Function of Diesel Fuel Injection

The fuel injection system lies at the very heart of the diesel engine. By pressurising and injecting the fuel, the system forces it into air that has been compressed to high pressure in the combustion chamber.

The diesel fuel injection system consists of:

  • fuel injection pump - pressurises fuel to high pressure
  • high-pressure pipe - sends fuel to the injection nozzle
  • injection nozzle - injects the fuel into the cylinder
  • feed pump – sucks fuel from the fuel tank
  • fuel filter - filtrates the fuel

Some types of fuel tanks also have a fuel sedimentor at the bottom of the filter to separate water content from the fuel.

Functions of the system

The diesel fuel injection system has four main functions:

Feeding fuel

Pump elements such as the cylinder and plunger are built into the injection pump body. The fuel is compressed to high pressure when the cam lifts the plunger, and is then sent to the injector.

Adjusting fuel quantity

In diesel engines the intake of air is almost constant, irrespective of the rotating speed and load. If the injection quantity is changed with the engine speed and the injection timing is constant, the output and fuel consumption change. Since the engine output is almost proportional to the injection quantity, this is adjusted by the accelerator pedal.

Adjusting injection timing

Ignition delay is the period of time between the point when the fuel is injected, ignited and combusted and when maximum combustion pressure is reached. As this period of time is almost constant, irrespective of engine speed, a timer is used to adjust and change injection timing – enabling optimum combustion to be achieved.

Atomising fuel

When fuel is pressurised by the injection pump and then atomised from the injection nozzle, it mixes thoroughly with air, thus improving ignition. The result is complete combustion.