Diesel Fuel Injection

A diesel engine high-pressure fuel injection system operates by injecting fuel into the cylinder at precisely the right time with the exact amount of fuel needed. The amount of fuel injected is varied to suit the load on the engine and to control engine speed.

With a normally aspirated diesel engine, the intake manifold is unrestricted for a full flow of air. Unlike most gasoline engines where an air/fuel mixture is drawn into the cylinder on the intake stroke, only air is drawn into the cylinder of diesel engines.

In a basic diesel fuel system, a fuel tank holds the diesel fuel Figure 54-1. The fuel is transferred to the injection system from the fuel tank by a lift pump. The lift pump is also called a transfer pump or a supply pump. Its job is to keep the injection pump full of fuel. A sedimenter, or separator, removes any water and large particles in the fuel. Additionally, a fuel filter removes minute particles that are measured in microns to ensure that contamination and damage of system components does not occur.

FIGURE 54-1
A basic fuel injection system.

An injection pump delivers fuel under very high pressure to the injectors through injector pipes. It must send the correct amount of fuel, and it must send it at the correct time in the engine cycle. An injector, at each cylinder, sprays fuel into each combustion chamber. Leak-off pipes, also called fuel return lines, return excess fuel used for lubrication and cooling from the injection pump and injectors back to the tank. They also help to remove air from the system. A governor controls engine speed, and a control lever on the governor is connected to the accelerator pedal.

The basic diesel fuel injection system is divided into two sections: the low-pressure side, or supply side, and the high-pressure side, or delivery side. The low-pressure side cleans the fuel and delivers it to the high-pressure side where the fuel is pressurized and delivered to the fuel injectors. The high-pressure side of the fuel injection system must raise the pressure of the fuel high enough to open an injector. This allows the fuel to be injected into the combustion chamber at the correct time.

Dirt and water can damage the diesel fuel injection system, which operates with very close tolerances and highly polished components; therefore, a very efficient filtration system is required to ensure all traces of dirt and water are removed. The polished finish is achieved by lapping two components together to form a matched set. Matched components must not be interchanged after lapping is completed.