FUEL INJECTION

DIESEL FUEL INJECTION SYSTEM

THE EFFICIENCY OF A DIESEL ENGINE DEPENDS UPON A NUMBER OF FACTORS. AMONG THEM FUEL-INJECTION SYSTEM PLAYS AN IMPORTANT ROLE ACCURATE METERING OF FUEL AND INJECTION, COUPLED WITH IGNITION PROCESS, INCREASES THE EFFICIENCY OF THE ENGINE TO THE MAXIMUM.

Some diesel engines in trucks operate on the two-stroke cycle. Others have both blowers and turbochargers. The following covers automotive diesel engines only. The diesel engine must be more heavily constructed because of the higher pressure in the combustion chambers and in the actions during the four strokes. Let us compare the strokes:

INTAKE STROKE

The intake valve is open and the piston is moving down, producing a partial vacuum in the cylinder. Atmospheric pressure pushes air through the air filter and intake manifold, past the open intake valve, and into the cylinder. There is no throttle valve or carburetor venturi to impede the movement of the air. The cylinder is filled completely with only air on the intake stroke. No fuel is present.

COMPRESSION STROKE

During the compression stroke, both valves are closed and the upward moving piston compresses the air. The compression ratio of the diesel engine is much higher than that of the petrol engine. It may be as high as 23.5:1. This contrasts with petrol engine compression ratios which average about 9:1. The reason the diesel engine can have such high compression. ratio is that air alone is compressed. Compressing air to one-twentieth of its original volume (compression ratio 20:1), increases the temperature to above 1000"F (537.8 DEGREE C). This temperature is high enough to ignite almost any fuel. That is the reason why we cannot have such high compression ratios with petrol engines. The air-fuel mixture would ignite before the piston reached top dead center (TDC)

POWER STROKE

As the piston approaches TDC on the compression stroke, the diesel-engine fuel-injection system starts to spray fuel into the cylinder. The high temperature of the compressed air ignites the fuel and the pressure rises rapidly. The power stroke then takes place.

EXHAUST STROKE

The exhaust stroke is similar to that of the petrol engine. The piston moves up and the exhaust gases flow out past the opened exhaust valve. To sum up, the diesel engine: Has no throttle valve to restrict air flow into the engine.

Compresses air only on the compression stroke. Has a much higher compression ratio. Does not have an electric ignition system. Instead, heat of compression ignites the fuel as it is sprayed into the cylinders. Engine power and speed are controlled only by the amount of fuel sprayed into the cylinders. For more power, more fuel is injected. For less power, less fuel is injected. Many diesel engines have glow plugs which make it easier to start a cold engine.

The diesel fuel system must :

Deliver the right amount of fuel to meet the operating requirements. Change the timing of fuel delivery as engine speed changes. As engine speed increases, the fuel delivery must start earlier. This compares with the advance of the spark in the petrol engine as engine speed increases. The purpose is the same, to get the ignition of the fuel started before the piston reaches TDC. The piston movement would keep ahead of the pressure rise so that most of the power in the fuel would be wasted. Deliver the fuel to the cylinders under high pressure. The pressure in the cylinder at the end of the compression stroke may be more than 500 psi (35.15 kg/cm2 or 3447 .4 kPa ). The fuel must be under pressure much higher than this in order for it to be sprayed into the compresses air.

TYPES OF FUEL-INJECTION SYSTEM

There are two basic types of fuel-injection systems used on the diesel engine. In one, a centrally located pump pressurises the fuel, meters it, times it, and delivers it at high pressure to the cylinders through tubes. In the other system the fuel is sent to the injectors under a relatively low pressure. The injectors have cam-operated plungers (like the valves) which are adjustable. At the proper instant the cams operate the plungers and they force the fuel at high pressure into the cylinders. The centrally located pump system used on a majority of diesel engines can be further divided into two types: the cam-operated in-line plunger type and the rotary distributor type. The rotary type is the most commonly used for passenger-car diesel engines

CAM- OPERATED IN LINE PLUNGER PUMP

This pump has a cylinder with plunger for each engine cylinder. It has six plungers working in six barrels, one for each engine cylinder. The camshaft is driven from the engine and it has a cam for each plunger. When the lobe of a cam comes up under a plunger, the plunger is raised and fuel is forced through a high-pressure tube to an injector nozzle in an engine cylinder. The injection pump has speed advance and metering systems which time the moment of injection and determine the amount of fuel to be injected.

ROTARY DISTRIBUTOR PUMP

This is the pump used on most automotive diesel engines. it is positioned between the cylinder banks. High-pressure tubes connect the pump to the injectors in each cylinder. The injectors are connected by a second set of tubes to the fuel tank. These are called the fuel leak-off return lines. If the injectors receive excess fuel, these lines carry this excess fuel back to the fuel tank. The fuel-injection pump includes a fuel-supply pump which delivers the fuel to the distributor part of the pump at high pressure. The distributor then sends the fuel to the engine cylinders in the proper firing order. This compares with the electric ignition system on petrol engines, sending high-voltage surges to the spark plugs in the proper firing order. Each nozzle of the fuel injector has a spring-loaded check valve that is closed except when high pressure is applied to the fuel. When this happens the check valve opens, allowing fuel to pass through. The fuel exits from the nozzle tip through small holes. The holes are located so as to send the fuel into the center of the compressed air. It ignites the instant it hits this hot air. When the fuel pressure drops, the check valve closes so the flow of fuel through the nozzle stops.

ROTARY- INJECTION PUMP OPERATION:

This is a cutaway view of the complete rotary injection pump. Although the pump looks complex, its operation is basically simple. The drive shaft operates at one-half crank- shaft speed. It is driven from the camshaft by a pair of bevel gears. The bevel gears are necessary because of the slight angle of drive from the camshaft to the pump drive shaft. The rotating members inside the pump include a transfer pump which builds up fuel pressure, and the injection pump rotor. The injection pump rotor, as it rotates, causes the two plungers to move in and out. The pump rotor rotates in a semi stationary internal cam ring. This ring has cam lobes on its inner surface. As the rotor rotates, the rollers ride up and down on these cam lobes. When they move in this way, they cause the two plungers to move up and down in their holes. This causes the size of the chamber between the inner ends of the plungers to increase and decrease in size. When the chamber increases in size, fuel from the transfer moves toward each other, the chamber decreases in size, forcing fuel to flow out of the chamber. The fuel flows through ports to the high-pressure line connected to the nozzle in the cylinder ready for ignition. That is, to the cylinder in which the piston is nearing TDC on the compression stroke. The action compares to the rotor in the ignition distributor. The rotor sends the high-voltage surge from the ignition coil through the high- voltage cable, to the sparkplug, in the cylinder that is ready to fire. In a similar manner, the rotor in the injection pump aligns holes that allows the fuel under high pressure between the two plungers to flow to the cylinder that is ready for ignition.

GOVERNOR

The governor allows the proper amount of fuel to flow through the system to allow the engine to operate at the proper speed. The governor has a pair of weights that are fastened to a rotating shaft. As the engine speed increases, the weights move out due to centrifugal force. This motion moves a thrust sleeve which, in turn, moves the governor arm. As the governor arm pivots, it actuates the fuel metering valve. This valve controls the amount of fuel that is fed to the pump rotor and therefore that amount of fuel, the cylinders receive. For example, when the car starts down a hill, the load on the engine decreases. The engine starts to speed up. Then the governor operates to prevent this by cutting back on the amount of fuel going to the engine. But suppose the car meets a hill, the engine tends to slow down. The governor therefore allows more fuel to flow so the engine produces more power. This maintains the action. AT this juncture, the throttle position plays a major role. Any particular throttle opening, in effect, presets the amount of fuel and engine speed. Any variation from this speed is countered by the governor as it changes the fuel flow to maintain the preset speed. The throttle movement puts more or less tension on the governor spring. With more tension (greater throttle opening), the engine and governor speed must go higher before the governor acts to cut back on fuel flow. With less tension (lighter throttle), the governor can cut back on fuel flow at lower engine and governor speed. The ignition system for gasoline engines has spark-advance mechanisms that move the spark ahead as engine speed increases. This allows the combustion to start earlier so it will be well advanced by the time the piston reaches TDC and starts down on the power stroke. The diesel engine fuel-injection system also includes a speed advance. It is built into the injection pump. The advance mechanism uses the hydraulic pressure from the transfer pump to control the position of the cam ring. As engine and transfer pump speed increase, the transfer pump hydraulic pressure also increases. This increasing pressure, acting on a piston located below the cam, forces the advance pin to move. As the cam moves, the rotor rollers meet the lobes and cause the plungers to be pushed together earlier. The fuel is then sent to the cylinder injection nozzles in advance required as per the speed of the engine. The advance is proportional to the transfer pump pressure which in turn depends upon the engine speed. The higher the speed, the greater the advance.

GLOW PLUG

For easy starting, especially in cold weather, many diesel engines use glow plugs. The glow plugs have electric heating elements that become very hot when connected to the battery. In a pre-combustion chamber, it is fitted close to the fuel-injection nozzle where the fuel is injected and the combustion starts. After combustion, the burning air-fuel mixture streams out of the pre-combustion chamber and into the main combustion chamber. There, it mixes with the air and combustion is completed. An excess of air is always available so that the combustion of fuel can be relatively complete. When the engine is cold and the air temperature is low, the glow plugs are turned on to put some heat into the pre-combustion chambers. This greatly improves starting because the fuel is sprayed into air that has been pre-heated by the glow plugs. On some engines, the glow plugs can be turned on manually if the driver feels the engine needs them. On others, such as the General Motors V-8 diesel used by Chevrolet, Oldsmobile, and Cadillac, the system is semi-automatic. The instrument panel has two special lights, WAIT and START. The starting procedure is as follows: Put the transmission lever in neutral. Turn the ignition switch to RUN not START. When you turn the switch to RUN, an amber WAIT light comes on (if engine is cold). This tells you that the glow plugs are on, heating the pre-combustion chambers in the engine. After the pre-combustion chambers have been sufficiently heated (usually only a few seconds, depending on the temperature) the WAIT light will go out and the START light will come on. Then, push the accelerator pedal halfway down to the floor and hold it there. Turn the ignition switch to START. Normally, the engine will start in only a few seconds. If it does not start in 15 seconds, release the ignition switch. If the WAIT light comes on again, leave the ignition switch in the RUN position until the WAIT light goes off and the START light comes on.