What is a Carnot cycle?

(N/A) Carnot cycle is a theoretical thermodynamic cycle proposed by Nicolas $L$éonard Sadi Carnot in $1824$. It describes the most efficient possible heat engine operating between two heat reservoirs at temperatures $T_H$ (hot) and $T_C$ (cold). The cycle consists of four reversible processes:
$1$. Reversible isothermal expansion: The gas expands at a constant high temperature $T_H$,absorbing heat $Q_H$ from the source.
$2$. Reversible adiabatic expansion: The gas continues to expand without heat exchange,and its temperature drops from $T_H$ to $T_C$.
$3$. Reversible isothermal compression: The gas is compressed at a constant low temperature $T_C$,rejecting heat $Q_C$ to the sink.
$4$. Reversible adiabatic compression: The gas is compressed without heat exchange,and its temperature rises from $T_C$ back to $T_H$,completing the cycle.