This invention provides heat engines based on the structure of internal combustion engines and employs a gaseous working fluid without combustion. The heat engine comprises at least a piston and cylinder assembly and each cylinder has at least an associated heating chamber with a heat exchanger unit being disposed therewithin. The chamber may have at least a chamber valve to establish or block the flow of the gaseous working fluid between the heating chamber and cylinder space. The engine is adapted to operate on cycles that enable heat transfer from a heat source to the working fluid while being enclosed within the heating chamber and provides substantially increased heat transfer duration before the power stroke. Therefore the engine may produce sufficiently high power output with reasonably high thermal efficiency.