Abstract: Each main cylinder (10) of an internal combustion engine is provided with a smaller idle cylinder (12). Pistons (14, 20) in the cylinders (10, 12) are operated substantially in unison and are driven by crank shafts (18, 24) which are mechanically connected (26). A fuel air charge is compressed within the idle cylinder (12). Then, it is ignited within an ignition passageway (60). The ignited mixture is injected into a center region of the combustion chamber (58) of the main cylinder (10). A relatively lean fuel-air mixture is delivered by a duct (54) into the combustion chamber (58) as a swirling stream. Air alone is delivered by duct (56) into the combustion chamber around the swirling fuel-air stream from duct (54). The ignited gases from passageway (60) ignites the fuel air mixture in combustion chamber (58).