Auxiliary power unit: Wikis


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The APU exhaust at the tail end of an Airbus A380

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 on some large ground vehicles.


Transport aircraft


Functions of APU

APIC APS3200 APU for Airbus 318/319/320/321

The primary purpose of an aircraft APU is to provide power to start the main engines. Turbine engines have large, heavy rotors that must be accelerated to a high rotational speed in order to provide sufficient air compression for self-sustaining operation. This process takes significantly longer and requires much more energy than starting a reciprocating engine. Smaller turbine engines are usually started by an electric motor, while larger turbine engines are usually started by an air turbine motor. Whether the starter is electrically, pneumatically, or hydraulically powered, however, the amount of energy required is far greater than what could be provided by a storage device (battery or air tank) of reasonable size and weight.

An APU solves this problem by powering up the aircraft in two stages. First, the APU is started by an electric or hydraulic motor, with power supplied by a battery, accumulator, or external power source (ground power unit). After the APU accelerates to full speed, it can provide a much larger amount of power to start the aircraft's main engines, either by turning an electrical generator or a hydraulic pump, or by providing compressed air to the air turbine of the starter motor.

APUs also have several auxiliary functions. Electrical and pneumatic power are used to run the heating, cooling, and ventilation systems prior to starting the main engines. This allows the cabin to be comfortable while the passengers are boarding without the expense, noise, and danger of running one of the aircraft's main engines. Electrical power is also used to power up systems for preflight checks. Some APUs are also connected to a hydraulic pump, allowing maintenance and flight crews to operate the flight controls and power equipment without running the main engines. This same function is also used as a backup in flight in case of an engine failure or hydraulic pump failure.


A gasoline piston engine APU was first used on the Pemberton-Billing P.B.31 Nighthawk 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.

Recent designs have started to explore the use of the Wankel engine in this role. The Wankel offers power-to-weight ratios that are superior to conventional piston engines and better fuel economy than a turbine engine.[1] [2]

APUs fitted to 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 while the aircraft is in flight, ETOPS-compliant APUs must be flight-startable at the 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. 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.

In case of APU failure an air starter unit (ASU) and a ground power unit (GPU) are needed for starting the main engines on ground and to provide electrical power to the aircraft prior to the main engine start.

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 combination starter/generator for APU starting and electrical power generation to reduce complexity.

Some APUs use an electronic control box (ECB), which is designed to control the APUs. It also serves as an interface between the subsystems of an APU and the aircraft.

With the Boeing 787 being an all electric aircraft, 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.

Onboard solid oxide fuel cell (SOFC) APUs are being researched.[3]

The 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 fighters 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, or JFS, is a small turboshaft engine designed to provide power to spool the main engine up to its self-accelerating RPM. Unlike the APUs used in transport aircraft, the JFS provides power through an output shaft connected through a gearbox to the main engine, rather than through bleed air.

Unlike the APUs in transport aircraft, which are started by electrical power, a JFS is spooled up for starting by a hydraulic motor with fluid from a hydraulic accumulator (a type of pressurized fluid reservoir). The advantages of this system over an electrically started APU are extra reliability and independence from ground support. Batteries may go dead if the aircraft isn't operated for a long period of time, while a hydraulic accumulator will stay charged indefinitely. Starting an aircraft with a JFS requires no external equipment or ground personnel, and only requires a small amount of battery power to operate the JFS controls and the electric valves in the hydraulic system. Once the main engine starts, the JFS accumulator will be almost instantly recharged by the engine-driven hydraulic pump, while a battery would take a much longer time to charge. In the event the main engine fails to start and the hydraulic accumulator is discharged, the accumulator may be recharged by a hand-operated pump onboard the aircraft.

All 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 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.


APUs are even more critical for Space Shuttle flight operations. Unlike aircraft APUs, 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 provide 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.


APUs are also fitted to some tanks to provide electrical power when stationary, without the high fuel consumption and large infrared signature caused by running 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 uses an Austin A-Series inline-4 as its auxiliary power unit.

Commercial vehicles

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. 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.

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 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 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. 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.

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 provide shore power connections in their parking areas.

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.

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.

As an alternative to the diesel units, APUs using an auxiliary battery system or hydrogen fuel cells[4] 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.

Other forms of transport

Where the elimination of exhaust emissions or noise is particularly important (such as yachts, camper vans), fuel cells and photovoltaic modules are used as APUs for electricity generation.

currently the most common APU units for Highway trucks are; Wabaso, Proheat, and Espar.

Ski lifts also use an APU if the main drive (usually electric) should fail or power be lost, enabling the lift to continue to operate. They can be either gas, diesel, or propane, and are connected to the main shaft or gearbox by means of chains or belts.

See also


External links


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