Abstract: A double-acting tandem free-piston engine comprising a housing including a cylinder having combustion chambers at opposite ends and one in its center with double-acting pistons displaceable between one end and the center chamber and between the other end and the center chamber. Each piston includes opposite piston heads with connecting rods between them together with balancing means to provide symmetrical piston movement. Each combustion chamber has a inlet port and valve and an exhaust valve, the valves timed to open and close to produce a pressure volume relationship therein wherein the pressure drops below atmospheric during at least a portion of the piston's displacement. The engine combined with a linear alternator in a hybrid vehicle powers electric wheel driving motors and stores power in its storage battery.