NOVELTY - The engine (1) has a first flow channel (2) communicating between an enclosure (20) and an outer side of the enclosure that include metallic bellows. A second flow channel communicates between another enclosure (30) and an outer side of the latter enclosure. A third flow channel communicates between the two enclosures. A heat exchanger heats or cools gas working fluid circulating in the third flow channel. A control unit (9) controls opening and closing of the three flow channels according to a predetermined operation cycle.    USE - Alternative heat engine for use as two-stage variable displacement compressor or a cogeneration system (all claimed) for producing electricity from recovered heat. The alternative engine may be used in a micro-cogeneration system using external heat from burning wood for small residential, commercial and industrial uses for locally producing electricity from biomass, but may also be used in large-size installations for collective use, without excluding the exploitation of fossil fuels.    ADVANTAGE - The engine maintains maximum efficiency of the thermodynamic working cycle by providing potential to exploit maximum pressures and temperatures of the cycle regardless of an operating point of the engine, and provides effective displacement variation of power induced by the amplitude variation in a rapid manner while optimizing primary energy consumption. The metallic bellows provide a large thermodynamic heat exchange surface and low thermal conductivity so as to limit heating of the fluid during fluid compression in the enclosure and support the heating of the fluid at the time of fluid relaxation in another enclosure. The design of the bellows improves heat exchange by increasing coolant path for heating or cooling the fluid between inlet and outlet channels of the bellows and reduces obstruction of the channels while preserving better homogeneity of heat transfer through walls of the bellows.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for: a two-cycle compressor; an engine/compressor including the alternative heat engine; and a cogeneration system using the alternative heat engine or the engine/compressor.    DESCRIPTION OF DRAWING(S) - The drawing shows a sectional view of an alternative heat engine.    Alternative heat engine (1)    Flow channel (2)    Control unit (9)    Enclosures (20, 30)    External guides (61)