Auxiliary power unit

An auxiliary power unit (English auxiliary power unit, APU ) is an auxiliary power unit, which is mainly used in aircraft.

Aeronautical engineering

The auxiliary power unit is not designed for the propulsion of the aircraft. It usually provides electrical power and compressed air for autonomous operation of the aircraft equipment on the ground, without the main engines have to run for it. This is the auxiliary power unit also required to start. Other systems on the aircraft that can be operated by the APU, the board electrics / electronics ( avionics), the air conditioning and de-icer. The auxiliary engine itself is started either by batteries or when the main engines are still running, by the on-board electrical / pneumatics. The auxiliary power unit is normally switched off after starting the main engines. However, there are exceptions. So it is provided in some situations to use the APU during the start phase for the operation of air conditioning, if a particularly high engine power is needed to start, as air conditioners gobble up a significant proportion of engine power. In some types of aircraft, it also serves as emergency unit. It must be started as soon as the aircraft is too far away from an emergency landing or alternate airport is located ( ETOPS) or at high altitude, ie at low temperatures and thin air.

An auxiliary power unit consists of an internal combustion engine, nowadays usually a gas turbine, used to be an internal combustion engine and a generator and various auxiliary equipment for control. The compact design is achieved speeds - depending on the model - from 22,000 to over 70,000 revolutions per minute. The installation space of the auxiliary power unit is equipped with a fire extinguishing system.

The auxiliary power unit is attached mostly at the rear, recognizable by a small exhaust pipe. When an aircraft has no functional APU on board or to reduce emissions of the operation of the APU is not allowed at an airport, then either ground power units ( engl. "ground power unit ", GPU), ground start equipment (English air starter unit, ASU) and air conditioning car back.

Vehicle Technology

As an auxiliary engine and auxiliary power units are referred to, which are incorporated in modern battle tanks. It can be either small gas turbines, but also piston engines. The APU (eg when M1A2 Abrams 6,5 kW thick) is used to operate the units of the carapace stand, without the (in this example 1119 kW ) main machine must be turned on. This not only saves supplies and wear, on the other hand it reduces the thermal and acoustic signature of the tank significantly. The necessary day-long maintaining a positive pressure in the fight room at ABC - contaminated environment is without APU also inconceivable.

Swell

• Willy JG Brunling: aircraft engines basics. Springer -Verlag, Berlin, 2000, 2004, 2009, 1647 pages online page 61

• Engine technology
• Combustion engine