A bypass duct (11) provides a return passage to the sump (12) for oil flow pump (3). The duct (11) is controlled by a valve (13) which is constructed to be open at oil pressures representing idle speed or lower. The orifice of the valve is designed to allow the return of enough oil flow to compensate the poor scavenging capability of the oil distribution system at idle engine speeds and to supply full oil flow at higher speeds. In order to prevent an accumulation of oil during the gradually declining speeds which occur at engine shutdown, a check valve (14) is placed in the main supply line (15) from oil pump (3) at a position downstream of the bypass valve (13). The check valve (14) is designed to close at a pressure which indicates that the engine is at low compressor rotor speed. Oil from the pump (3), which flows during the later stages of engine deceleration, is returned through bypass duct (11) and a build-up within bearing housing (5) is avoided.
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