fuel it is necessary that a little excess air is required to be supplied to ensure complete combustion. It is observed that air-fuel ratios in the range of 16.5 to gives maximum economy of fuel i.e. it gives minimum specific fuel consumption Fig. : Effect of air-fuel ratio on specific fuel consumption at various throttle openings Prof. Yogesh Sonawane( )

(3) Starting, Idling and Low Load Running: • The engine is said to idle when no external load is applied on the engine and at the throttle valve is almost closed. • Under idling conditions the power developed by the engine is just sufficient to overcome the various friction losses of the engine. • Low load running is usually taken in the range of zero to 20% of the rated power the engine. • At the time of starting and idling the engine, the working temperatures are low Therefore, the carburettor is not able to vaporize the fuel and the mixture reaching the cylinder is lean. This may lead to non-initiation of combustion in the cylinder. • To ensure minimum fuel vapour in the cylinder rich mixtures are necessary to initiate the combustion. • Under the conditions of idling and low load running the throttle valve almost closed due to which the pressures in the intake manifolds are much lower than atmospheric pressure while the pressure inside the cylinder is approximately atmospheric pressure at the end of exhaust stroke. • When the inlet valve opens during the suction stroke, there may be back flow of residual gases into the intake manifold. Prof. Yogesh Sonawane( )

When the piston moves outwards, the residual gases are drawn along with the fresh charge. Therefore the actual mixture inside the cylinder would contain large percentage of residual gases in the fresh charge i.e. the cylinder mixture is too diluted and it is at low temperatures. This diluted mixture is not able to initiate proper combustion. In order to offset the dilution of fresh charge due to residual gases and low temperatures, it is necessary to supply rich mixtures during starting, idling and low load running of engines. Usual air-fuel ratio requirements is about 11 to 12: 1 as represented by the curve (a-b) in Fig. Prof. Yogesh Sonawane( )

(4) Acceleration: Under normal running of engine the fuel that leaves the carburettor is not completely vaporized and a part of the liquid remains in the intake manifolds as liquid film because the liquid particles have larger inertia compared to vaporized fuel. It does not create any problems under steady state running of engine since the fuel of previous stroke in the intake manifolds vaporizes and supplied to the engine in the subsequent stroke. When the engine is to be accelerated suddenly by opening the throttle valve, the liquid fuel lags behind in the intake manifolds due to its large inertia. As a result the mixture of fuel and air reaching the cylinder is lean due to instant opening of throttle valve which is contrary to the requirement of rich mixture during acceleration. In order to compensate this ill effect and to provide the needed rich mixtures during acceleration, a suitable mechanism called acceleration pump is provided in the carburettors. Prof. Yogesh Sonawane( )

(5) Part Load Running - Cruising Range: Curve (b-c) of shows the part load running of engine which is in the range of 20 to 75% of rated power. As the load on the engine is increased beyond 20% of load, the throttle valve is opened gradually with the increase in load. It reduces the inlet pressure and the problem of dilution of fresh charge by the residual gases is also reduced. The air-fuel ratio increases and it ensures economical running of the engine. Usually air-fuel ratio of about 17 : 1 is kept in cruising range for a single cylinder engine and slightly rich mixtures with air-fuel ratio 16: 1 in case of multicylinder engines. Prof. Yogesh Sonawane( )

Requirements of a Good Carburettor
To meter the liquid fuel so as to produce the required air-fuel ratios at all operating conditions like during idling, low load running, cruising range and maximum power range. (b) To provide energy to be supplied to change the fuel from liquid to vapour state since the fuel in liquid form or drops will not burn efficiently in an engine. (c) To prepare the homogeneous mixture of fuel and air as far as possible. (d) To provide rich mixtures for ease of starting the engine. (e) To provide the required rich mixture during acceleration. Prof. Yogesh Sonawane( )

Simple Carburettor Prof. Yogesh Sonawane( )

Simple Carburettor It consists of a float in float chamber, venturi and the main fuel jet. Float chamber is open to atmosphere so pressure in float chamber is atmospheric pressure. Fuel is supplied to the float chamber through strainer from fuel tank with the help of fuel pump. The jet tube consists of main nozzle to which fuel is supplied from the float chamber through a main fuel jet. The suction of the engine draws air through the choke tube and passes through the venturi. Since the area of cross-section at the throat of venturi reduces, the pressure at the main nozzle reduces and the velocity of air increases. Due to pressure differential caused at the main nozzle and the pressure in the float chamber, the fuel from float chamber is supplied to the main nozzle which mixes with the incoming air. Prof. Yogesh Sonawane( )

Simple Carburettor The velocity of air past the venturi vaporizes the petrol fuel partially which is then evaporated by the heat in the intake manifolds and the cylinder walls. A petrol engine is quantity governed. It means that the amount of charge delivered- is according to power delivered by the engine at a particular speed. This is achieved by a throttle valve of butterfly type. When the throttle valve opens, more air flows through the venturi tube and more quantity of fuel and air is delivered to the engine, therefore, engine develops more power. When the throttle valve closes, reverse is the action. Prof. Yogesh Sonawane( )

Simple Carburettor Nozzle lip (h) The pressure at the throat under fully open throttle condition lies between 4 to 6 mm of Hg below atmospheric. In order to avoid overflow of the fuel from nozzle, the main nozzle tip is kept slightly higher than the level of fuel in float chamber. The difference of level of tip of main nozzle and fuel level in float chamber is called nozzle lip. If h is the nozzle lip and (Δp)a is the pressure drop due to flow of air, then the pressure drop available for flow of fuel will be, Prof. Yogesh Sonawane( )

Drawbacks of a simple Carburettor
It provides increasing richness of A/F mixture as the speed of the engine increases because as the throttle valve is opened gradually, the pressure the venturi throat decreases, which decreases density of air with increase in its velocity. Whereas, the quantity of fuel flow remains constant. Therefore, A/F ratio decreases with increase in speed of engine. 2. If the speed is too low, we get very lean mixtures which may not be sufficient ignite the mixture. Prof. Yogesh Sonawane( )

Modifications of Simple Carburettor

Starting Choke: Rich mixture is required at the time of starting of the engine due to cold conditions of the engine. A butterfly valve called choke is incorporated before the venturi for this purpose. At the time of cold starting of the engine the choke is almost closed and It lowers the pressure at the venturi. This large pressure drop between the pressure in the float chamber and at the venturi increases the mass flow rate of fuel thereby ensuring that a very rich mixture is supplied to the engine. Prof. Yogesh Sonawane( )

2) Metering and Idling System Idling of the engine means no load running of the engine which requires a rich mixture. At no load, the throttle valve is almost closed and due to this the airflow through the venturi is greatly reduced. The pressure drop at the venturi is very low and the main jet is not able to supply any fuel. In order to supply rich mixture an idling circuit is introduced in the carburettor, located below the throttle valve. The low pressure existing in the intake manifolds past the throttle valve allows the fuel to be supplied from the float chamber through the idle jet thereby enriching the mixture of fuel and air. The air-fuel mixture discharged into the air stream past the throttle valve is controlled by the idling adjustment screw. Air bleeds prevent the fuel to be supplied from the float chamber when the engine is shut-off. At part load running the idle jet becomes ineffective. Prof. Yogesh Sonawane( )

Arrangement of Acceleration Condition Prof. Yogesh Sonawane( )

(3) Acceleration: Simple carburettor is not able to supply the required mixture momentarily due to inertia of liquid fuel particles when the engine is to be accelerated by opening the throttle valve suddenly. To overcome this problem an accelerating pump is used. It consists of a piston-cylinder arrangement with a ball valve in the cylinder and a spring. The piston is forced downwards into cylinder simultaneously when the throttle valve is opened. This forces the extra petrol fuel into the venturi and the amount of fuel is controlled by metering orifice. This way it supplies rich mixture temporarily. The piston is raised again due to the spring force when the throttle valve is again partly closed. Prof. Yogesh Sonawane( )

Part load running-metering pin method Prof. Yogesh Sonawane( )

(4) Part Load Running-Economic Range (Metering Pin Method) A simple carburettor supplies rich mixture when the engine speed increases, in the range of 20 to 75% load. In order to run the engine under maximum economy, a metering pin in the main metering orifice is provided. The movement of the pin rod in the metering orifice is controlled by a control lever by changing the coefficient of discharge and area of flow of fuel into the main jet. It controls the supply of fuel flow into the venturi according to the load on the engine. Prof. Yogesh Sonawane( )

(5) Pressure Reduction Method: • This method provides the required air-fuel ratios in the economic and power range. • An orifice is provided which communicates the float chamber with the venturi above the throat on the choke side of the carburettor. • An automatically operated control valve is provided which regulates the pressure in the float chamber as shown in Fig. • The maximum and minimum pressures in the float chamber correspond to fully opened and almost closed positions of the control valve. This variation in pressure in the float chamber regulates the rate of flow of fuel into the venturi. Prof. Yogesh Sonawane( )

Pressure Reduction Method Prof. Yogesh Sonawane( )

Compensating Devices A simple carburettor supplies rich mixture with increase in speed. A carburettor is also required to supply nearly constant A/F ratio over wide range of speed and load for its economic operation. Air-fuel ratio can be maintained either by increasing the supply of air or by supplying less fuel with increase in speed. The devices used for maintenance of constant A/F ratio are called compensating devices. Prof. Yogesh Sonawane( )

Compensating Devices (I) Compensating jet method: Prof. Yogesh Sonawane( )

Compensating Devices (ii) Auxiliary air valve method: Prof. Yogesh Sonawane( )

Compensating Devices (iii) Auxiliary port method: Prof. Yogesh Sonawane( )

Classification of Carburetors
Depending upon working Construction Constant Choke Carburetor – Choke area is const, Pressure diff varied by venturies like Solex, Carter Constant Vacuum Carburetor – Choke area is varied and depression/vacuum is kept constant; SU Carburetor Depending upon direction of Airflow Up Draught Carburetor Down Draught Carburetor Side/Horizontal/Cross Draught Carburetor

Types of Carburettors:
Depending upon the direction of air and fuel flow, the carburettors are classified as: (a) Updraught carburettors (b) Downdraught carburettors (c) Side draught or horizontal carburettors. Prof. Yogesh Sonawane( )

Analysis of a single jet Carburettor

Automobile Carburettors: (Solex Carburettor)

Automobile Carburettors: (Carter Carburettor)

Automobile Carburettors: (S. U. Carburettor)

Problems in Carburettors
Ice Formation: During atomization and vaporization process of fuel at the venturi, it takes its latent heat of vaporization from the incoming air. It lowers the temperature of mixture of fuel and air. In case its temperature becomes lower than the dew point of water vapour associated with air, it may result into formation of ice on the throttle plate. Vapour Lock: The vaporization of fuel may be caused due to high volatility of fuel or heating of incoming air to avoid ice formation or due to heating of fuel pipes being too near the engine. In case the fuel pipe is small in cross-section and the amount of fuel drawn is large, the resulting high velocity of fuel will cause a considerable pressure drop in line and a vacuum. This may cause the formation of vapour bubbles at the highest point of a tube bend and interrupt the gasoline flow from the fuel tank or fuel pump. Prof. Yogesh Sonawane( )

Mechanical Fuel Pump for SI Engines

Electrical Fuel Pump Prof. Yogesh Sonawane( )

Drawbacks of Carburettor System
(1) Problem of ice formation at low temperatures. (2) Non-supply of exact A.F ratio at all loads. (3) Distribution of mixture is non-uniform to cylinders in case of multicylinder engines due to resistance to mixture flow in unequal length of intake manifolds. (4) Economy of fuel is affected during idling and low load running of the engine. (5) It limits the use of compression ratio. (6) Possibility of back firing at low speeds particularly in multi-cylinder engines. A gasoline injection system eliminates most of the above drawbacks of a carburettor Prof. Yogesh Sonawane( )

Classification of Fuel Injection Systems

Continuous Injection System

M.P.F.I. System Prof. Yogesh Sonawane( )

D-M.P.F.I. System Prof. Yogesh Sonawane( )

L-M.P.F.I. System Prof. Yogesh Sonawane( )

Components of M.P.F.I. System

Advantages of M.P.F.I. System
1 . More uniform A.F mixture will be supplied to each cylinder. 2. No need to crank the engine twice or thrice in case of cold starting as happens in the carburettor system. 3. Provides immediate response in case of sudden acceleration/deceleration. 4. Since the engine is controlled by ECU (Engine Control Unit), more accurate amount of A.F mixture will be supplied and as a result complete combustion will take/place. 5 . Power developed by the engine is better compared to carburetion system. 6. Specific fuel consumption is low and mileage of the vehicle improves. Prof. Yogesh Sonawane( )

Combustion in SI Engines
The combustion is defined as the rapid and high temperature oxidation of fuel with liberation of heat energy. When a mixture is ignited by spark plug, it develops a flame defined as gas rendered luminous by liberation of chemical energy, which starts from the point of ignition and spreads continuously in outward direction. If the flame travels from the point of ignition up to the end of combustion chamber without any change in speed and shape, the combustion is said to be normal combustion. In case of normal combustion the forward boundary of reaction zone of a flame is called flame front. Prof. Yogesh Sonawane( )

Combustion in SI Engines
Frame Front is defined as the surface or area between the luminous region and the dark region of the unburned charge. If the mixture of fuel and air ignites prior to reaching the flame front, this phenomenon of combustion is called auto-ignition. The temperature at which the fuel will ignite itself without a flame is called self ignition temperature (S.I.T.). The velocity of flame by which it moves in space is called spatial velocity which depends upon the shape and size of the combustion chamber. Prof. Yogesh Sonawane( )

Combustion Stages in SI Engines

Combustion Stages in SI Engines
Curve ABMN represents the motoring curve i.e. when the engine is not firing and ABCD represents the actual combustion curve. Point A represents the point of ignition where the spark is supplied by a spark plug. The crank angle before T.D. C. at the instant spark is given is called angle of spark advance. Three stages of combustion are as follows: 1 . Period of ignition lag or preparation phase 2. Flame propagation phase 3. After burning or flame termination phase. Prof. Yogesh Sonawane( )

Effect of Engine Variables on Ignition Lag
Pressure and temperature Compression ratio The residual gases in the combustion chamber Nature of fuel Mixture strength Speed Prof. Yogesh Sonawane( )

Effect of Engine Variables on Flame Prorogation
Compression ratio Intake or suction pressure Intake temperature Air-fuel ratio: Prof. Yogesh Sonawane( )

Detonation in SI Engines

Detonation in SI Engines
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 termed as detonation The detonation will take place in case of S.I. engines if: (a) End part of the charge reaches to its self ignition temperature and its delay period is over. (b) The flame does not reach the end part of the charge. Prof. Yogesh Sonawane( )

Effect of Detonation in SI Engines
1. Mechanical failure like erosion of piston crown, pitting of cylinder head and valves etc. 2. Local melting of piston and rings Because of high temperature gases produced compared to normal combustion. 3. Overheating of spark plug due to violent gas vibrations, increases its temperature to a great extent. 4. In case of detonating engine the power output and its thermal efficiency decreases. 6. Pre-ignition: Due to high rate of heat transfer and increased temperature of hot gases, it may cause local heating at certain spots in the combustion chamber and especially the spark plug may become red hot. In such a case it will act as ignition source to ignite the mixture of fuel and air before the supply of spark, thus causing pre-ignition. This pre-ignition may lead to auto-ignition and detonation of the engine causing severe damage to the engine before the pre-ignition is detected. Prof. Yogesh Sonawane( )

Effect of Engine Variables on Detonation in SI Engines
The engine variables which tend to increase the ignition lag and increase the flame speeds would tend to reduce the detonation tendency. 1 . Intake temperature: increases the detonation tendency. 2. Intake pressure: reduces the delay period but increases the flame speed. The overall effect is to increase the detonation tendency. 3. Compression ratio: By increasing CR, tendency to detonation increases. 4. Engine load: The increased loads increases the detonation tendency of the engine. 5. Engine speed: Increased speed of the engine reduces the detonation tendency. 6. Air-fuel ratio: The effect of slightly rich mixtures on delay period is more dominant compared to flame speeds due to which the detonation tendency increases. 7. Engine size : The larger engines have more tendency to detonate compared to smaller engines. 8. Combustion chamber design: More the compact combustion chambers, shorter will be flame travel and combustion time, hence, it will give better anti-knock characteristics. The combustion chamber are designed nearer to spherical shape. Prof. Yogesh Sonawane( )

Effect of Engine Variables on Detonation in SI Engines

Combustion Chambers for S I Engines
Factors to be considered while designing the combustion chambers of SI engines: 1 . The length of flame travel from, the spark plug to the farthest point should be kept minimum to avoid detonation problem. 2. The spark plugs are located at the central location or in some cases dual spark plugs are used. 3. The shape of combustion chambers should be as far as possible spherical to reduce the length of flame travel. 4. To achieve high speed of flame propagation, an adequate amount of turbulence is ensured. 5. It should provide large area to the inlet and exhaust valves with ample clearance around the valve head. 5 . Exhaust valves should not be located near the end gas location of combustion chamber to reduce the possibility of detonation since these valves are hottest spot in the combustion chamber. 9. Thickness of cylinder walls should be uniform to avoid non-uniform expansion. Prof. Yogesh Sonawane( )

Types of Combustion Chambers for S I Engines
T-Head Combustion Chamber : L-Head or Side Valve Combustion Chamber: Prof. Yogesh Sonawane( )

Ricardo Turbulent Combustion Chamber: Bath Tub Combustion Chamber: Prof. Yogesh Sonawane( )

Wedge Head Combustion Chamber F-Head Combustion Chamber Prof. Yogesh Sonawane( )

UNIT II.

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