small engine used to provide power to an aircraft's primary systems (e.g. ventilation and electrical systems) whilst the main engines are shut down.
A small turbine engine used to provide electrical power and bleed air for ground operations only.
a small diesel fuel-powered generator mounted outside the cab that provides the operator heat, air-conditioning, and electrical power to run in cab appliances
A separate motor mounted on a motor vehicle using the fuel supply tank of the motor vehicle.
Equipment, either on-board an aircraft or on the ground, that is used to generate electric power for operating on-board equipment, or for engine starts, when the aircraft is at rest.
Some units have Auxiliary Power Units to supply additional Kreigs. These are secondary engines used solely to produce power, and have no effect on movement. An APU can be any engine type.
An auxiliary power unit (APU) is a relatively small, self-contained generator used in aircraft 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.