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.
Functions of APU
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 electrica
lly, pneumatically, or hydraulically powered, however, the energy required is far greater than 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 enough 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 pn
eumatic power 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.
Sections of APU
A typical gas turbine APU for commercial transport aircraft comprises three main sections:
The power section is the gas generator portion of the engine a
nd produces all the shaft power for the APU. The load compressor is generally a shaft-mounted compress
or 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.
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 a pneumatic system simplifies the design, but the demand for hundreds of kilowatts (kW) of electricity requires heavier generators and unique system requirements.
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.