The Electric Motor is used initially to start the machine. Thereafter the engine is moved by the air pressure itself and during which time, the engine motor retains the speed by acting as a brake. Since work is done by air in rotating the Flywheel, it loses its heat content (enthalpy). Thus the air gets cooled. This cooling is more than that of an expansion in an expansion valve.
The Expansion Engine can be considered as three major units:
a. The drive unit.
b. The Cylinder Unit for air expansion and
c. The Hydraulic Systems for operating the Valves.

The High Pressure Expansion Engine is used for liquid Plants.