Oil Pump: Description and Operation

GENERAL





Using the compound oil pump in place of the conventional gear oil pump reduces the hydraulic power losses caused by the bypass operation and also improves noise characteristics while offering the same delivery capacity.
The compound oil pump channels some of the oil under zero pressure via parallel pump chambers. Only the oil quantity required for the particular oil pressure is pumped (see oil pressure operating curves).

DESIGN AND FUNCTION





The compound oil pump consists of a primary and secondary stage each having a pair of externally toothed gears (2a and 3a) and also (6 and 7). The gears (2a) and (6) are fixed on the drive shaft (2). The gear (3a) is fixed and the gear (7) sits loose on the oil pump shaft (3). The primary oil flow passes directly into the main oil gallery (arrow), while the secondary oil flow passes through the timing case (9) before entering the main oil gallery.





Primary and secondary stages have a common suction chamber (E) and separate pressure chambers (primary stage P), (secondary stage S).





Primary and secondary oil flow are led to the control piston (10) separately.





The pressure relief bore (G) enables the control piston (10) to operate in the piston guide without oil back-pressure.

CONTROL PISTON IN REST Position





The primary oil flow (P) is passed directly into the main oil gallery (F) while the secondary oil flow (S) passes through the timing case (9) before entering the main oil gallery.
Benefit:
Greater delivery during starting and hot idling.

CONTROL PISTON IN BLOW-OFF POSITION





The pressurized oil is passed through the damper bore (15) in the control piston (10) into the working chamber (16) and moves the control piston (10). The secondary flow (S) is led back into the suction chamber (E) (only flow losses). All of the primary oil flow (P) passes into the main oil gallery.
Benefit:
Low hydraulic losses at high engine speeds.

Note:
The plate valve (5) ensures a pressure balance between primary and secondary flows when the control piston (10) is moving from the rest into the blow-off position.

CONTROL PISTON IN BYPASS POSITION





The secondary oil flow (S) is passed completely into the suction chamber (E). Part of the primary oil flow (P) is passed via the front piston edge of the control piston into the suction chamber (E) (bypass control).