connecting rod 140 thereby acting upon the crank shaft 150 and producing rotation lower pressure gas 104 having converted to mechanical energy some of its potential energy from being at a higher pressure inlet gas is expelled through gas outlet 160 reciprocating piston gas engines are well known in the art and may contain single or double acting pistons as well as single or multiple cylinders fig2 a is a schematic that illustrates how a piston type gas expansion engine converts pressure drop from the inlet gas in the piston cylinder into power by driving a piston attached to a rotatable shaft fig2 b is a schematic that illustrates how a conventional turbo expander u201c turbine engine u201d 200 converts pressure drop in the inlet gas 202 flowing through the turbine vanes 265 into mechanical energy through a rotatable shaft 270 attached to the turbine blades turbo expander engines are well known in the art pressurized inlet gas 202 enters inlet 210 passes through vanes 265 converting some of its potential energy to mechanical energy is expelled as lower pressure gas 204 through gas outlet 260 applications of the present invention may be initially divided into two groups open systems and closed systems fig3 illustrates an example of an open system such a system might be located at a natural gas regulation station where natural gas comes in from a high pressure pipeline 300 and exits to a lower pressure distribution network 310 mechanical energy is recovered from the potential energy of the inlet by.