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[[Image:A380 APU P1230093.jpg|thumb|The APU exhaust at the tail end of an [[Airbus A380]]]]

An '''auxiliary power unit''' ('''APU''') is a device on a vehicle that provides energy for functions other than propulsion. They are commonly found on large aircraft, as well as some large land vehicles.

==Transport aircraft==
===Functions of APU===
[[Image:APIC-Honeywell-APU-Airbus.jpg|thumb|APIC APS3200 [[Auxiliary Power Unit|APU]] for [[Airbus]] 318/319/320/321]] 

The primary purpose of an aircraft APU is to provide power to start the main engines. [[Turbine engine]]s must be accelerated to a high rotational speed in order to provide sufficient air compression for self-sustaining operation. Smaller jet engines are usually started by an electric motor, while larger engines are usually started by an air turbine motor. Before engines are to be turned, the APU is started, generally by a battery or [[hydraulic machinery|hydraulic]] accumulator. Once the APU is running, it provides power (electric, [[bleed air|pneumatic]], or hydraulic, depending on the design) to start the aircraft's main engines.

APUs are also used to run accessories while the engines are shut down. This allows the cabin to be comfortable while the passengers are boarding before the aircraft's engines are started. Electrical power is used to run systems for preflight checks. Some APUs are also connected to a hydraulic pump, allowing crews to operate hydraulic equipment (such as [[flight controls]] or [[flap (aircraft)|flaps]]) prior to engine start. This function can also be used, on some aircraft, as a backup in flight in case of engine or hydraulic failure.

Aircraft with APUs can also accept electrical and pneumatic power from ground equipment when an APU has failed or is not to be used.

APUs fitted to [[ETOPS/LROPS|ETOPS]] (Extended-range Twin-engine Operations) aircraft are a critical safety device, as they supply backup electricity and compressed air in place of the dead engine or failed main engine generator.  While some APUs may not be startable in flight, ETOPS-compliant APUs must be flight-startable at altitudes up to the aircraft [[service ceiling]].  Recent applications have specified starting up to 43,000 ft. (≈ 13 000 m) from a complete cold-soak condition such as the [[Hamilton Sundstrand]] APS5000 for the [[Boeing 787]] Dreamliner. If the APU or its electrical generator is not available, the airplane cannot be released for ETOPS flight and is forced to take a longer non-ETOPS route.

===History===

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. The APU can be identified on many modern airliners by an exhaust pipe at the aircraft tail.

===Sections of APU===
A typical gas turbine APU for commercial transport aircraft comprises three main sections:  
*Power section
*Load compressor section and
*Gearbox section
The power section is the gas generator portion of the engine and produces all the shaft power for the APU.  The load compressor is generally a shaft-mounted compressor that provides pneumatic power for the aircraft, though some APUs extract [[bleed air]] from the power section compressor. There are two actuated devices: the inlet guide vanes that regulate airflow to the load compressor and the surge control valve that maintains stable or surge-free operation of the turbo machine. The third section of the engine is the [[gearbox]].  The gearbox transfers power from the main shaft of the engine to an oil-cooled generator for electrical power.  Within the gearbox, power is also transferred to engine accessories such as the fuel control unit, the lube module and cooling fan. In addition, there is also a starter motor connected through the gear train to perform the starting function of the APU.  Some APU designs use a combination starter/generator for APU starting and electrical power generation to reduce complexity.

On the [[Boeing 787]] all-electric aircraft, the APU delivers only electricity to the aircraft. The absence of a pneumatic system simplifies the design, but high demand for electricity requires heavier generators.

Onboard solid oxide fuel cell ([[SOFC]]) APUs are being researched.<ref>  [http://www.boeing.com/news/frontiers/archive/2004/july/ts_sf7a.html 2004 - SOFC fuel cell APU]</ref>

===Manufacturers===

Three main corporations compete in the aircraft APU market: [[Goodrich Corporation]], [[United Technologies Corporation]] (through its subsidiaries [[Pratt & Whitney Canada]] and [[Hamilton Sundstrand]]), and [[Honeywell]] International Inc.

==Military aircraft==

Smaller military aircraft, such as [[Fighter aircraft|fighter]]s and [[attack aircraft]], feature auxiliary power systems which are different from those used in transport aircraft. The functions of engine starting and providing electrical and hydraulic power are divided up among two units, the '''jet fuel starter''' and the '''emergency power unit'''.

===Jet fuel starter===
A jet fuel starter is a small [[turboshaft]] engine designed to drive a jet engine to its self-accelerating RPM. Rather than supplying [[bleed air]] to a starter motor in the manner of an APU, a JFS output shaft is mechanically connected to an engine. As soon as the JFS begins to turn, the engine turns; unlike APUs, these starters are not designed to produce electrical power when engines are not running.

Jet fuel starters use a free power turbine section, but the method of connecting it to the engine depends on the aircraft design. In single-engine aircraft such as the [[A-7 Corsair II]] and [[F-16 Fighting Falcon]], the JFS power section is always connected to the main engine through the engine's accessory gearbox. In contrast, the twin-engine [[F-15 Eagle]] features a single JFS, and the JFS power section is connected through a central gearbox which can be engaged to one engine at a time.

===Emergency power unit===
Emergency hydraulic and electric power are provided by a different type of gas turbine engine. Unlike most gas turbines, an emergency power unit has no [[gas compressor]] or ignitors, and uses a combination of [[hydrazine]] and [[water]], rather than [[jet fuel]]. When the hydrazine and water mixture is released and passes across a [[catalyst]] of [[iridium]], it spontaneously ignites, creating hot expanding gases which drive the turbine. The power created is transmitted through a gearbox to drive an electrical generator and hydraulic pump.

The hydrazine is contained in a sealed, nitrogen charged [[Hydraulic accumulator|accumulator]]. When the system is armed, the hydrazine is released whenever the engine-driven generators go off-line, or if all engine-driven hydraulic pumps fail.

==Spacecraft==

The [[Space Shuttle]] APUs provide [[Hydraulic machinery|hydraulic]] pressure. The Space Shuttle has three [[redundancy (engineering)|redundant]] APUs, powered by [[hydrazine]] fuel. They function during powered ascent, [[re-entry]], and landing. During ascent, the APUs provide hydraulic power for [[gimbal]]ling of Shuttle's [[rocket engine|engines]] and [[flight control surface|control surfaces]]. During landing, they power the control surfaces and [[brake]]s. Landing can be accomplished with only one APU working.  On [[STS-9]], two of [[Space Shuttle Columbia|''Columbia'''s]] APUs caught fire, but the craft landed successfully.

==Armor==

APUs are fitted to some [[tank]]s to provide electrical power without the high fuel consumption and large [[infrared signature]] of the main engine. Both the [[M1 Abrams]] and variants of the [[Leopard 2]] such as the Spanish and Danish variants carry the APU in the rear right hull section. The British [[Centurion tank]] used an [[BMC A-Series engine|Austin A-Series]] inline-4 as its auxiliary power unit. The Turkish self-propelled howitzer [[T-155 Fırtına]] uses a 2-stroke diesel engine located at the rear right hull to supply power to fire control computers and turret hydraulics.

==Commercial vehicles==

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.

In the United States, federal [[United States Department of Transportation|Department of Transportation]] regulations require 10 hours of rest for every 11 hours of driving. When stopped, drivers often idle their engines to provide heat, light, and power. Idling burns fuel and puts wear on engines. Some trucks carry an APU designed to eliminate these long idles. An APU can save up to 20 gallons (Cat 600 - 10 hours downtime @ 2 gallons per hour idling) (≈ 76 litres) of fuel a day, and can extend the useful life of the main engine by around 100,000 miles (≈ 160,000 kilometres), by reducing non-productive run time.{{Fact|date=November 2007}}

On some older diesel engines, an APU was used instead of an electric motor to start the main engine. These were primarily used on large pieces of construction equipment.

===Diesel APUs===
<gallery>
Image:TriPacAPU.jpg|Diesel-powered APU on truck</gallery>
The most common APU for a commercial truck is a small [[diesel engine]] with its own cooling system, heating system, generator or alternator system with or without inverter, and air conditioning compressor, housed in an enclosure and mounted to one of the frame rails of a [[semi-truck]].  Other designs fully integrate the auxiliary cooling, heating, and electrical components throughout the chassis of the truck. The APU generator engine is a fraction of the main engine's displacement and so uses a fraction of the fuel; some models can run for eight hours on one US gallon (≈ 4 litres) of diesel.  The generator also powers the main engine's block and fuel system heaters, so the main engine can be started easily right before departure if the APU is allowed to run for a period beforehand.  These units are used to provide climate control and electrical power for the truck's sleeper cab and engine block heater during downtime on the road.

===Electric APUs===
[[Image:ifs_f1.JPG|thumb|150px|right|An electric APU installed on a truck]]
Electric APUs have started gaining acceptance. These electric APUs use battery packs instead of the diesel engine on traditional APUs as a source of power. The APU's battery pack is charged when the truck is in motion. When the truck is idle, the stored energy in the battery pack is then used to power an air condtioner, heater, and other devices ([[television]], [[microwave oven]],..etc) in the bunk.

===Other APUs===

As an alternative to the diesel and electric units, APUs using hydrogen fuel cells<ref>[http://www.hydrogen.energy.gov/pdfs/review05/fcp_32_brodrick.pdf Fuel cell TRU]</ref> as a source of power have also been designed.  [[Freightliner LLC|Freightliner]] has shown a demonstration model of a fuel cell APU, run on a tank of liquid [[hydrogen]] mounted to the truck, on one of their Century Class S/T road tractors.

Some APUs can also use solar pv and wind power as an option for power generation that is stored in batteries for later use. Unlike other APUs renewable energy APUs use the sun and wind for power instead of a fuel to produce power to operate air conditioning and heating and other semi truck accessories. Hybrid APUs are able to replace fuel APUs.

==Other forms of transport==

Where the elimination of exhaust emissions or noise is particularly important (such as yachts, camper vans), [[fuel cell]]s and [[photovoltaics in transport|photovoltaic modules]] are used as APUs for electricity generation.

== See also==
{{commonscat|Auxiliary power units (aircraft)}}
*[[Air start system]]
*[[Coffman engine starter]] - A similar system which uses an explosive cartridge to supply gas pressure.

==References==
{{reflist}}

==External links==
* [http://www.memagazine.org/backissues/august97/features/armor/armor.html "Armor-plated auxiliary power"] design of a modern gas turbine APU
* [http://spaceflight.nasa.gov/shuttle/reference/shutref/orbiter/apu/ "Space Shuttle Orbiter APU"]
* [http://www.freesound.org/samplesViewSingle.php?id=69203 "Sound of an APU from inside a Boeing 737 cabin"]

{{Aircraft components}}
{{Aircraft piston engine components}}
{{Aircraft gas turbine engine components}}
{{aviation lists}}

[[Category:Electrical generators]]
[[Category:Aircraft components]]

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