Abstract: A closed cycle heat engine is provided. The heat engine includes first and second expansion pistons that are fluidly coupled to a heater. The expansion pistons are also fluidly and operably coupled to a compression piston. A regenerator extracts heat from a working fluid flowing from the expansion cylinders to the compression cylinders to preheat the working fluid flowing to the heater. A cooler is arranged in between the regenerator and the compression cylinder to remove additional heat before the working fluid reaches the compression cylinder. The heat engine is arranged such that the working fluid travels unidirectionally within the engine. The heat engine may further include one or more actuated valves for controlling the flow of the working gas. In one embodiment, the pressure of the crankcase is controlled to be at or less than the heat engine minimum pressure.