Auxiliary power unit

Auxiliary power unit


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

The APU exhaust at the tail end of an Airbus A380

Auxiliary power unit (APU) of a Boeing 737


Functions of APU [/center]




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 or pneumatically 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 motor, with power supplied by a battery 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 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



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


History

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 SOFC fuel cell APUs are being researched.

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