Rankine cycle systems, using steam as a working fluid, are not well suited to the recovery of power from heat sources in the 300–450 °C temperature range, such as internal combustion engine exhaust gases, mainly due to the relatively large enthalpy of vaporization of water. Admitting the steam to the expander as vapour approximately 50 per cent dry, would be preferable but turbines cannot be used to expand vapours from this state. However, screw expanders can operate well in this mode. It is shown that, apart from being environmentally benign and free from flammability risks, a screw-driven wet steam cycle system can recover power from engine exhaust gases, with comparable efficiencies to turbine-driven systems using organic fluids at a significantly lower cost per unit output.