Carnot Engine
The most efficient heat engine possible between two temperatures. Four reversible stages trace a closed loop on the PV diagram — the theoretical ceiling that no real engine can exceed.
Thermodynamics
The four stages
1. Isothermal expansion — Gas absorbs heat QH from the hot reservoir at temperature TH and expands slowly. Temperature stays constant; pressure drops as volume increases. PV = nRTH.
2. Adiabatic expansion — No heat exchange. The gas continues expanding, doing work, and its temperature drops from TH to TC. The curve follows PVγ = const.
3. Isothermal compression — The gas is compressed at TC, rejecting heat QC to the cold reservoir. Temperature stays at TC.
4. Adiabatic compression — No heat exchange. Compression raises the temperature back to TH, completing the cycle.
The efficiency is η = 1 − TC/TH, depending only on the two reservoir temperatures. This is the maximum efficiency any heat engine can achieve — the second law of thermodynamics in action.