Abstract: An engine is provided which converts pressurized gas into useful mechanical energy. The engine uses pressurized gas in such a way to avoid contact between the gas and sliding metal parts. As such, the engine is particularly adapted for use with naturally pressurized geothermal gases which may contain substances harmful to metal. In the preferred form, the engine includes a plurality of reciprocating plungers which serve as energy transfer means. Deformable enclosures such as spheroidal hollow balls made of an elastomer material are positioned on opposite sides of each plunger along its axis of movement. The deformable enclosures exert a force against the plungers to move the plungers when filled with pressurized gas. Each plunger alternately collapses each of the deformable enclosures as it reciprocates. A valve system for controlling the ingress and egress of pressurized gas to the deformable enclosures is responsive to the motion of the plungers.