What is oil used for?
There are two basic classifications of turbine
engine lubrication systems. These are:
• The wet-sump lubrication system was used in
some early jet engines. But today only on the
smaller engines such as auxiliary power units
(APU) are designed with a wet-sump system.
• In a wet-sump system, pressurized oil is used
to lubricate the bearings and gears.
• This oil is carried in the gear housing that
serves as the oil reservoir.
• Oil that has lubricated the bearings and gears
drains by gravity and is collected and directed
back to the gear housing, where it is stored
until it re circulates through the system.
• The oil, after serving its lubrication and
cooling functions, is returned by scavenger
pumps to a tank outside of the engine.
• There are two types of dry-sump lubrication
• Hot-tank system
• Cold-tank system
• In a hot-tank lubrication system, the oil cooler
is in the pressure systems and the scavenged
oil is not cooled before it is returned to the
• In the lubrication system the oil is carried in the oil tank
and flows by gravity to the main pressure pump.
• From there, the pressurized oil flows through the filter
and through the oil cooler, to the bearings.
• When the oil temperature is low, the temperature
bypass valve is open and the oil flows directly to the
• When the temperature is high enough to require
cooling, the valve restricts the flow of oil and forces it
through the cooler.
• After leaving the oil cooler, the oil flows to jets which
spray the oil into the bearings.
• The oil drains from the bearings and is collected by
scavenger pumps and returned to the oil tank.
• The cold-tank system is the same as the hot-tank
system, except for the placement of the oil cooler and
the temperature bypass and cooler bypass valves.
• The oil flows directly from the pressure pump through
the main filter to the bearings.
• From the bearings, it drains down and is collected and
returned to the oil tank by the scavenger pumps.
• If the oil is cool enough, it returns to the tank through
the temperature bypass valve.
• If it is too hot, this valve closes, forcing the oil to flow
through the cooler where it gives up the excess heat.