A Low engine speeds
B Increasing engine speeds
C High engine speeds
1 Transfer pump pressure
2 Time
3 Compression spring
4 Orifice
5 To the high-pressure chambers
6 Control piston
7 Lubrication/cooling/ventilation โ high-pressure pump
8 High-pressure pump cooling bypass
9 Return bypass to transfer pump
High-pressure generation (up to 1600 bar) means high thermal load on the individual components of the high-pressure pump. The mechanical components of the high-pressure pump must also be lubricated sufficiently to ensure durability.
The overflow reducing valve is designed to ensure optimum lubrication or cooling for all operating conditions.
At low engine speeds (low transfer pump pressure) the control piston is moved only slightly out of its seat. The lubrication/cooling requirement is correspondingly low. A small amount of fuel is released to lubricate/cool the pump via the restriction at the end of the control piston.
NOTE: The high-pressure pump features automatic venting. Any air in the high-pressure pump is vented through the restriction.
With increasing engine speed (increasing transfer pump pressure), the control piston is moved further against the compression spring.
Increasing engine speeds require increased cooling of the high-pressure pump. Above a certain pressure, the high-pressure pump cooling bypass is opened and the flow rate through the high-pressure pump is increased. At high engine speeds (high transfer pump pressure), the control piston is moved further against the compression spring. The high-pressure pump cooling bypass is now fully open (maximum cooling). Excess fuel is transferred to the intake side of the transfer pump via the return bypass.
In this way, the internal pump pressure is limited to a maximum of 6 bar.