Abstract: An improvement for a diesel engine uses a vapor to purge a gap between a piston and cylinder wall. The diesel engine includes at least one cylinder, a piston reciprocatingly mounted in the cylinder, and means for establishing a compression seal between the piston sidewall surface and cylinder wall surface. A chamber is disposed in the piston crown. Means are provided for conducting a liquid into the chamber and containing the liquid under pressure in the chamber. Further provided are means situated in the piston sidewall surface for conducting a vapor between the chamber and the gap between the piston and cylinder wall. In operation, a liquid such as water is conducted into the chamber disposed in the piston crown. The heat generated in the cylinder during the normal operation of the engine is absorbed through the piston crown by the liquid in the chamber.