Abstract: The invention concerns reciprocating-piston engines, especially hot-gas engines or compressors, wherein thin plates (2,4) having a height corresponding to the piston stroke are arranged within the working chamber on the piston and on the cylinder head and mesh with one another during the entire piston stroke. The heating or cooling medium is conducted outside of the engagement zone of the plates (6) and heats or cools the plates at the cylinder head by heat conduction (arrows 7), wherein the plates on the piston are likewise heated or cooled by direct action, and the operating medium assumes the temperature of the plates by being exposed to strong turbulence in the narrow gaps. Argon is indicated as an economical operating gas for hot-gas engines.