Abstract: An engine system using refrigerant fluid is capable of utilizing the heat produced by an external high-efficiency hydrocarbon fuel combustion process. The heat from that process is utilized to transform the refrigerant fluid from a liquid state to a gaseous state in a cycle which includes extracting work from the fluid in the gaseous state in a high-compression-ratio piston engine. The cycle further includes transforming the fluid in the gaseous state back to the liquid state in a condenser, and then feeding that fluid under pressure to a heating chamber where the combustion process heat again returns it to the gaseous state at high pressure and temperature. The engine system has a higher efficiency than hydrocarbon fuel combustion engines, and has particular application to use in automobiles. One preferred refrigerant fluid for this engine system is 2,2,Dichloro-1,1,1,Trifluoro-Ethane.