DV11
Operation and Maintenance
121
Common Rail Fuel-injection System
Printed in Mar. 2005 PS-MMA0608-E1A
2) Construction of the fuel high pressure pump
The fuel is compressed with three radially arranged pump pistons which are at an
angle of 120° to each other. Since three delivery strokes take place for every
revolution, only low peak drive torques are generated so that the stress on the
pump drive remains uniform. The driving torque of the common rail is only about
1/9 of torques to drive existing pump system.
3) Method of operation
The fuel supply pump fuel from the tank to the high pressure pump through the
fuel inlet and the safety valve. It forces the fuel through the safety valve into the
high pressure pump. The driveshaft with its eccentric cams moves the piston of
the pump up and down in accordance with the shape of the cam.
As soon as the delivery pressure exceeds the safety valve's opening pressure
(0.5 ~ 0.5 bar), the fuel supply pump can force fuel through the high pressure
pump's inlet valve into the pumping-element chamber whose pump piston is
moving downwards(suction stroke). The inlet valve closes when the pump piston
passes through the bottom dead center (BDC) and, since it is impossible for the
fuel in the pumping-element chamber to escape, it can now be compressed
beyond the delivery pressure. The increasing pressure opens the outlet valve as
soon as the rail pressure is reached, and the compressed fuel enters the high
pressure circuit. The pump piston continues to deliver fuel until it reaches TDC,
after which the pressure collapses so that the outlet valve closes. The fuel
remaining in the pumping-element chamber relaxes and the pump piston moves
downwards again. As soon as the pressure in the pumping-element chamber
drops below the presupply pump pressure, the inlet valve opens and the pumping
process starts again.
4) Fuel delivery rate
Since the high-pressure pump is designed for large delivery quantities, excess
high-pressure fuel is delivered during idle and part load operation. This excess
fuel is returned to the tank via the pressure control valve.
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