Variable displacement free piston engine

A variable displacement engine (2) includes a free piston (4) reciprocal in a cylinder (6) between a combustion chamber (8) and a fluid filled work chamber (10). The work chamber has a work output port (12) expelling fluid (14) during the power stroke of the piston. A fluid motor (24) is operatively coupled to the work chamber and driven by fluid from the work output port. A valve (28) is selectively actuated during the power stroke of the piston to stop expulsion of fluid from the work output port to the fluid motor to halt movement of the piston, providing variable displacement according to actuation timing of the valve. A control (72) is coupled to the valve and to the fuel supply (16) to the combustion chamber, and the supplied fuel may be matched to the variable length power stroke of the piston.