In an adiabatic internal combustion engine with a piston movably disposed in a cylinder to which air and fuel are admitted for combustion therein and from which the gases generated during combustion are discharged after extraction of energy therefrom, the cylinder includes a piston guide and seal structure having piston seal rings associated therewith and has a wall structure of heat resistant material and the piston has a guided section adapted to move back and forth through the guide and seal structure and a cap of heat resistant material disposed on top of the guided piston section and having a cylindrical side wall slightly spaced from the heat resistant cylinder wall.