The Carnot cycle sets the fundamental upper bound on heat engine efficiency. Explore PV diagrams, TS diagrams, and how temperature differences determine maximum work extraction.
Carnot efficiency: η = 1 − T_C/T_H (maximum possible for any heat engine)
Cycle stages:
1→2: Isothermal expansion at T_H (absorbs Q_H)
2→3: Adiabatic expansion (T drops from T_H to T_C)
3→4: Isothermal compression at T_C (releases Q_C)
4→1: Adiabatic compression (T rises from T_C to T_H)
Second law: No real engine exceeds Carnot efficiency. The entropy of the universe never decreases: ΔS_universe = Q_C/T_C − Q_H/T_H ≥ 0.