The paper is devoted to the innovative device consisting of the frees-winging piston internal or external combustion engine and oscillating rotary electrical generator. Advantages and possibilities of such unit are presented to compare with shortcomings of the conventional units, as well as with units of the linear free-piston internal combustion engine and oscillating linear generator. Principles of modelling an simulation of the considered device are presented too. Usage of the proposed unit as a source of energy in the hybrid electric vehicles enables to create competitive hybrid vehicle.