power plants on the basis of heat engines except for reciprocating internal combustion engines and jet propulsion engines are classified by the authors of the paper as non-conventional ones. The article also presents the results of research carried out in this field by scientists of the Institute of Aeronautics of Riga Technical University. The article describes the two patents. The first patent to be considered was received for a rotary-piston engine, the second one – for the Stirling engine.