Engine having external combustion chamber

A method of making electricity and recovering the compression energy in an engine comprising:

• supplying compressed natural gas from a high pressure main line to a combustion chamber;
• supplying compressed air from a high pressure air vessel to the combustion chamber from the high-pressure air vessel;
• burning said fuel and air in said combustion chamber to form a compressed combustion gas;
• opening an intake valve and supplying said compressed combustion gas to a positive displacement chamber containing a reciprocating piston such that said compressed combustion gas expands forcing said piston in a direction that increases the volume of the positive displacement cylinder to form an expanded gas;
• closing said intake valve and opening an exhaust valve and allowing the expanded gas to exit said displacement chamber while said piston is moving in a direction which decreases the volume of the positive displacement chamber to provide a exhaust gas ad thereby produce rotational energy;
• and
• driving an electrical generator with said rotational energy to produce electricity.