Abstract: Stirling cycle heat engine adapted for use as space heating and/or air conditioners. One embodiment features a Stirling engine prime mover providing a hybrid Stirling machine. Another embodiment utilizes an electric drive motor as a prime mover and a final embodiment is an open drive Stirling thermal engine particularly adapted for automotive belt driven applications. Enhancements in performance are provided by using pressure ratios significantly lower than that ordinarily provided for Stirling cycle engines. Decreases in pressure ratios are provided by intentionally adding dead volume to the cycle and particularly adding this dead volume strategically in the regenerator of the device which has been found to provide performance benefits. The systems according to this invention can be used either as space heaters or air conditioners by appropriately directing heat absorbed and rejected from the heat exchangers of the device to the approrpriate environment.