Page 84 - The Jet Engine

Lubrication

Pressure relief valve system 6 Fig. 8-1 shows the pressure relief valve system
5. In the pressure relief valve system the oil flow to for a turbo-propeller engine and indicates the basic
the bearing chambers is controlled by limiting the components that comprise an engine lubrication
pressure in the feed line to a given design value. This system. The oil pressure pump draws oil from the
is accomplished by the use of a spring loaded valve tank through a strainer which protects the pump
which allows oil to be directly returned from the gears from debris which may have entered the tank,
pressure pump outlet to the oil tank, or pressure Oil is then delivered through a pressure filter to the
pump inlet, when the design value is exceeded. The pressure relief valve which maintains a constant oil
valve opens at a pressure which corresponds to the delivery pressure to the feed jets in the bearing
idling speed of the engine, thus giving a constant chambers. Some engines may have an additional
feed pressure over normal engine operating speeds. relief valve (pressure limiting valve) which is fitted at
However, increasing engine speed causes the the oil pressure pump outlet. This valve is set to open
bearing chamber pressure to rise sharply. This at a much higher value than the pressure relief valve
reduces the pressure difference between the bearing to return the oil to the inlet side of the oil pressure
chamber and feed jet, thus decreasing the oil flow pump in the event of the system becoming blocked.
rate to the bearings as engine speed increases. To A similar valve may also be fitted across the pressure
alleviate this problem, some pressure relief valve filter to prevent oil starvation of the bearing chambers
systems use the increasing bearing chamber should the filter become partially blocked or the oil
pressure to augment the relief valve spring load, This having a high viscosity under cold starting conditions
maintains a constant flow rate at the higher engine preventing sufficient flow through the filter. Provision
speeds by increasing the pressure in the feed line as is also made to supply oil to the propeller pitch
the bearing chamber pressure increases. control system, reduction gear and torquemeter

Fig. 8-1 A pressure relief valve type oil system.

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