An externally heated thermodynamic engine having a closed working-fluid circuit, the engine comprising: • a working fluid comprising at least two miscible, different boiling point constituent fluids, • a thermodynamic expander for extracting work from vaporised working fluid fed to a feed for it, • a condenser downstream of the expander for condensing expanded vaporised working fluid exhausting from the expander, • a liquid tank downstream from the condenser, • pump means downstream from the liquid tank for pumping out condensed working fluid from it, and • means for heating and at least partially vaporising working fluid pumped to it from the pump means and feeding the heated working fluid to the expander, • the heating means having at least one inlet for working fluid pumped to it and at least one output from which the working fluid is fed to the expander; wherein: • the pump means is adapted to pump from the liquid tank to the heating means both the different boiling point constituent fluids in a determined ratio as liquids; characterized in that • the relative boiling points of the different boiling point constituent fluids are such that in use: • on feeding of the working fluid to the expander, it is in at least partially vaporised state, • vapour and/or liquid of the higher boiling point fluid releases heat energy in the expander to vapour of the lower boiling point constituent fluid for production of work in the expander and • the higher boiling point fluid is liquid on exit from exhaust of the thermodynamic expander. • a working fluid comprising at least two miscible, different boiling point constituent fluids, • a thermodynamic expander for extracting work from vaporised working fluid fed to a feed for it, • a condenser downstream of the expander for condensing expanded vaporised working fluid exhausting from the expander, • a liquid tank downstream from the condenser, • pump means downstream from the liquid tank for pumping out condensed working fluid from it, and • means for heating and at least partially vaporising working fluid pumped to it from the pump means and feeding the heated working fluid to the expander, • the heating means having at least one inlet for working fluid pumped to it and at least one output from which the working fluid is fed to the expander; • the heating means having at least one inlet for working fluid pumped to it and at least one output from which the working fluid is fed to the expander; • the pump means is adapted to pump from the liquid tank to the heating means both the different boiling point constituent fluids in a determined ratio as liquids; characterized in that • the relative boiling points of the different boiling point constituent fluids are such that in use: • on feeding of the working fluid to the expander, it is in at least partially vaporised state, • vapour and/or liquid of the higher boiling point fluid releases heat energy in the expander to vapour of the lower boiling point constituent fluid for production of work in the expander and • the higher boiling point fluid is liquid on exit from exhaust of the thermodynamic expander. • on feeding of the working fluid to the expander, it is in at least partially vaporised state, • vapour and/or liquid of the higher boiling point fluid releases heat energy in the expander to vapour of the lower boiling point constituent fluid for production of work in the expander and • the higher boiling point fluid is liquid on exit from exhaust of the thermodynamic expander.