A drive train for a vehicle having a pair of mechanically driven wheels, a pair of hydraulically driven wheels, and a mechanically driven accessory is disclosed. The drive train includes a transmission system connected to the pair of mechanically driven wheels and to the accessory. A first drive source including an internal combustion engine is connected to the transmission system. A second drive source, operating through a source of stored mechanical energy, is also connected to the transmission system. At least one of the first and second drive sources is capable of driving the pair of mechanically driven wheels by driving the transmission system. One or both of the first and second drive sources can be disengaged from the transmission system so that the vehicle's kinetic energy is transmitted through the transmission system to drive the accessory. The internal combustion engine of the first drive source has a main piston with an inertia piston slidable received therein to increase the volumetric efficiency of the engine.