Carnot cycle
/ kar-NOH /
Imagine the most perfect steam engine that could ever exist — one with no friction, no leaks, no wasted heat, running so gently it could be reversed at any moment. The Carnot cycle is that imaginary best-possible engine, sketched out on paper in 1824 by a young French officer, Sadi Carnot, before anyone even had the word 'entropy.'
The cycle has four reversible steps: a gas absorbs heat from a hot reservoir while expanding at constant temperature; it then expands further without exchanging heat, cooling down; it dumps heat to a cold reservoir while being compressed at that low temperature; and finally it is compressed back without heat exchange, warming up to where it began. Two 'isothermal' legs and two 'adiabatic' legs, and the loop closes.
Why it matters: because every step is reversible, the Carnot cycle wrings the absolute maximum work out of a given temperature gap — no real engine can beat it. Its efficiency depends only on the two temperatures, not on the working fluid, which is what first revealed that temperature ratios have a deep, absolute meaning. The caveat: it is a thought experiment. Its infinitely slow steps would produce zero power in practice, so real engines trade some efficiency for actually getting somewhere.
A Carnot engine running between boiling water (373 K) and ice water (273 K) has a ceiling efficiency of 1 − 273/373 ≈ 27%. No cleverness of gears or fuel can push a real engine across those same two temperatures past that line — it is set by the temperatures alone.
Carnot efficiency = 1 − (cold temperature ÷ hot temperature), in kelvin.
All reversible engines working between the same two temperatures have identical efficiency, regardless of what they run on. That surprising fact is Carnot's theorem, and it is what lets the Carnot cycle define an absolute temperature scale.