the combustion gas that would otherwise be wasted by cylinder blow down when an exhaust valve opens on an engine operating according to the conventional four stroke cycle in this embodiment the two isentropic expansion half strokes in the expansion cylinder 14 from 90 degrees to 180 degrees and from 450 degrees to 540 degrees together comprise a volume equal to the displacement of the power cylinder 5 which effects a doubling of the engine expansion ratio as compared to an equivalent conventional four stroke cycle engine fig7 shows the timing of the engine valves effected by the profile of the valve cams 6 7 25 28 29 30 and 31 for the fig1 two cylinder engine embodiment fig7 shows that at 180 degrees there is no opening position overlap between the fig1 power cylinder intake valve cam 6 and power cylinder exhaust valve cams 7 when compound mode is selected this is required during compound mode operation because if both the power cylinder intake valve 42 and the power cylinder exhaust valves 33 were to be open at the same time some of the compressed exhaust gas charge could be lost by reverse flow through the power cylinder exhaust valves 33 through the power cylinder 5 and out the port for the power cylinder intake valve 42 if valve overlap is desired to enhance power output for high engine speed wide open throttle operation while compound mode is de selected a conventional camshaft phase shifting mechanism or a conventional variable cam lift mechanism may be added to modify the timing of the power cylinder intake valve 42 or power cylinder exhaust valves 33 such that power.