Unit Injector (Pumpe Deuse) systems

The pump injector system has been in use in basic form for some years on larger direct injection diesel engines. Recent developments in electronic engine control systems have enabled the system to be refined for use on smaller car and light commercial engines, with VW/Audi, and Land Rover among the major manufacturers selling vehicles equipped with this system.

As its name implies, a  pump injector consists of a fuel injection pump, combined with a fuel injector. Each cylinder of the engine has its own pump injector, which eliminates the need for a separate high-pressure fuel pump, and the associated high-pressure fuel lines and rail.

The pump injectors are operated by the engine camshaft, and are able to generate extremely high fuel pressures (up to 2000 bar on some systems). The pump injectors are mounted in the cylinder head, and are supplied with fuel via a distributor pipe mounted in the cylinder head. A fuel lift pump pumps fuel from the fuel tank to the distributor pipe. Each pump injector is individually controlled via signals from the vehicles electronic control unit (ECU).

The pump injectors are electromagnetically-operated. Pressure limiting valves maintain constant fuel pressures in the fuel supply and return lines.

Because of the extremely high fuel injection pressure, the fuel in the return line becomes very hot, and a fuel cooling system is used to cool the excess fuel before it is returned to the tank. Besides the obvious effect on safety, if the fuel was not cooled, the fuel temperature in the tank would rise, which in turn means that the temperature of the fuel supplied to the injectors would also rise. Under high-pressure injection conditions, hot fuel reduces fuel delivery from the injectors; although the ECU can compensate to a reasonable degree for fuel temperature variations, cool fuel gives improved combustion and hence improved engine efficiency.