APUs are used on a vast number of commercial and military vehicles. Applications include, but are not limited to, aircraft, tanks and other tracked vehicles. The functions of an APU will vary from vehicle to vehicle, but in general most are used as a means to provide electrical power to operate the vehicle's electronic systems at a significantly lower fuel consumption rate than is possible when running the vehicle's main engines. Other APU functions can include charging the vehicle's batteries, operation of the vehicle's hydraulic system, and providing the compressed air sometimes needed to start the vehicle's main engines.
A simplified block diagram of a typical APU system is shown in FIG. 7. An APU generally consists of one or more operator control panels 110, a gas turbine or diesel engine 111, a generator 112, an electronic control unit (ECU) 113 (also called electronic sequencing control unit (ECSU)), and numerous servos, valves, relays, and sensors 114 to control engine operation. Any component of the APU that is individually replaceable in a maintenance operation is commonly referred to as a line replaceable unit (LRU).