Auxiliary power units are engines, motors, and power units that provide vehicles with energy for functions other than propulsion. They are used in larger vehicles, such as aircraft, marine vessels, and some larger land vehicles to perform tasks such as starting main engines, heating motor blocks, and charging batteries. They can provide power in electric, pneumatic, and hydraulic forms.
In aircraft, APUs assist in starting the primary engine or engines, generate power for the aircraft during pre-flight checks, and energize cabin amenities such as lights and heating while the engines are off. APUs come in various different types, serving different purposes, and draw power from multiple kinds of sources, including batteries, hydraulic accumulators, and combustion engines.
APUs consist of three basic sections. The first is the power section, which is typically a gas generator for producing the device’s shaft power. Next is the load compressor, a shaft-mounted compressor that provides pneumatic power (some APUs extract bleed air from the power section compressor). Lastly is the gearbox section, which transfers power from the main shaft of the engine to a generator for electrical power. Power is transferred from the gearbox to the fuel control units, lubrication modules, and cooling fans.
Auxiliary power units in commercial airliners take the form of a small turbine engine, usually mounted in the tail. The APU functions just like the turbine engines that provide thrust for the jet, but unlike them, the APU does not provide thrust to the aircraft. The engine drives a generator, which in turn powers the electrical systems onboard like lights and heating while the aircraft is on the ground. In flight, power for these systems is provided by the main engines, and the APU is shut off to save on fuel.
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