Functional description

Depending upon conditions, one of the following 3 available closed-loop control strategies is used to adjust the rail pressure to the correct level:

• 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 cylinder is not completely filled with fuel. The higher the control signal current, the lower is the rail pressure that is generated. The rail-pressure regulating valve is not supplied with current for the maximum pressure. The rail-pressure regulating valve is supplied with slightly higher current than would be necessary for setting the target pressure.

• Pressure regulation by the rail pressure control valve:

  The high-pressure pump provides a constant supply of highly-pressurized fuel to the rail. The rail pressure regulating valve diverts excess fuel arriving in the rail into the return line. The higher the control signal current, the higher the rail pressure that is maintained. The flow regulating valve is set to maximum flow.

• Combined closed-loop control with simultaneous regulation from the rail pressure regulating valve and the fuel-quantity control valve:

  At extremely low fuel-injection quantities of less than roughly 4 mg (on trailing throttle/overrun) the rail pressure regulating valve must discharge a certain amount of fuel from the high-pressure system. The reason is that the high-pressure pump cannot run with zero delivery. This means that the high-pressure pump supplies fuel to the high-pressure system even when the flow regulating valve is closed. This would lead to excessive rail pressure and thus to a control deviation.

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

• During engine starts: Pressure regulation is always active.
• When the engine is running: At a coolant temperature below 1 °C, the pressure regulation is always active. Once the coolant temperature rises above 15 °C the combined closed-loop control system assumes operation.
  Fig 1: Fuel-Quantity Control Valve And High-Pressure Pump - Circuit Diagram

  Courtesy of BMW OF NORTH AMERICA, INC.