Diesel Engine Fuel System

The type of fuel available for use in diesel engine varies from highly volatile jet fuels and kerosene, to the heavier furnace oil. Automotive engines are capable of burning a wide range of fuel between these two extremes. How well a diesel engine can operate with different types of fuel is dependent upon engine operation conditions, as well as fuel characteristics.

Diesel fuel is a mixture of kerosene, gas oil and solar oil fractions obtained after distillation of gasoline fraction from petroleum. The main characteristics of diesel fuel are ignitability estimated in certain numbers, viscosity, pour point, purity, etc. Diesel fuel is produced in different grades, such as summer fuel, winter fuel, arctic fuel, which differ mainly in the pour points, flashing points and viscosity values.

A diesel engine fuel system consists of a fuel tank, a primary filter, a secondary filter, a fuel supply pump with a hand primer, an injector pump with a speed governor and automatic injection timing clutch, nozzle holders with nozzles, low-and high-pressure fuel lines (Fig. 8)

During operation of the engine, the fuel supply pump draw fuel from the tank, forces it through the primary filter and delivers through the secondary filter to the injector pump. From the injector pump the fuel is fed through the high-pressure lines to the nozzles; the fuel atomized by the nozzles is injected into the cylinders according to the engine firing order. Surplus fuel is returned from the injector pump and nozzles to the fuel tank. The air is supplied to the cylinders through the air cleaner.

Fig. 8 Diesel engine fuel sysem.

The fuel injector pump is intended to inject fuel under high pressure to the engine cylinders in a particular sequence. The injector pump a disposed between the cylinder banks and is driven from the camshaft by means of a gear train. The pump comprises a body, a camshaft, eight sections (according to the number of cylinders), and a plunger control mechanism. The front part of the injector pump carries a positive speed governor which meters the fuel in accordance with the load thus maintaining the engine speed preset by the driver.

The rear end of the pump camshaft mounts an injection timing clutch which is used to change the instant of fuel injection depending on the engine speed.

A section of the injector pump consists of a plunger and a barrel, a roller tappet and a delivery valve.

The barrel has two ports located at different levels; the plunger top is also provided with two ports and a helix. The plunger is lapped to the barrel.

When the plunger moves down forced by the spring, the fuel under a slight pressure created by the fuel supply pump flows, through the longitudinal inlet passage in the body filling the space above the plunger. As the plunger is moved upward by the cam and the tappet, the fuel is by-passed to the fuel passage till the plunger edge seals off the barrel port. As the plunger continues to move upward the pressure in the space over it will rise. When the pressure reaches the delivery valve limit, the plunger lifts slightly and the fuel is discharged through the high-pressure line to the nozzle. The plunger keeping on moving builds up a pressure that overcomes the nozzle stem spring load. The stem lifts up and the fuel injection begins. The injection continues until the edge of the plunger helix opens the port in the barrel; now the fuel pressure drops the relief band of the delivery valve is lowered to the seat by the spring thus jncreasiflg the volume in the fuel line between the nozzle and the valve, hence insuring positive shutoff of the fuel. When the rack is moved, the plunger rotates and the helix edge opens the barrel port in advance or with delay so that the time of port opening and the quantity of fuel injected into the cylinder are changed.

The fuel metering is controlled from the pedal in the driver’s cab through a system of rods and levers acting up-on the positive speed governor.

The nozzle is used to inject metered quantities of finely atomized fuel under pressure into the cylinders. The closed-type nozzle consists of a steel nozzle holder, a cap nut, a spray nozzle, a stem (or needle), a spindle and.a filter. The fuel passes through the filter, a vertical passage, annular slot to the, fuel space of the spray nozzle. When the pressure in the fuel space overcomes the spring, the stem is lined from the seat and the fuel in injected into the combustion chamber. As the pressure in the fuel line drops, the stem is shut off. Surplus fuel is by-passed via a return tine to the tank. The nozzle is adjusted for the injection pressure of 115-185 kgf/cm2. 5-18. 5MPa)

All the units of the diesel fuel system are inter connected by high-and low-pressure lines, the low pressure lines are made from transparent oil-gasoline resistant plastics tubing, while the high-pressure lines are made from thick-walled steel pipes.

The automatic injection timing clutch serves to change the instance of fuel injection depending on the engine speed for improving engine starting conditions and economy at acceleration.

The injection timing clutch consists of a driving half and a driven half. The driven half is secured to the shank of the injector pump camshaft. The driving half runs freely on a bub of the driven half and is rotated from the timing gear through flexible couplings. Hinged on the pivots of the driven half are the weights urged to the initial position by two springs. When the engine speed increases, the centrifugal forces move the weights apart, the shaped lugs on the weights turn the driven half, hence the camshaft for-ward (along the direction of its rotation) thus advancing timing. When the engine speed decreases, the springs return the cams to the initial position so that the driven half turns in the opposite direction to slow timing.