There is disclosed a heat engine 10 comprising: a heat exchanger 12 to transfer heat from a heat source 100 to a working fluid; a positive displacement expander 16 configured to receive inlet working fluid from the heat exchanger 12 and discharge expanded working fluid as a multiphase fluid so that there is an overall volumetric expansion ratio between the expanded working fluid and the inlet working fluid which is a function of an inlet dryness of the inlet working fluid; a variable expansion valve 14 disposed between the heat exchanger 12 and the expander 16, the valve being configured to introduce a variable pressure drop in the working fluid to vary the inlet dryness; and a controller 30 configured to maintain the overall volumetric expansion ratio by controlling the valve 14 to compensate for variable heat transfer to or from the working fluid.