Abstract: An open-cycle internal-combustion Stirling engine having two pistons coupled by a rhombic drive which define a combustion chamber and a compression chamber, within either a single cylinder or two cylinders, and a manifold enabling flow of the working fluid between the compression chamber and the combustion chamber with a plurality of engine valves controlling such flow, the dead space of the manifold being minimized and provision of the working fluid (i.e., air) being provided by an intake valve and an exhaust valve with corresponding manifolds for providing the open-cycle characteristics of the engine while approximating an ideal thermo-dynamic system.