Mixture Preparation

The vehicles are equipped with two fuel pumps, a pre-fuel pump and a system fuel pump. The pre-fuel pump is located in the fuel tank. It sucks in fuel and delivers it with a mild pressure to the system fuel pump. The system fuel pump, located at the lower side of the vehicle close to the rear axle, builds up injection pressure and delivers fuel to the injection valves.

The pre-fuel pump is a vane-type pump whereas the system fuel pump is of the roller-cell type.

A hose line links the pressure regulator and the air collector so that the difference between the fuel pressure and the air collector pressure remains constant. In this way the amount of fuel provided by the injection valve only depends on the duration of opening of the valve. From the pressure regulator, excess fuel flows back into the fuel tank.

To reduce the complexity of electronic components in the control unit, all injection valves are connected parallel in the electric circuit. The injection valves, therefore, inject fuel all at the same time but at various phases of the work cycle for each individual cylinder.

To achieve sufficiently uniform combustion, half of the fuel quantity required for the work cycle is injected twice for every revolution of the camshaft (corresponds to one for every revolution of the crankshaft).

For cold starts which necessitate an especially rich mixture preparation, a cold start valve is provided. If engine temperature is below a certain value, the cold-start valve will inject fuel for a certain time during starting. This injection phase is determined by the coolant temperature and controlled by the thermo-time switch.

During the warming up period the NTC sensor in the coolant controls the fuel enrichment via the control unit. As engine temperature increases, the additional amount of fuel injected is constantly reduced until it reaches zero at normal operating temperatures.