What is the primary function of the combustion and turbine assembly in an APU?

The primary function of the combustion and turbine assembly in an Auxiliary Power Unit (APU) is to convert heat energy into torque. In an APU, the combustion assembly ignites fuel, releasing combustion gases that generate heat. This heat energy is then utilized by the turbine assembly, which extracts energy from the hot gases and converts it into mechanical energy, or torque. This torque drives an output shaft that powers various systems, such as electrical generators or hydraulic pumps, providing necessary auxiliary power for the aircraft when the engines are not running.

This conversion process is essential because it allows the APU to efficiently generate power without needing the main engines to be operational. By focusing on how heat energy from combustion is transformed into usable mechanical energy, one can understand the fundamental operational role of the combustion and turbine assembly in supporting various aircraft functions.