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Thermodynamics... one of the most subtle branches of science, and often counter-intuitive. But not as bad in that respect as aerodynamics.
You might think that a liquid with a low boiling point- preferably just above ambient temperature- would be ideal for use in a heat engine. Less thermal energy would be required to boil it and produce a vapour under pressure that could drive some kind of motor. However, you would be wrong. It has been known since Frenchman Sadi Carnot published his famous book in 1824 that the efficiency of any thermodynamic cycle, no matter what the working fluid used, increases as the temperature difference between the temperature at which heat is put in (as in the boiler of a steam engine) and the temperature at which it is taken out (as in a condenser) is increased.
Where Th is the input temperature and Tc the heat output temperature. (Absolute temperatures, in degrees Kelvin)
The Carnot efficiency is a maximum limit. In practice efficiency is always lower.
This is sometimes called Carnot's Law: see here for much more info. (External site)