A diagram of a basic fuel system for a diesel engine is shown in Figure 19.1. This consists of the following parts:
- A fuel tank – for the diesel fuel.
- A fuel lifts or supply pump – to supply the system with fuel from the tank.
- Fuel filters – capable of filtering minute particles from the fuel.
- An injection pump – which delivers an accurate amount of fuel at high pressure to each injector at the right time.
- Injectors – one for each cylinder, which spray finely atomized fuel into combustion chambers.A control device – to enable the amount of fuel delivered to the injectors to be controlled and so control the engine.
- The control device, which is connected to the governor, is not shown in the illustration.
- A governor – to control engine speed to suit load conditions.
- Leak off pipes – to return excess fuel from the fuel pump and injectors to the tank, and to assist in priming and bleeding the system.
The fuel system shown in the figure has a jerk-type injection pump with six separate pumping elements. Each element of the pump supplies fuel to one of the injectors. For automotive engines, the elements are contained within a simple pump housing in a line, and so are referred to as “in-line pumps”. For large diesel engines such as those used in marine work, each element is made into a separate pump.
Figure 19.2 shows the same fuel system but this time with the components installed on the engine. The fuel tanks, and the pipes connecting it to the engine, are not shown.
In the system illustrated, fuel is taken from the fuel tank by the lift pump or supply pump that is mounted on the side of the injection-pump housing. Fuel is supplied through the fuel filters to the injection pump. The injection pump both meters the fuel and pumps it at high pressure to the appropriate injector at the right time. Overflow pipes, also referred to as leak-off pipes, carry surplus fuel back to the fuel tank. They also maintain an air-free system.
The injection pump is driven by the engine and timed to deliver fuel to each cylinder at the correct time. An automatic timing device is fitted to some injection pumps to advance injection timing as the speed of the engine increases.
A governor, fitted to the rear of the injection pump, controls engine speed. A diesel engine must be governed to maintain idle speed and to prevent in from reaching excessively high speed.
A number of different fuel systems are in use, but they all need the above parts in one form or another. The parts may differ in design, for example a distributor-type pump can replace the in-line pump shown, or the function of two parts can be combined, such as that of the pump and injector, where both pumping and injection are carried out within the injector unit. Irrespective of design, the purpose of the fuel system is to provide a finely atomized spray of clean fuel into the combustion chamber – the right amount of fuel in the right form at the right time.