Abstract: An ignition plug provided with a tubular center electrode through which open air is introduced into the engine cylinder to accelerate exhaust of the combustion gas from the cylinder in the exhaust stroke of an engine cycle. The tubular electrode has a double-check valve fitted therein to prevent leakage of the high-pressure gas from the cylinder when the cylinder pressure is above the ambient pressure. The valve is composed of two differently-weighted floats serially disposed in the center electrode, one of the floats being in operative position even if the other resonates with engine vibration.