CARBURETION, INJECTION AND SUPERCHARGING

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Topics Include: Carburation, Injection and supercharging

Simple carburettor

Essential Parts of a Carburettor

Introduction, MPFI system.

Injection system of C.I. engines,

Functional Divisions of MPFI System

Introduction of supercharging,

Types Of Super charging

Methods Of Super Charging

CARBURETION, INJECTION AND SUPERCHARGING

Introduction:

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

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

Factors Affecting Carburetion:

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

a) The engine speed

b) The vaporization characteristics of the fuel

c) The temperature of the incoming air, and

d) The design of the carburetor

Principle of Carburetion:

Both air and gasoline are drawn through the carburetor and into the engine cylinders by the suction created by the downward movement of the piston. This suction is due to an increase in the volume of the cylinder and a con¬sequent decrease in the gas pressure in this chamber. It is the difference in pressure between the atmosphere and cylinder that causes the air to flow into the chamber. In the carburetor, air passing into the combustion cham-ber picks up fuel discharged from a tube. This tube has a fine orifice called carburetor 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 venturi and on the area of the outlet of the tube. In order that the fuel drawn from the nozzle may be thoroughly atomized, the suction effect must be strong and the nozzle outlet comparatively small. In order to produce a strong suction, the pipe in the carburetor carrying air to the engine is made to have a restriction. At this restriction called throat due to increase in velocity of flow, a suction effect is created. The restriction is made in the form of a venturi as shown in

Fig.3.5 to minimize throttling losses. The end of the fuel jet is located at the venturi or throat of the carburetor.

Fig.3.5: Operation of the venturi tube

The geometry of venturi tube is as shown in Fig.3.5. It has a narrower path at the centre so that the flow area through which the air must pass is considerably reduced. As the same amount of air must pass through every point in the tube, its velocity will be greatest at the narrowest point. The smaller the area, the greater will be the velocity of the air, and thereby the suction is proportionately increased

As mentioned earlier, the opening of the fuel discharge jet is usually located where the suction is maximum. Normally, this is just below the narrowest section of the venturi tube. The spray of gasoline from the nozzle and the air entering through the venturi tube are mixed together in this region and a combustible mixture is formed which passes through the intake manifold into the cylinders. Most of the fuel gets atomized and simultaneously a small part will be vapourized. Increased air velocity at the throat of the venturi helps the rate of evaporation of fuel. The difficulty of obtaining a mixture of sufficiently high fuel vapour-air ratio for efficient starting of the engine and for

uniform fuel-air ratio in different cylinders (in case of multicylinder engine) cannot be fully met by the increased air velocity alone at the venturi throat

The Simple Carburetor:

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

During suction stroke air is drawn through the venturi. As the air passes through the venturi the velocity increases reaching a maximum at the venturi throat. Correspondingly, the pressure decreases reaching a minimum. From the float chamber, the fuel is fed to a discharge jet, the tip, of which is located in the throat of the venturi. Because of the differential pressure between the float chamber and the throat of the venturi, known as carburetor depression, fuel is discharged into the air stream. The fuel discharge is affected by the size of the discharge jet and it is chosen to give the required air-fuel ratio. 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

Essential Parts of a Carburettor: A carburettor consists essentially of the following parts, viz.

a) fuel strainer

b) float chamber

c) main fuel metering and idling nozzles

d) choke and throttle

a) The Fuel Strainer

As the gasoline has to pass through a narrow nozzle exit there is every pos-sibility that the nozzle may get clogged during prolonged operation of the engine. To prevent possible blockage of the nozzle by dust particles, the gaso¬line is filtered by installing a fuel strainer at the inlet to the float chamber (Fig.3.7). The strainer consists of a fine wire mesh or other type of filtering device, cone shaped or cylindrical shaped. The strainer is usually removable so that it can be taken out and cleaned thoroughly. It is retained in its seat by a strainer plug or a compression spring.

b) The Float Chamber

The function of a float chamber in a carburetor is to supply the fuel to the nozzle at a constant pressure head. This is possible by maintaining a constant level of the fuel in the float bowl. The float in a carburetor is designed to control the level of fuel in the float chamber. This fuel level must be maintained slightly below the discharge nozzle outlet holes in order to provide the correct amount of fuel flow and to prevent leakage of fuel from the nozzle when the engine is not operating. The arrangement of a float mechanism in relation to the discharge nozzle is shown in Fig.3.8. When the float rises with the fuel coming in, the fuel supply valve closes and stops the flow of fuel into the chamber. At this point, the level of the fuel is correct for proper operation of the carburetor.

As shown in Fig.3.8, the float valve mechanism includes a fuel supply valve and a pivot. During the operation of the carburetor, the float assumes a position slightly below its highest level to allow a valve opening sufficient for replacement of the fuel as it is drawn out through the discharge nozzle.

c) The Main Metering and Idling System The main metering system of the carburettor controls the fuel feed for cruising and full throttle operations (Fig.3.9). 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 main 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

Figure 3.9 shows a schematic diagram of a carburetor highlighting the main metering and idling system. Usually air-fuel ratio of about 12:1 is required for idling. In order to provide such rich mixture, during idling, most of the modern carburetors incorporate special idling system is their construction. This consists of idling fuel passage and idling ports as shown in Fig.3.9. This system gets operational at starting, idling and very low speed running of the vehicle engine and is non-operational when throttle is opened beyond 15% to 20%.

With the opening of throttle and the engine passing through the idling range of operation, the suction pressure at the idle discharge port is not sufficient to draw the gasoline through the idling passage. And the idling system goes out of action. There after main air flow increases and the cruising range of operation is established. The desired fuel-air ratio for idling can be regulated by idling adjustment shown in Fig.3.9.

The Choke and the Throttle

When the vehicle is kept stationary for a long period during cool winter seasons, may be overnight, starting becomes more difficult. At low cranking speeds and intake temperatures a very rich mixture is required to initiate combustion. Sometimes air-fuel ratio as rich as 9:1 is required. The main reason is that very large fraction of the fuel may remain as liquid suspended in air even in the cylinder. For initiating combustion, fuel-vapour and air in the form of mixture at a ratio that can sustain combustion is required. It may be noted that at very low temperature vapour fraction of the fuel is also very small and this forms combustible mixture to initiate combustion. Hence, a very rich mixture must be supplied. The most popular method of providing such mixture is by the use of choke valve. This is simple butterfly valve located between the entrance to the carburetor and the venturi throat as shown in Fig.3.10. When the choke is partly closed, large pressure drop occurs at the venturi throat that would normally result from the quantity of air passing through the venturi throat. The very large depression at the throat inducts large amount of fuel from the main nozzle and provides a very rich mixture so that the ratio of the evaporated fuel to air in the cylinder is within the combustible limits. Sometimes, the choke valves are spring loaded to ensure that large carburetor depression and excessive choking does not persist after the engine has started, and reached a desired speed. This choke can be made to operate automatically by means of a thermostat so that the choke is closed when engine is cold and goes out of operation when engine warms up after starting. The speed and the output of an engine is controlled by the use of the throttle valve, which is located on the downstream side of the venturi. The more the throttle is closed the greater is the obstruction to the flow of the mixture placed in the passage and the less is the quantity of mixture delivered to the cylinders. The decreased quantity of mixture gives a less powerful impulse to the pistons and the output of the engine is reduced accordingly.

Multi-Point Fuel Injection (MPFI) System The main purpose of the Multi-Point Fuel Injection (MPFI) system is to supply a proper ratio of gasoline and air to the cylinders. Different types of MPFI systems are, namely

Port injection

Throttle body injection

D-MPFI system

L-MPFI system

a) Port Injection:

In the port injection arrangement, the injector is placed on intake manifold near the intake port as shown in Fig.3.11. The injector sprays gasoline into the air, inside the intake manifold. The gasoline mixes with the air in a reasonably uniform manner. This mixture of gasoline and air then passes through the intake valve and enters into the cylinder.

b) Throttle Body Injection System

This throttle body is similar to the carburettor throttle body, with the throttle valve controlling the amount of air entering the intake manifold.

An injector is placed slightly above the throat of the throttle body. The injector sprays gasoline into the air in the intake manifold where the gasoline mixes with air. This mixture then passes through the throttle valve and enters into the intake manifold.

As already mentioned, fuel-injection systems can be either timed or continuous. In the timed injection system, gasoline is sprayed from the injectors in pulses. In the continuous injection system, gasoline is sprayed continuously from the injectors. The port injection system and the throttle-body injection system may be either pulsed systems or continuous systems. In both systems, the amount of gasoline injected depends upon the engine speed and power demands.

D- MPFI SYSTEM

The D-MPFI system is the manifold fuel injection system. In this type, the vacuum in the intake manifold is first sensed. In addition, it senses the volume of air by its density. Figure 3.14 gives the block diagram regarding the functioning of the D-MPFI system. As air enters into the intake manifold, the manifold pressure sensor detects the intake manifold vacuum and sends the information to the Electronic Control Unit (ECU). The speed sensor also sends information about the rpm of the engine to the ECU. The ECU in turn sends commands to the injector to regulate the amount of gasoline supply for injection. When the injector sprays fuel in the intake manifold the gasoline mixes with the air and the mixture enters the cylinder.

d) L-MPFI System:

The L-MPFI system is a port fuel-injection system. In this type the fuel metering is regulated by the engine speed and the amount of air that actually enters the engine. This is called air-mass metering or air-flow metering. The block diagram of an L-MPFI system is shown in Fig.3.15. As air enters into the intake manifold, the air flow sensor measures the amount of air and sends information to the ECU. Similarly, the speed sensor sends information about the speed of the engine to the ECU. The ECU processes the information received and sends appropriate commands to the injector, in order to regulate the amount of gasoline supply for injection. When injection takes place, the gasoline mixes with the air and the mixture enters the cylinder.

Functional Divisions of MPFI System: The MPFI system can be functionally divided into

Electronic control system,

Fuel system, and

Air induction system.

The MPFI-electronic control system

The MPFI-electronic control system is shown in the form of block diagram in Fig.3.16. The sensors that monitor intake air temperature, the oxygen, the water temperature, the starter signal and the throttle position send signals to the ECU. The air-flow sensor sends signals to the ECU regarding the intake air volume. The ignition sensor sends information about the engine speed.

The ECU processes all these signals and sends appropriate commands to the injectors, to control the volume of the fuel for injection. When necessary the cold-start injector timing switch off the ECU operates the cold start injector which is a part of the fuel system.

b) MPFI-Fuel System:

The MPFI-fuel system is shown in the form of block diagrams in Fig.3.17. In this system, fuel is supplied by the fuel pump. At the time of starting, the cold start injector is operated by the cold start injector time switch.

The cold start injector injects fuel into the air intake chamber, thus enriching the air-fuel mixture. The pressure regulator regulates the pressure of the fuel. The injectors receive signals from the ECU and inject the fuel into the intake manifold

c) MPFI-Air Induction System:

The MPFI-air induction system is shown in the block diagram in Fig.3.18. The air cleaner, the air-flow meter, the throttle body and the air valve supply a proper amount of air to the air intake chamber and intake manifold. The quantity of air supplied is just what is necessary for complete combustion.

Mechanical Injection Systems:

The fuel-injection system is the most vital component in the working of CI engines. The engine performance viz., power output, economy etc. is greatly dependent on the effectiveness of the fuel-injection system. The injection system has to perform the important duty of initiating and controlling the combustion process.

When the fuel is injected into the combustion chamber towards the end of compression stroke, it is atomized into very fine droplets. These droplets vaporize due to heat transfer from the compressed air and form a fuel-air mix¬ture. Due to continued heat transfer from hot air to the fuel, the temperature reaches a value higher than its self-ignition temperature. This causes the fuel to ignite spontaneously initiating the combustion process.

Classification of Injection Systems: In a constant-pressure cycle or diesel engine, only air is compressed in the cylinder and then fuel is injected into the cylinder by means of a fuel-injection system. For producing the required pressure for atomizing the fuel either air or a mechanical means is used. Accordingly the injection systems can be classified as:

Air injection systems

Solid injection systems

Air Injection System

In this system, fuel is forced into the cylinder by means of compressed air. This system is little used nowadays, because it requires a bulky multi-stage air compressor. This causes an increase in engine weight and reduces the brake power output further. One advantage that is claimed for the air injection system is good mixing of fuel with the air with resultant higher mean effective pressure. Another is the ability to utilize fuels of high viscosity which are less expensive than those used by the engines with solid injection systems. These advantages are off-set by the requirement of a multistage compressor thereby making the air-injection system obsolete.

Solid Injection System

In this system the liquid fuel is injected directly into the combustion chamber without the aid of compressed air. Hence, it is also called airless mechanical injection or solid injection system. Solid injection systems can be classified as:

Individual pump and nozzle system

Unit injector system

Common rail system

Distributor system

a) Individual Pump and Nozzle System:

The details of the individual pump and nozzle system are shown in Fig.3.20(a) and (b). In this system, each cylinder is provided with one pump and one injector. In this arrangement a separate metering and compression pump is provided for each cylinder. The pump may be placed close to the cylinder as shown in Fig.3.20(a) or they may be arranged in a cluster as shown in Fig.3.20(b). The high pressure pump plunger is actuated by a cam, and pro¬duces the fuel pressure necessary to open the injector valve at the correct time. The amount of fuel injected depends on the effective stroke of the plunger.

Unit Injector System:

The unit injector system as shown in Fig.3.21, is one in which the pump and the injector nozzle are combined in one housing. Each cylinder is provided with one of these unit injectors. Fuel is brought up to the injector by a low pressure pump, where at the proper time; a rocker arm actuates the plunger and thus injects the fuel into the cylinder. The amount of fuel injected is regulated by the effective stroke of the plunger. The pump and the injector can be integrated in one unit as shown in Fig

c) Common Rail System:

In the common rail system as shown in Fig,a HP pump supplies fuel, under high pressure, to a fuel header. High pressure in the header forces the fuel to each of the nozzles located in the cylinders. At the proper time, a mechanically operated (by means of a push rod and rocker arm) valve allows the fuel to enter the proper cylinder through the nozzle. The pressure in the fuel header must be that, for which the injector system was designed, i.e., it must enable to penetrate and disperse the fuel in the combustion chamber. The amount of fuel entering the cylinder is regulated by varying the length of the push rod stroke. A high pressure pump is used for supplying fuel to a header, from where the fuel is metered by injectors (assigned one per cylinder).

d) Distributor System:

In fig shows a schematic diagram of a distributor system. In this system the pump which pressurizes the fuel also meters and times it. The fuel pump after metering the required amount of fuel supplies it to a rotating distributor at the correct time for supply to each cylinder. The number of injection strokes per cycle for the pump is equal to the number of cylinders. Since there is one metering element in each pump, a uniform distribution is automatically ensured. Not only that, the cost of the fuel-injection system also reduces to a value less than two-thirds of that for individual pump system.

Fuel Injector: Quick and complete combustion is ensured by a well-designed fuel injector. By atomizing the fuel into very fine droplets, it increases the surface area of the fuel droplets resulting in better mixing and subsequent combustion. Atomization is done by forcing the fuel through a small orifice under high pressure. The injector assembly consists of

a needle valve

a compression spring

a nozzle

an injector body

A cross sectional view of a typical Bosch fuel injector is shown in Fig.3.24. When the fuel is supplied by the injection pump it exerts sufficient force against the spring to lift the nozzle valve, fuel is sprayed into the combustion chamber in a finely atomized particles. After, fuel from the delivery pump gets exhausted; the spring pressure pushes the nozzle valve back on its seat. For proper lubrication between nozzle valve and its guide a small quantity of fuel is allowed to leak through the clearance between them and then drained back to fuel tank through leak off connection. The spring tension and hence the valve opening pressure is controlled by adjusting the screw provided at the top

Supercharging

The power output of an engine depends upon the amount of air inducted per unit time, the degree of utilization of this air and the thermal efficiency of the engine. The amount of air inducted per unit time can be increased by increasing the engine speed or by increasing the density of air at intake. The increase in engine speed calls for rigid and robust engine as the inertia loads increase. The engine friction and bearing loads also increase and the volumetric efficiency decreases when the speed is increased. The method of increasing the inlet air density, called supercharging, is usually employed to increase the power output of the engine. This is done by supplying air at a pressure higher than the pressure at which the engine naturally aspirates air from the atmosphere by using a pressure boosting device called a supercharger.

The power output can also be increased by increasing the thermal efficiency of the engine, say, by increasing the compression ratio. However, this increases the maximum cylinder pressure. The rate of increase of maximum cylinder pressure is less than the rate of increase of break mean effective pressure in case of a supercharged engine. This means that for a given maximum cylinder pressure more power can be obtained by supercharging as compared to that obtained by increase in compression ratio. The rate of increase of maximum temperature is also low in case of supercharging. This results in lower thermal loads.

Types of superchargers:

Supercharger is a pressure-boosting device which supplies air (or mixture) at a higher pressure. A centrifugal or axial flow or displacement type compressor is normally used. If the supercharger is driven by the engine crankshaft, then it is called mechanically driven supercharger. Some superchargers are driven by a gas turbine, which derives its power from the engine exhaust gases. Such a supercharger is called turbocharger. There are three types of superchargers

Centrifugal type

Root's type

Vane type

a) Centrifugal Type Supercharger:

The centrifugal type supercharger is commonly used in automotive engines and is as shown in Fig.3.28. A V-belt from the engine pulley runs the supercharger. First, the air-fuel mixture enters the impeller at the centre. It then passes through the impeller and the diffuser vanes. Finally, air or mixture enters the volute casing and then goes to the engine from the casing. The mixture will come out at higher pressure and this condition is called supercharged condition.

Because of higher pressure more air-fuel mixture is forced into the cylinder. About 30% more air-fuel mixture can be forced into the combustion chamber. The impeller runs at very high speeds, about 80,000 revolutions per minute. Therefore the impeller should be able to withstand the high stresses produced at this speed. Impellers are usually made of duralumin, or alloy steels, to withstand the high stresses.

b) Root's Supercharger:

The details of Root's supercharger is shown in Fig.3.29. The Root's super-charger has two rotors of epicycloids shape, with each rotor keyed to its shaft. One rotor is connected with the other one by means of gears. The gears are of equal size and therefore both the rotors rotate at the same speed. The Root's supercharger operates like a gear pump. The mixture at the outlet of this supercharger will be at much higher pressure than the inlet.

c) Vane Type Supercharger

Details of a typical vane type supercharger is shown in Fig.3.30. A number of vanes are mounted on the drum which is inside the body of the supercharger. The vanes can slide in or out, against the force of the spring. Because of this arrangement, the vanes are always in contact with the inner surface of the body. The space between the inner surface of the body and the drum decreases from the inlet to the outer side. In this way, the quantity of the mixture which enters at the inlet, decreases in volume, because of which the pressure of the mixture will increase as it reaches the exit.

Methods of Supercharging:

Necessary amount of compressed air (or mixture) can be supplied to the engine in the following ways.

Independently driven compressor or blower, usually driven by an electric motor.

Ram effect.

Under piston supercharging.

Kadenacy system (applied to two stroke engines).

Engine driven compressor or blower.

a) Electric Motor Driven Supercharging:

In this type the compressor is driven independently usually by an electric motor. The speed of the supercharger can be varied independent of engine speed and therefore control is comparatively easier.

b) Ram Effect of Supercharging:

The ram effect of supercharging system consists primarily of tuned inlet pipes. These pipes induce resonant harmonic air oscillations. The kinetic energy of these oscillations provides a ramming effect. For the efficient operation of this system, 'the engine speed must be kept constant.

c) Under Piston Supercharging:

Under piston method of supercharging has so far been confined to large marine four stroke engines of the crosshead type. It utilizes the bottom side of the piston for compressing the air. The bottom ends of the cylinder are closed off and provided with suitable valves. This system gives an adequate supply of compressed air, as there are two delivery strokes to each suction stroke of the cycle.

Kadenacy System of Supercharging: The kadenacy system utilizes the energy in the exhaust system to cause a depression of pressure in the cylinder. This depression makes the scavenge air to flow into the cylinder. A blower may also be used with this system, but it is not an essential requirement.

The kadenacy system is based on the following principle: When the exhaust ports or valves are opened rapidly during the end of expansion stroke, there is, within the first interval of a few thousandths of a second, an urge or impulse in the gases to escape very rapidly from the cylinder. The escaping gases leave behind a pressure depression.

At the above moment, the fresh charge of air (or mixture) is allowed to enter the cylinder behind the exhaust gases by suitable timing of the admission valve or ports.

Note

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