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.
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Aircraft
An aircraft APU is a relatively small gas turbine used to produce shaft power 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 through a generator installed on the gearbox. 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. 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 (Extended-range Twin-engine Operations) 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 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 route. 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. 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 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. 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.
Armor
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.
Commercial Vehicles
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, heating system, generator or alternator system, and air conditioning compressor, mounted to one of the frame rails of a semi-truck along with an internally mounted inverter in some cases. 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 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 already provide shore power connections in their parking areas. 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 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.
External links
- "Armor-plated auxiliary power" design of a modern gas turbine APU
- "Space Shuttle Orbiter APU"
