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A Carnot heat engine[2] is a theoretical engine that operates on the Carnot cycle. The basic model for this engine was developed by Nicolas Léonard Sadi Carnot in 1824. The Carnot engine model was graphically expanded by Benoît Paul Émile Clapeyron in 1834 and mathematically explored by Rudolf Clausius in 1857, work that led to the fundamental thermodynamic concept of entropy.

Every thermodynamic system exists in a particular state. A thermodynamic cycle occurs when a system is taken through a series of different states, and finally returned to its initial state. In the process of going through this cycle, the system may perform work on its surroundings, thereby acting as a heat engine.

A heat engine acts by transferring energy from a warm region to a cool region of space and, in the process, converting some of that energy to mechanical work. The cycle may also be reversed. The system may be worked upon by an external force, and in the process, it can transfer thermal energy from a cooler system to a warmer one, thereby acting as a refrigerator or heat pump rather than a heat engine.

(9)

where ${\displaystyle \eta ={\frac {W}{Q_{\text{H}}}}}$

(9)

where ${\displaystyle \eta ={\frac {W}{Q_{\text{H}}}}}$ is the efficiency of the real engine, and ${\displaystyle \eta _{\text{I}}}$ is the efficiency of the Carnot engine working between the same two reservoirs at the temperatures ${\displaystyle T_{\text{H}}}$ and ${\displaystyle T_{\text{C}}}$. For the Carnot engine, the entire process is 'reversible', and Equation (7) is an equality.

Hence, the efficiency of the real e

Hence, the efficiency of the real engine is always less than the ideal Carnot engine.

Equation (7) signifies that the total entropy of the total system (the two reservoirs + fluid) increases for the real engine, because the entropy gain of the cold reservoir as ${\displaystyle Q_{\text{C}}}$ flows into it at the fixed temperature ${\displaystyle T_{\text{C}}}$, is greater than the entropy loss of the hot reservoir as ${\displaystyle Q_{\text{H}}}$ leaves it at its fixed temperature ${\displaystyle T_{\text{H}}}$. The inequality in Equation (7) is essentially the statement of the Clausius theorem.

According to the second theorem, "The efficiency of the Carnot engine is independent of the nature of the working substance".