The Expansion Engine is a vertical, Single acting, reciprocating type of engine. This produces the cold required for the plant. High-pressure air enters through inlet valve at the start of downward stroke of the piston. On further downward motion, the inlet valve closes and entrapped air expands. During upward stroke, Outlet valve remains open and inlet valve remains closed.

Therefore, in downward stroke air enters the cylinder and expands. In the upward stroke the expanded air is pushed outside. The electric motor is used 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.