Carnova

Meet Carnova, the ideal heat engine. Even she has to pay a tax to the cold side.

A heat engine takes heat from a hot reservoir, turns part of it into work, and dumps the rest into a cold reservoir. In 1824 Sadi Carnot showed there is a ceiling on how well this can be done. No engine running between temperatures T_h and T_c can have an efficiency higher than 1 − T_c/T_h, with both temperatures in kelvin. Only an idealised reversible engine reaches that limit.

Example: steam at 500 K with cooling at 300 K gives a maximum of 1 − 300/500 = 0.40, or 40 percent. The other 60 percent of the heat must go to the cold side, whatever the design.

The limit explains why power plants run their steam as hot as materials allow, and why car engines waste much of their fuel as heat.

Real engines fall well short of Carnot's limit because of friction and heat leaks, and because a truly reversible cycle would run infinitely slowly and deliver no power.