Abstract: An engine-flywheel hybrid in which the engine power is varied mainly by the frequency of power cycles, not by the power per power cycle. The flywheel absorbs the energy from the power cycles and delivers energy. The rotary stop start mechanism stops and starts a crankshaft efficiently with minimal energy loss. This is accomplished by varying the offset of a double crank four bar linkage. The invention allows an engine's flywheel to be quickly be brought up to speed in about a half of a revolution, followed by combustion, expansion, and stopped one revolution after starting. Further, utilizing offset, the engine crankshaft will turn faster than the input shaft when the piston is at top dead center, shortening the time of highest heat transfer, making the engine more efficient. Also, that when used as an auto engine, the part load efficiency would be near the maximum efficiency of the engine, nearly doubling the miles per gallon of the auto.