Auxiliary power unit

A jet fuel starter (JFS) is a similar device to an APU but directly linked to the main engine and started by an onboard compressed air bottle.

[2] During World War I, the British Coastal class blimps, one of several types of airship operated by the Royal Navy, carried a 1.75 horsepower (1.30 kW) ABC auxiliary engine.

[Note 1][3] One of the first military fixed-wing aircraft to use an APU was the British, World War 1, Supermarine Nighthawk, an anti-Zeppelin night fighter.

A hole in the extreme nose of the cone contained a manual pull-handle which started the piston engine, which in turn rotated the compressor.

Two spark plug access ports existed in the Jumo 004's nose cone to service the Riedel unit's cylinders in situ, for maintenance purposes.

[9] The Boeing 727 in 1963 was the first jetliner to feature a gas turbine APU, allowing it to operate at smaller airports, independent from ground facilities.

[11] In this section of the engine, air and fuel are mixed, compressed and ignited to create hot and expanding gases.

Within the gearbox, power is also transferred to engine accessories such as the fuel control unit, the lubrication module, and cooling fan.

The 2018 market share varied according to the application platforms:[16] On June 4, 2018, Boeing and Safran announced their 50–50 partnership to design, build and service APUs after regulatory and antitrust clearance in the second half of 2018.

[Note 2] APUs are fitted to some tanks to provide electrical power without the high fuel consumption and large infrared signature of the main engine.

[26] A refrigerated or frozen food semi trailer or train car may be equipped with an independent APU and fuel tank to maintain low temperatures while in transit, without the need for an external transport-supplied power source.

The exhaust path of the pony engine was typically arranged so as to warm the intake manifold of the diesel, to ease starting in colder weather.

[32] Delphi has said the 5 kW system for Class 8 trucks will be released in 2012,[needs update] at an $8000–9000 price tag that would be competitive with other "midrange" two-cylinder diesel APUs, should they be able to meet those deadlines and cost estimates.

A Honeywell GTCP36 APU mounted in the tail of a business jet
The APU exhaust in the tailcone of an Airbus A380
The intake diverter of the Jumo 004, with pullcord starter handle for Riedel APU and its sparkplug access ports
The Riedel 2-stroke engine used as the pioneering example of an APU, to turn over the central shaft of both World War II-era German BMW 003 and Junkers Jumo 004 jet engines (pullcord starter variant shown).
The Riedel APU installed on a preserved BMW 003 jet engine (electric starter variant shown).