Rail pressure control

The following components belong to the common rail system and influence the rail pressure control:
  • High pressure pump (HDP)
  • Rail Pressure Control Valve (DRV)
  • Flow regulating valve (MRV)
  • Rail pressure sensor
  • High-pressure accumulator (fuel rail)
  • High pressure lines
  • Fuel injectors

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 actuator 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

Rail pressure monitoring

The rail pressure is monitored during engine start and engine operation by way of setpoint/actual value comparison. The rail pressure sensor supplies the actual value to the DDE. The setpoint is calculated by the DDE depending on the operating status. If the DDE detects a discrepancy between the specified and actual level that is outside the permissible limits, a fault is stored in the fault memory.

An impermissible deviation of the rail pressure from the setpoint can be caused by the following:

  • External leaks at components in the common rail system
  • Flow regulating valve not opening
  • Internal leaks in common rail system, e.g.
    • In the high pressure pump (e.g. worn piston)
    • on rail pressure control valve (diverting too little or too much fuel into the return line)
    • At the fuel injectors (too much volume in the return flow)
  • External influences such as defective fuel supply, clogged fuel return system, air in fuel system

Troubleshooting

If the DDE detects a rail-pressure control discrepancy that is outside the permissible limits, its first action is to limit the injection volume. If this does not eliminate the control discrepancy or keep it to a low level, the engine cuts out or, as the case may be, cannot be started.
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