Abstract: The heat efficiency of a piston combustion engine is increased by addition of a steam-gas cycle to the working cycle of the combustion engine. After the working cycle of the engine is finished the combustion products of the engine are again compressed in the cylinder, and water is injected into the cylinder near the end of the working stroke and at the start of the following additional compression stroke. This cools down the combustion products and causes transmission of heat accumulated in the walls of the engine to the mixture of combustion products and water mist, which mixture is expanding in the following expansion stroke.