Carnot efficiency defines the maximum possible efficiency that any heat engine can achieve when operating between two thermal reservoirs — a hot source and a cold sink.
Key principles:
- It was formulated by Sadi Carnot in the 19th century.
- The Carnot efficiency depends only on the temperatures of the hot and cold reservoirs (measured in absolute units like Kelvin).
- No real engine can exceed or even reach this efficiency due to irreversibilities (like friction and heat loss).
Why it matters:
Carnot efficiency provides a benchmark for all real-world heat engines. It shows that to improve engine efficiency, one must either increase the hot reservoir temperature or lower the cold reservoir temperature — but both come with practical limits. This principle underscores the second law of thermodynamics and the inevitable loss of usable energy in heat transfer.