Liquid fuel and water pressurized by fuel and water pumps are forced into aaporizer which is heated by hot exhaust gases of a piston engine. The resulting vapor mixture is blown into the combustion chamber of a cylinder head of the engine at a speed up to that of sound near the end of the engine's compression stroke where it mixed with precompressed air. The resulting explosive mixture is ignited by self-ignition or by spark plug and the water vapor content mixed with the fuel vapor lowers the combustion temperature, resulting in a significant reduction of nitrogen oxide content in the exhaust gases. The ratio of water vapor to fuel vapor is preferably 1:1 to 3:1 and the temperature at which the vapor mixture is introduced into the cylinder is above the dew point of the fuel in the fuel vapor and water vapor mixture. The mixture of fuel with air is approximately stoichiometric. Diesel oil is useable, and also such bio-mass fuels as vegetable oils.