A Carnot cycle is an extremely basic set of thermodynamic processes on a simple compressible system, which we will use to understand entropy. It is, in fact, simply a more complete description of the power, refrigeration, and heat pump cycles we have already considered.
We may describe a Carnot cycle on a $p,v$ chart. When the processes of a Carnot cycle occur in a clockwise manner, it is a power cycle. When the process occur in a counter-clockwise manner, it is a refrigeration/heat-pump cycle. The ideal carnot cycle is completely reversible, so the ideal coefficients of performance for the particular thermal reservoir temperatures are obtained.
$p,v$ chart for a Carnot Cycle
Simple Compressible System Undergoing a Carnot Cycle
Process 1-2 is an adiabatic compression from $T_C$ to $T_H$. The piston is retracted, doing work on the ideal gas by compressing it, such that the temperature increases. No heat transfer occurs.
Process 2-3 is an isothermal expansion at $T_H$. The piston is extended, the gas doing work on it, while heat is transferred from the the hot thermal reservoir to keep the temperature constant.
Process 3-4 is an adiabatic expansion from $T_H$ to $T_C$. The piston continues to extend, the gas doing work on it, such that the ideal gas cools from $T_H$ to $T_C$. No heat transfer occurs.
Process 4-1 is an isothermal compression at $T_C$. The piston is retracted back to the original position, doing work on the gas by compressing it, while heat is transferred to the cold reservoir to keep the temperature constant.