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An auxiliary power unit (APU) is a device on a vehicle whose purpose is to provide energy for functions other than propulsion. Different types of APU are found on aircraft, as well as some large ground vehicles.

Contents 1 Aircraft 2 Armor 3 Commercial Vehicles 4 External links

An aircraft APU is a relatively small, self-contained generator used to start the jet engines, usually with compressed air, and to provide electricity, hydraulic pressure and air conditioning while the aircraft is on the ground. In many aircraft, the APU can also provide electrical power in the air.

A gasoline piston engine APU was first used on the Pemberton-Billing P.B.31 Night Hawk Scout aircraft in 1916. The Boeing 727 in 1963 was the first jetliner to feature a gas turbine APU, allowing it to operate at smaller, regional airports, independent from ground facilities.

Although APUs have been installed in many locations on various military and commercial aircraft, they are usually mounted at the rear of modern jet airliners. The APU exhaust can be seen on most modern airliners as a small pipe exiting at the aircraft tail.

In most cases the APU is powered by a small gas turbine engine that provides compressed air from within or drives an air compressor (load compressor). Recent designs have started to explore the use of the Wankel engine in this role. The Wankel offers power-to-weight ratios better than normal piston engines and better fuel economy than a turbine.

APUs fitted to ETOPS airplanes are more critical than others, as they supply backup electricity and compressed air in place of the dead engine during emergencies. While some APUs may not be startable while the aircraft is in flight, ETOPS compliant APUs must be flight-startable at all altitudes. Recent applications have specified starting up to 43,000 ft from a complete cold-soak condition. 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 route.

APUs are even more critical for space shuttle flight operations. Unlike aircraft APU's, they provide hydraulic pressure, not electrical power. The space shuttle has three redundant APUs, powered by hydrazine fuel. They only function during powered ascent and during re-entry and landing. During powered ascent, the APUs provides hydraulic power for gimballing of shuttle's engines and control surfaces. During landing, they power the control surfaces and brakes. Landing can be accomplished with only one APU working. On STS-9, two of Columbia's APUs caught fire, but the flight still landed successfully.

A typical gas turbine APU for commercial transport aircraft comprises three main sections:

• Power section
• Load compressor
• Gearbox

The power section is the gas generator portion of the engine and produces all the power for the APU. The load compressor is generally a shaft‐mounted compressor that provides all pneumatic power for the aircraft. 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.

With the Boeing 787 being an all electric airplane, the APU delivers only electricity to the aircraft. The absence of pneumatic system simplifies the design, but the demand for hundreds of kilowatts (kW) of electricity requires heavier generators and unique system requirements.

Two main corporations compete in the aircraft APU market: United Technologies Corporation, through its subsidiaries Hamilton Sundstrand and Pratt & Whitney Canada, and Honeywell International Inc.

In case of APU failure an air start unit (ASU) and ground power unit (GPU), respectively is needed.

APUs are also fitted to some tanks to provide electrical power when stationary, without the high fuel consumption caused by running the main engine.

Some commercial vehicles now mount auxiliary power units of their own. A typical APU for a commercial truck is a small diesel engine with its own cooling system, generator, and air conditioning compressor, mounted to one of the frame rails of a semi-truck. This unit is used to provide climate control and electrical power for the truck's sleeper cab and engine block heater during downtime on the road.

In the United States, federal Department of Transportation regulations require 10 hours of rest for every 11 hours of driving. During these times, truck drivers often idle their engines to provide heat, light, and power for various comfort items. Although diesel engines are very efficient when idling, it is still financially and environmentally costly to idle them like this, from a fuel consumption and an engine wear perspective. The APU is designed to eliminate these long idles. Since the generator engine is a fraction of the main engine's displacement, it uses a fraction of the fuel; some models can run for eight hours on a gallon 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. An APU can save five gallons of fuel a day, and can extrend the useful life of the main engine by around 100,000 miles, by reducing non-productive run time.

Some vehicle APUs can also use an external shore power connection for their heating and cooling functions, thus eliminating fuel consumption during rest periods altogether. Many truck stops already provide shore power connections in their parking areas.

As an alternative to the diesel units, APUs using an auxiliary battery system or hydrogen fuel cells as a source of power have also been designed. 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.

• "Armor-plated auxiliary power" design of a modern gas turbine APU
• "Space Shuttle Orbiter APU"