A method for operating a four-stroke internal combustion engine and a four-stroke combustion engine configured to be operated by the same are disclosed. The method comprises injecting compressed gas into a cylinder during the exhaust stroke of the combustion cycle. In another aspect of the invention, the injection occurs at a time in the combustion cycle near the end of the exhaust stroke, the cylinder approaching Top Dead Center. In a still further aspect of the invention, the gas comprises air.