A heat transfer engine having cooling and heating modes of reversible operation, in which heat can be effectively transferred within diverse user environments for cooling, heating and dehumidification applications. The heat transfer engine of the present invention includes a rotor structure which is rotatably supported within a stator structure. The stator has primary and secondary heat exchanging chambers in thermal isolation from each other. The rotor has primary and secondary heat transferring portions within which a closed fluid flow circuit is embodied. The closed fluid flow circuit within the rotor has a spiraled fluid-return passageway extending along its rotary shaft, and is charged with a refrigerant which is automatically circulated between the primary and secondary heat transferring portions of the rotor when the rotor is rotated within an optimized angular velocity range under the control of a temperature-responsive system controller.