Abstract: A cylinder has a head and a bottom. A piston is reciprocable in the cylinder and defines a power chamber with said head and an air compression chamber with said bottom. Controlled intake and exhaust ports are formed in said cylinder and adapted to communicate with said power chamber. Igniting means are operable to initiate a combustion of a fuel-air mixture in the power chamber. An inlet is formed in the cylinder and adapted to communicate with said compression chamber. A valve controls said inlet. The piston is adapted to perform in response to the combustion a power stroke toward a dead center near the bottom, whereby air is compressed in the compression chamber. Transfer means are provided to transfer compression air from said compression chamber to said exhaust port after said power stroke.