S.I Engine Mixture Formation (Chapter 5C & 6C)

S.I Engine Mixture Formation And Thermo-chemistry of mixtures  Introduction  Engine induction and fuel system must prepare a fuel-air mixture that satisfies the requirements of the engine over its entire operating regime  Preparation of Fuel-air...

S.I Engine Mixture Formation And Thermo-chemistry of mixtures  Introduction  Engine induction and fuel system must prepare a fuel-air mixture that satisfies the requirements of the engine over its entire operating regime  Preparation of Fuel-air mixture can be done  In side the Engine Diesel Engine (Fuel Injection Pump)  Out side the Engine Gasoline Engine (Carburetor)  Optimum air-fuel ratio for an SI engine is that which gives 1. Required power output 2. With lowest fuel consumption 3. Consistent with smooth and reliable operation Introduction  The constraints of emissions may dictate a different air fuel ratio and also require recycling some exhaust gas.  Relative proportions of fuel and air that give the above requirements depend on engine speed and load.  The purpose of carburetion is to provide a combustible mixture of fuel and air in the required quantity and quality for efficient operation of the engine under all conditions. Definition of Carburetion  The process of formation of a combustible fuel-air mixture by mixing the proper amount of fuel with air before admission to engine cylinder is called carburetion and the device which does this job is called a carburetor.  Carburetor = Car + burette  the carburetor “meters” the appropriate quantity of liquid fuel (like a burette) and mixed it with air before sending the mixture into the engine cylinder. FACTORS AFFECTING CARBURETION  the process of carburetion is influenced by  the engine speed  the velocity of the air stream at the point where the fuel is injected has to be increased. This is achieved by introducing a venturi section in the path of the air.  the vaporization characteristics of the fuel  Will require a volatile fuel for quick evaporation and mixing with air  the temperature of the incoming air and  Higher atmospheric air temperature increases the vaporization of fuel and produces a more homogeneous mixture.  the design of the carburetor  Proper design of carburetor elements alone ensures the supply of desired composition of the mixture under different operating conditions of the engine. Mixture strength  Mixture strength is given in terms of air-fuel or fuel-air ratio or equivalence ratio.  Fuel and air are mixed to form three different types of mixtures.  Chemically correct mixture  Rich mixture and  Lean mixture  Chemically correct or stoichiometric mixture is one in which there : is just enough air for complete combustion of the fuel Mixture strength  A mixture which contains less air than the stoichiometric requirement is called a rich mixture  A mixture which contains more air than the stoichiometric requirement is called a lean mixture  There is a limited range of AI F ratios in a homogeneous mixture, only within which combustion in an SI engine will occur. Too rich Combustible Too lean To burn Range To burn 9 19  A/F ratio for Stoichiometric  Air By Volume By mass  79 % N2 77 % N2  21 % O2 23% O2  Each mole of oxygen entering a combustion chamber will be accompanied by 79/21= 3.76 moles N2  1k moles O2 + 3.76 k moles N2 = 4.76 k moles of air  For complete y zcombustion  of any fuel y of C xH O  y yz z C x H y Oz  x    O2  3.73N 2  xCO2  H 2O   x    3.76N 2   4 2 2  4 2  y z Let U  x     4 2 y C x H y Oz U [O2  3.76 U N 2 ] xCO2  H 2O  3.76 U N 2  2  A/F ratio for Stoichiometric  Stoichiometric Air/Fuel Ratio  y z    x    32  28 3.76  A Mass of Air   4 2       F  st Mass of fuel  12 x  y  16 z        U 32  28 3.376   137.28U       12 x  y  16 z    12 x  y  16 z   y  y  y Let U  x   C x H y   x   O2  3.347 N 2  xCO 2    H 2O  4  2  2  y z  y z  y 32 x    4.347 C x H y   x    4.347 O2  xCO 2    H 2O  A   2 2  4 2  2     F  st 12 x  y  16 z   A/F ratio for Stoichiometric  Ex. Stoichiometric Air/Fuel ratio of Ethanol (C2H5-OH)  C=2, H=6, O=1  y z  6 1 U  x     2    3  2 2  4 2  A U 32  28 3.76 332  28 3.76     8.95  F  st 12 x  y  16 z  24  6  16  Ex.2 C8H18 x=8 y=18 and z=0  18 0  U  8    12.5  4 2  A 12.532  (28  3.76)     15.05  F  st 12 8  18 1  16 0  A/F ratio for Non-Stoichiometric  y z U  x     2 2   - Equivalent ratio (F/A)  R - Number of Moles of CO2  W - Number of moles of excess O2  V - Number of moles of CO  Stoichiometric Lean Mixture Rich Mixture Φ=1 Φ1 U  y z U  x     y z U  x     y z U  x     2 2  2 2  2 2 R (CO2)  1 x x R  x  2U  1      1 V(CO) 0 0 V 2U  1     1  W (O2) 0 W U   1 0   Example  Combustion of Isooctane (C8H18O0)  y z   18 0  U  x     8    12.5  2 2  4 2  If Φ=1, combustion occurs in stoichiometric condition,  R=8, V=0 W=0 N2=3.76(12.5)=47 and H2O=(18/2)=9  If Φ=0.8 1, rich condition, not enough O2 to convert all C to CO2 with some CO formation with  1  1  V 2U  1   2 12.5 1   5 R=(8-5)=3,       1.25  W=0 and N2=47/1.25=37.6 Example  Combustion of Isooctane (C8H18O0) with 20% excess oxygen C8 H 18  15(O2  3.76 N 2 ) 8CO2  9 H 2 O  15 3.76 N 2  2.5O2 A 15 32  56.4 28  18.1 F 8 12 18  Mixture Requirements  The A/F ratio has a considerable influence on its performance.  For Full Throttle and constant speed with varying A/F ratio. Under this condition the A/F ratio affects  The power output Best Power Mixture  The specific fuel consumption  Best Power Mixture at A/F=12:1  Best Economy Mixture at A/F=16:1 Best economy mixture  Mixture Requirements  At full load, complete utilization of inducted air to obtain maximum power for a given displaced volume is the critical issue. For such operation slightly rich mixture is necessary.  At part-load at a given speed, efficient utilization of fuel is the critical issue. Maximum power is not required and economy is desired weak mixture is necessary  For control of NO, HC and CO, operating the engine with stoichiometric mixture is advantageous  General Ranges of Throttle Operation  For successful operation of the engine, the carburetor has to provide mixtures which follow the general shape of the curve ABCD (single cylinder) and A'B'C'D' (multi- cylinder) The carburetor must be suitably designed to meet the various engine requirements. The three general ranges of throttle operation Idling (Enriched Mixture) Cruising (Lean mixture) High power (Enriched Mixture  General Ranges of Throttle Operation  Idling  An idling engine is one which operates at no load and with nearly closed throttle.  Under idling conditions, the engine requires a rich mixture, as indicated by point A. The reasons are  When the intake valve opens, the pressure differential between the combustion chamber and the intake manifold results in initial backward flow of exhaust gases into the intake manifold.  The exhaust gas pressure at the end of the exhaust stroke does not vary greatly regardless of the throttle position  which cause exhaust gas dilution of the fresh charge  General Ranges of Throttle Operation  The amount of fresh charge brought in during idling 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  The presence of this exhaust gas tends to obstruct the contact of fuel and air particles as a result, in loss of power It is necessary to provide more fuel particles by richening the air-fuel mixture. General Ranges of Throttle Operation very small opening of the throttle Significant Pressure difference B/n The comb. Chamber & Inlet manifold Backflow Exhaust gas Dilution with The fresh charge obstruct the contact of fuel and air particles Less power Enrich the mixture to increase the probability of contact of Fuel and air  General Ranges of Throttle Operation  Cruising Range  In the cruising range from B to C, the exhaust gas dilution problem is relatively insignificant The primary interest of this range is obtaining the maximum fuel economy In this range it is desirable that the carburetor provides the engine with the best economy mixture  General Ranges of Throttle Operation  Power Range  During peak power operation the engine requires a richer mixture, as indicated by the line CD for the following reasons (i) To provide best power: (ii) To prevent overheating of exhaust valve and the area near it Enrichening the mixture reduces the flame temperature and the cylinder temperature  PRINCIPLE OF CARBURETION  The ideal state for the fuel to be in when it reaches the cylinder is to be vaporized completely  Good intake manifold design will help to vaporize the fuel, but the carburetor must properly atomize the fuel beforehand  Because of the downward movement of the piston, there 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 PRINCIPLE OF CARBURETION  The rate at which fuel is discharged into the air depends on  the pressure difference or pressure head between the float chamber and the throat of the venturi and  the area of the outlet of the tube  In order that the fuel drawn from the nozzle may be thoroughly atomized,  the suction effect must be strong and  the nozzle outlet comparatively small PRINCIPLE OF CARBURETION  In order to produce a strong suction  Create restriction (venturi ) on the flow of air in to the engine.  At this restriction (throat) due to increase in velocity of flow, a suction effect is created.  The smaller the area, the greater will be the velocity of the air, and thereby the suction is proportionately increased Venturi or choke Tube.  venturi is a tube of decreasing cross-section with a minimum area at the throat, to  Increase the velocity  Decrease the pressure  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.  To avoid overflow of fuel through the jet, the level of the liquid in the float chamber is maintained at a level slightly below the tip of the discharge jet. This is called the tip of the nozzle  PRINCIPLE OF CARBURETION  The spray of gasoline from the nozzle and the air entering through the venturi tube are mixed together in this region and a combustible mixture is formed which passes through the intake manifold into the cylinders  Most of the fuel gets atomized and simultaneously a small part will be vapourized.  Increased air velocity at the throat of the venturi helps the rate of evaporation of fuel  PRINCIPLE OF CARBURETION  The difficulty of obtaining a mixture of sufficiently high fuel vapour-air ratio for efficient starting of the engine and for uniform fuel-air ratio in different cylinders (in case of multi- cylinder engine) cannot be fully met by the increased air velocity alone at the venturi throat.  The carburetor must provide the engine with the correct mixture for all driving conditions. This is very difficult to accomplish  Different carburetor circuits or fuel pathways are used to achieve a smooth operation Air-Fuel Ratio Chart Summary of the Deficiencies of the Elementary Carburetor 1. At low loads, the mixture becomes leaner; the engine requires the mixture to be enriched at low loads. The mixture is richest at idle. 2. At intermediate loads, the equivalence ratio increases slightly as the air flow rate increases; the engine requires an almost constant equivalence ratio. 3. As the air flow approaches the maximum (WOT) value, the equivalence ratio remains essentially constant; the engine requires an equivalence ratio of about 1.1(rich mixture) at maximum engine power. 4. The elementary carburetor cannot compensate for transient phenomena in the intake manifold. It also cannot provide a rich mixture during engine starting and warm-up. 5. It cannot adjust to changes in ambient air density due to changes in altitude.  Modern Carburetor Design The changes required in the elementary carburetor so that it provides the equivalence ratio required at various air flow rates are as follows. 1. The main metering system must be compensated to provide a constant lean or stoichiometric mixture over 20 to 80% of the air flow range. 2. An idle system must be added to meter the fuel flow at idle and light loads to provide a rich mixture. 3. An enrichment system must be provided so that the engine can get a rich mixture as WOT conditions is approached and maximum power can be obtained. 4. An accelerator pump must be provided so that additional fuel can be introduced into the engine only when the throttle is suddenly opened. 5. A choke must be added to enrich the mixture during cold starting and warm-up to ensure that a combustible mixture is provided to each cylinder at the time of ignition. 6. Altitude compensation is necessary to adjust the fuel flow which makes the mixture rich when air density is lowered. 7. Increase in the magnitude of the pressure drop available for controlling the fuel flow is provided by introducing boost venturis (Venturis in series) or Multiple-barrel carburetors (Venturis in parallel).  Carburetor Circuit  Major Circuits of Carburetors  Float Circuit  Idle Circuit  Low speed circuit  Main Circuit  Power Circuit  Acceleration Circuits  Chock circuits  SIMPLE CARBURETOR  Let us first understand the working principle of a simple or elementary carburetor which provides an air-fuel mixture for cruising or normal range at a single speed The simple carburetor mainly consists of 1. Float chamber, 2. Fuel discharge nozzle and a metering orifice, 3. Venturi, 4. Throttle valve and 5. Choke Valve  Float Circuit  Purpose: The float circuit maintains a steady working supply of gasoline at a constant level in the carburetor  Operation: If the amount of fuel in the float chamber falls below the designed level, the float goes down, thereby opening the fuel supply valve and admitting fuel  Float Circuit  Venting: The pressure in the float bowl must be regulated to assure the proper delivery of fuel and purging of vapors.  Balance Tube.  Idle Vent. The pressure in the float bowl must equal that of the air horn in order for the carburetor to provide fuel delivery The Fuel Strainer  The gasoline has to pass through a narrow nozzle exist there is every possibility that the nozzle may clogged during prolonged operation of the engine.  To prevent possible blockage of the nozzle by dust particles, the gasoline is filtered by installing a fuel strainer at the inlet to the float chamber  Throttle valve  govern or vary the amount of charge to the engine when power output is to be varied at a particular speed  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.  Idle and Low-Speed System  Purpose: The idle and low-speed system provides the proper air- fuel mixture when the engine is at idle and during other periods of small throttle opening  During these periods, there is not enough air flowing through the throat to make the discharge nozzle work.  The idle circuit sustains the engine at idle operational when throttle is opened beyond 15% to 20%. And Usually air-fuel ratio of about 12:1  Operation: The idle and the low-speed portions of the system are really separate circuits in operation.  As the throttle begins to open, the effectiveness of the idle circuit falls off gradually as the low-speed circuit takes over. Idle and Low-Speed System  As throttle valve is almost closed (Idling Circuit)  This creates a high vacuum in the area of the carburetor under the throttle valve. This high vacuum causes atmospheric pressure to push gasoline through the idle port from the float bowl.  As the throttle valve is opened (Low Speed Circuit)  the vacuum under it begins to fall off, causing less gasoline to be drawn from the idle port. As more air flows through the throat, the gasoline will begin flowing through the low speed or off-idle discharge port, which is usually in the shape of a rectangular slot or a series of two or three holes.  During the low-speed system operation, there is still not enough airflow through the throat for the discharge nozzle to work. Idle and Low-Speed System Idling Circuit Low Speed Circuit Idle and Low-Speed System Idle and Low-Speed System  Idle Mixture Screw: A needle shaped screw is used in the carburetor to regulate the idle port opening. The air-fuel ratio of the idle system can be adjusted by turning the screw in or out.  The CO content is regulated by the mixture control screw  Air Bleeds: Air bleeds also are used in the idle and low-speed circuits to help atomize the fuel.  Passage to Float Bowl: The passage that supplies the idle and low-speed circuits must (at some point) be higher than the level of the gasoline in the float bowl. If this passage went straight to the idle and low-speed ports, the float bowl would be able to drain through them.  Idle shut-off valve: Even if the heat from the combustion chamber and valves is sufficient to ignite the incoming fuel mixture so that the engine runs on, this situation is some times known as 'dieselling' Main Metering System  The main metering system of the carburetor controls the fuel feed for cruising and full throttle operations. It consists of three principal units:  the fuel metering orifice through which fuel is drawn from the. float chamber  the main discharge nozzle  the passage leading to the idling system  The three functions of the main metering system are  to proportion the fuel-air mixture  to decrease the pressure at the discharge nozzle exit  to limit the air flow at full throttle Main Metering System  The main circuit functions after the throttle open about 25 % or more Acceleration Circuit  In automobile engines situations arise when it is necessary to accelerate the vehicle. This requires an increased output from the engine in a very short time  If the throttle is suddenly opened there is a corresponding increase in the air flow. However, because of the inertia of the liquid fuel, the fuel flow does not increase in proportion to the increase in air flow. This results in a temporary lean mixture causing the engine to misfire and a temporary reduction in power output.  To prevent this condition, all modern carburetors are equipped with an accelerating system  Acceleration Circuit  The pump comprises of a spring loaded plunger which takes care of the situation with the rapid opening of the throttle valve  The plunger moves into the cylinder and forces an additional jet of fuel at the venturi throat Economizer or Power Enrichment Circuit  At the maximum power range of operation from 80% to 100% load, richer air-fuel ratio of about 12 to 14 is required  An economizer is a valve which remains closed at normal cruise operation and gets opened to supply rich mixture at full throttle operation.  Vacuum operated metering rod is used to provide more fuel through the main circuit  Choke System  Purpose: When the engine is cold, the gasoline tends to condense into large drops in the manifold rather than vaporizing. By supplying a richer mixture (8:1 to 9:1) there will be enough vapor to assure complete combustion.  The choke system provides a very rich mixture to start the cold engine. It then gradually makes the mixture less rich as the engine reaches operating temperature.  This is simple butterfly valve located between the entrance to the carburetor and the venturi throat Choke System  Operation: When the choke is partly closed, large pressure drop occurs at the venturi throat that would normally result from the quantity of air passing through the venturi throat.  The very large depression at the throat inducts large amount of fuel from the main nozzle and provides a very rich mixture so that the ratio of the evaporated fuel to air in the cylinder is within the combustible limits.  This is because cold fuel does not atomize well and tends to pool on the walls and floor of the intake manifold Manuel Choke System  the choke valve is operated by a flexible cable that extends into the driver's compartment. As the control is pulled out, the choke valve will be closed so that the engine can be started. As the control is pushed back in, the position of the choke valve is adjusted to provide the proper mixture.  Two features are incorporated into manual choke systems to reduce the possibility of engine flooding by automatically admitting air into the engine:  Manuel Choke System  A spring-loaded poppet valve that is automatically pulled open by the force of the engine intake strokes.  A choke valve that is pivoted off center on its shaft. This will create a pressure differential between the two sides of the choke valve when it is subjected to the engine intake, causing it to be pulled open against the force of spring-loaded linkage. Manual operated choke (strangler)  Semi-Automatic choke  Semi-automatic choke with offset strangler spindle  The effectiveness of the simple manual strangler valve choke may be improved by slightly offsetting the valve plate spindle to one side of the centre line of the intake tube  When the cable is pulled out from the dashboard, the relay lever will be rotated anti- clockwise which closes the strangler valve and partially opens the throttle valve.  Semi-automatic choke with offset strangler spindle and pull-down diaphragm  In addition to the automatic strangler by the offset spindle, a diaphragm unit included, this device is sensitive to the venturi depression when the engine is from cranking to fast-idle conditions  Semi-automatic choke with offset strangler spindle Semi-automatic choke with offset strangler spindle and pull-down diaphragm Automatic Choke System  The automatic choke control system is centered around a thermostatic coil spring.  The spring exerts pressure to hold the choke valve closed. Heat is applied to the coil after the engine is started.  The heat causes the coil to expand, allowing the choke to open.  The four methods of providing controlled heat to the automatic choke thermostatic spring are:  Electricity  Engine Coolant  Intake Manifold Crossover  Exhaust Manifold  Automatic Choke System ENGINE COOLANT HEATED CHOKE ELECTRIC CHOKE WELL-TYPE EXHAUST HEATED CHOKE. EXHAUST HEAT-TUBE TYPE CHOKE  Attitude Compensation  At higher altitudes, density of air is less and therefore the mass of the air taken into engine decreases and the power is reduced in approximately the same proportion. Since, the quantity of oxygen taken into the engine decreases, the fuel-air mixture becomes too rich  Mixture-control systems may be classified according to their principles of operation as  back suction type, which reduces the effective suction on the metering system  needle type, which restricts the flow of fuel through the metering system; and  the air-port type, which allows additional air to enter the carburetor between the main discharge nozzle and the throttle valve The Size of carburetor  The size of a carburetor is given in terms of the diameter of the venturi tube in mm and the jet size in hundredths of a millimeter  The calibrated jets have a stamped number which gives the flow in ml/min under a head of 500 mm of pure benzol.  For a venturi of 30 to 35 mm size (having a jet size which is one sixteenth of venturi size)  the pressure difference (PI -P2) is about 50 mm of Hg.  The velocity at throat is about 90 -100 m/s and  the coefficient of discharge for venturi Cda is usually 0.85. Thanks!