A piston for an internal combustion engine is provided with a reaction chamber for generating radical fuel species during a combustion cycle for use during the next succeeding combustion cycle. The reaction chamber is located adjacent a piston crown recess and is in communication with the main combustion chamber of the engine through discrete orifices that are intended to separately control the supply of fuel and air to the reaction chamber for fuel injected engines and to enhance reaction chamber function in fuel aspirating engines. The discrete fuel and air control orifices are particularly beneficial for use in high swirl, direct injected combustion chambers.