a propulsion system for a vehicle provides for a drive shaft to be powered by an electric motor or an internal combustion engine , the motor being permanently coupled to the drive shaft and the engine engageable with the shaft by a clutch . engine speed is matched with that of the drive shaft before engagement of the clutch by electrically loading the engine , preferably by coupling a generator to the engine and varying the generator field current . the clutch is automatically engaged when the speeds are equal .