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at a much higher value than the pressure relief valve
to return the oil to the inlet side of the oil pressure
pump in the event of the system becoming blocked.
A similar valve may also be fitted across the pressure
filter to prevent oil starvation of the bearing chambers
should the filter become partially blocked or the oil
having a high viscosity under cold starting conditions
preventing sufficient flow through the filter. Provision
is also made to supply oil to the propeller pitch
control system, reduction gear and torquemeter
Lubrication
74
Fig. 8-1 A pressure relief valve type oil system.
system. Scavenge pumps return the oil to the tank
via the oil cooler. On entering the tank, the oil is deaerated
ready for recirculation.
Full flow system
7. Although the pressure relief valve system
operates satisfactorily for engines which have a low
bearing chamber pressure, which does not unduly
increase with engine speed, it becomes an
undesirable system for engines which have high
chamber pressures. For example, if a bearing
chamber has a maximum pressure of 90 lb. per sq.
in. It would require a pressure relief valve setting of
130 lb. per sq. in. to produce a pressure drop of 40
lb. per sq. in. at the oil feed jet. This results in the
need for large pumps and difficulty in matching the
required oil flow at slower speeds.
8. The full flow system achieves the desired oil flow
rates throughout the complete engine speed range
by dispensing with the pressure relief valve and
allowing the pressure pump delivery pressure to
supply directly the oil feed jets. Fig. 8-2 shows an
example of this system which may be found on a
turbo-fan engine. The pressure pump size is
determined by the flow required at maximum engine
speed. The use of this system allows smaller
pressure and scavenge pumps to be used since the
large volume of oil which is spilled by the pressure
relief valve system at maximum engine speed is
obviated.
Lubrication
75
Fig. 8-2 A full flow type oil system.
9. To prevent high oil pressures from damaging
filters or coolers, pressure limiting valves are fitted to
by-pass these units. These valves normally only
operate under cold starting conditions or in the event
of a blockage. Advance warning of a blocked filter
may be indicated in the cockpit by a differential
pressure switch which senses an increase in the
pressure difference between the inlet and outlet of
the filter.
Total loss (expendable) system
10. For engines which run for periods of short
duration, such as booster and vertical lift engines,
the total loss oil system is generally used. The
system is simple and incurs low weight penalties
because it requires no oil cooler, scavenge pump or
filters. On some engines oil is delivered in a
continuous flow to the bearings by a plunger-type
pump, indirectly driven from the compressor shaft; on
others it is delivered by a piston-type pump operated
by fuel pressure (fig. 8-3). In the latter, the oil supply
is automatically selected by the high pressure fuel
shut-off valve (cock) during engine starting and is
delivered as a single shot to the front and rear
bearings. On some engines provision is made for a
Lubrication
76
Fig. 8-3 A total loss (expendable) oil system.
second shot to be delivered to the rear bearing only,
after a predetermined period.
11. After lubricating the fuel unit and front bearings,
the oil from the front bearing drains into a collector
tray and is then ejected into the main gas stream
through an ejector nozzle. The oil that has passed
through the rear bearing, drains into a reservoir at
the rear of the bearing where it is retained by
centrifugal force until the engine is shut down. This
oil then drains overboard through a central tube in
the exhaust unit inner cone.
OIL SYSTEM COMPONENTS
12. The oil tank (fig. 8-4) is usually mounted on the
engine and is normally a separate unit although it
may also be an integral part of the external gearbox.
It must have provision to allow the lubrication system
to be drained and replenished. A sight glass or a
dipstick must also be incorporated to allow the oil
Lubrication
77
Fig. 8-4 An oil tank.
system contents to be checked. The filler can be
either the gravity or pressure filling type; on some
engines both types are fitted. Provision is also made
for a continuous supply of oil to be made available in
aircraft which are designed to operate during
inverted flight conditions. Since air is mixed with the
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