A medium-power, free-piston electric current generator is disclosed in which the piston is slidable in a vertical cylinder open at its top end, with a combustion chamber at its lower end. Induction coils are provided along the length of the cylinder. The piston is formed at least in part of magnetic material and the piston return is effected by the force of gravity. Valves for exhausting combustion gases are provided in the lower part of the cylinder and the opening and closing of the valves is synchronized with the movement of the piston. The cylinder and piston are cooled appropriately. The generator may employ gasoline, heating oil, a gas fuel or explosive substance.