The Carnot cycle is an idealized thermodynamic cycle that provides the maximum possible efficiency for a heat engine.

• The efficiency ($\eta$) of a Carnot engine is given by: $\eta =1-\frac{T_{cold}}{T_{hot}}$ where $T$ must be in Kelvin.
• Significantly, this efficiency is independent of the working substance (it doesn't matter if it's air, steam, or an ideal gas) and depends only on the temperatures of the source and the sink.