Rail Pressure Control DDE 7.3 In M57TU2TOP U: Function

The high pressure pump constantly generates the system pressure held in the fuel rail. The fuel-rail pressure is adjusted to the correct level according to engine operating parameters by one of the following two possible control methods (dual controller concept):

• Pressure regulation by the rail pressure control valve:

  The high pressure pump constantly delivers fuel at high pressure to the rail. The rail pressure control valve diverts any excess fuel volume delivered to the rail into the return line, thereby maintaining the rail pressure at the required level. The higher the control signal current, the higher the rail pressure that is maintained.

  The flow regulating valve is set to maximum flow.

• Volumetric flow regulation by the flow regulating valve:

  The flow regulating valve only allows the amount of fuel to flow into the high pressure pump from the low-pressure side that is required in order to generate the required fuel rail pressure. The high-pressure pump cylinders are not completely filled with fuel. The higher the control signal current, the lower is the rail pressure that is generated.

  The rail pressure control valve is kept closed, i.e. set to maximum rail pressure.

The two control methods are used under the following engine operating conditions:

• When the engine is started, rail pressure control is always active.
• When the engine is running:
  • at coolant temperatures below about 19 °C rail pressure control is always active
  • at coolant temperatures above about 19 °C volumetric flow regulation is active