Full flow system,A full flow type oil system.

Full flow system

Fig. 8-2 A full flow type oil 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 theneed 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.


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.