To provide appropriate generating power corresponding to a required power generation quantity.

Electric power supply to a travel motor 1 for driving drive wheels 2R, 2L is done by a first generator 12 and a second generator 13. The first generator 12 is established to show good power generation efficiency in high speed operation. The second generator 13 functions as a weight for changing inertia mass of pistons 41, 42 of a free piston engine 11 and is established to show good power generation efficiency in low speed operation. The second generator 13 is connected to and disconnected from the pistons 41, 42 via a clutch 14. Inertia mass of the pistons 41, 42 are increased by connecting the clutch 14, the engine is operated at low speed and electric power is efficiently generated by the second generator 13 when demand electric power is low. Inertia mass of the pistons 41, 42 are decreased by disconnecting the clutch 14, the engine is operated at high speed and electric power is efficiently generated by the first generator 12 when demand electric power is high.

COPYRIGHT: (C)2008,JPO&INPIT