To operate an engine in an optimum range while driving a vehicle by the engine and power from both power accumulating source of a battery and a flywheel.

This compound drive system consists of a differential generator that generates electric power by the relative rotations of an input shaft interlocked with the output shaft of the engine and the flywheel, and another differential generator that generates electric power by the relative rotations of the flywheel and the output shaft interlocked with the drive wheels of the vehicle.