The electric fuel pump (EKP) is located inside the fuel tank. It supplies the fuel to the two high pressure pumps. A roller cell pump with increased feed pressure of 6 bar is used for the N73 engine. This increased feed pressure is required for adequate supply of the two high pressure pumps. As with the N62 engine, the electric fuel pump is located in the fuel tank and is activated by the DME as the engine requires.
The HDP compresses the fuel to a pressure of between 50 and 120 bar and delivers it via a high pressure pipe to the rail. Due to the function, a small amount (maximum 1 litre per hour) of fuel runs past the pump plunger and flows through the leak pipe back to the fuel tank.
The flow regulating valve is fitted in the high pressure pump and it regulates the delivery volume of the high pressure pump. To this end, it is activated by the DME with vehicle voltage. At the BDC position of the pump plunger, the MSV is supplied with current and is thus closed. During the upward movement of the pump plunger, as soon as the injection pressure calculated by the DME is reached, the MSV is de-energised. The MSV is now opened and the excess fuel is returned to the inlet.
In the rail, the fuel is accumulated with a pressure between 50 and 120 bar and distributed to the fuel injector valves.
Each rail contains a pressure limiting valve that is connected by a line with the fuel feed of the HDP. The pressure limiting valve opens as of a pressure of 125 bar to prevent damage to the injection system. The valve can open briefly if no fuel is required by the fuel injector valves during the overrun fuel cut-off or during the afterheating phase with the warm engine switched off.
There is a rail-pressure sensor on each rail. The sensor is a pressure-dependent resistor and is supplied by the DME with a voltage of 5 V. The system pressure that builds up changes the sensor resistance. In accordance with the present system pressure, the rail-pressure sensor outputs a voltage signal to the DME. The sensor signal rises in a linear form with increasing rail pressure of 0 V (0 bar) to 4.5 V (140 bar). In the case of a rail-pressure sensor failure, the flow regulating valve is activated with an emergency operation function by the DME.
The return shut-off valve is located in the leak line; it is supplied with vehicle voltage via terminal 87 and activated on the earth side by the DME. While the engine is running, it enables the return flow of the leaked fuel into the fuel tank. After engine start-up, the valve is supplied with current with a delay to prevent pressure drop in the feed area of the HDP. Pressure drop in the HDP would lead to build-up of vapour bubbles.
For each cylinder group, an HDEV control unit is used; it is supplied with vehicle voltage via terminal 87. Data transmisson from the DME to the HDEV control units is across a separate line for each high pressure fuel injection valve. The high pressure fuel injection valves are activated by the HDEV control units with a voltage of 100 V and supplied with approx. 80 V during the nozzle opening.
The design of the HDEV corresponds largely to that of conventional fuel injector valves. To open the HDEV, the nozzle pintle is raised from its seat by the solenoid coil. Due to the high injection pressure of up to 120 bar, the compression springs have been configured for 30 Newtons of pressure force (5 Newtons in the case of conventional fuel injector valves). During the closing operation, the compression spring quickly presses the nozzle pintle onto its seat with adequate contact pressure.