I am currently building a rotary thermal motor prototype for a hybrid car. In a sealed cylinder what looks like a rotary vane pump operates as a simple heat engine and the torque is leveraged by the condensation and boiling of water. The compression ratio is set to allow the wet saturated vapor and any remaining steam to easily transfer back to the starting point. Heat stored in a thermal battery or directly from an engine is used to power the unit. Then because it is also a heat exchanger multiple units could be installed in series and parallel. With any luck the mileage will more than double.