A piston compression system converts energy from a conventional combustion cycle engine driving a piston to displace a working gas for flow through a turbine for output power. The working gas is derived by diverting a portion of the charge during combustion at near peak combustion pressure (PCP) into a closed working volume. The working gas is maintained at high pressure within the working volume. The working volume has a first displacement compartment and a second displacement compartment, a supply manifold connected for receiving pressurized working gas alternately from the first and second compartments and connected to an inlet of the turbine, and a return manifold connected to an outlet of the turbine and alternately returning working gas to the second and first compartments. The engine is configured with first and second pistons housed in first and second combustion cylinders respectively powering a first displacing surface for displacement of working gas in the first compartment and a second displacing surface for displacement of working gas in the second compartment.