How this calculator works
The Carnot cycle is reversible, so it sets a theoretical ceiling for engines exchanging heat with two reservoirs. Dividing absolute cold temperature by absolute hot temperature gives the heat-rejection fraction. Finite temperature differences, friction and other irreversibilities lower actual efficiency. Equal reservoir temperatures give zero efficiency; 100% would require a zero-kelvin sink and is excluded here.
Formula
eta = 1 - Tc/Th; Th = Tc/(1 - eta); Tc = Th(1 - eta).
Worked example
Between 600 K and 300 K, the ceiling is 50%. To obtain a 25% reversible efficiency with a 300 K sink requires a 400 K hot reservoir.
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Frequently asked questions
Is this an actual engine efficiency?
It is an upper limit. Use measured heat and work for actual performance.
Can I use Celsius?
Yes. Both temperatures are converted to kelvins before their ratio is taken.
Why is 100% rejected?
It would require a zero-kelvin cold reservoir or an unbounded hot temperature, outside this model.