Britannica Sources used in this answer

How does entropy limit the efficiency of a heat engine?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Logs burning in a fireplace with bright flames. — heat What's the relationship between heat and temperature?

Entropy limits the efficiency of a heat engine by dictating that some heat must always be exhausted into the environment, preventing 100 percent conversion of heat into work.123

According to the second law of thermodynamics, the net entropy change of a system and its reservoirs must be greater than or equal to zero. A heat engine operating in a cycle absorbs heat $Q_{1}$ from a high-temperature reservoir and exhausts heat $Q_{2}$ to a lower-temperature reservoir. If an engine were to convert all heat into work ( $Q_{2} = 0$ ), the net entropy change would be negative, which is physically impossible.132

The fundamental limit on efficiency is defined by the condition where the net entropy change is zero, known as a reversible process. This leads to the maximum theoretical efficiency, which depends on the temperatures of the reservoirs. Because real-world factors like friction and imperfect insulation further increase entropy, actual efficiency is always lower than this theoretical maximum.13