to thereafter commence the power stroke further supply ports not shown are provided for introducing the fuel and air into the chamber after ignition and an exhaust port may be provided for suctioning out ignited residual gases preferably the volume of the air fuel mixture swept by the central piston 6 within the central cylinder 10 is about twice that swept by each of the outer pistons 4a 4b within the respective outer cylinders 8a 8b for instance the volume swept by the central piston 6 may be 125 cubic units while the volume swept by each of the outer pistons 4a 4b may be 64 cubic units when the 34 bore and stroke 34 ratio of the central piston 6 to each outer piston 4a 4b is 5 4 i e the cube of 5 4 is 125 64 the crank throw ratio would also be 5 4 for the central piston with respect to each outer piston the outer pistons 4a 4b should each sweep with the same overall compression ratio as the central piston the central piston 6 sweeps twice the volume as that swept by each of the outer pistons 4a 4b in this manner the volumetric displacement of each half size chamber 24a 24b is about one half that of the.