What is described as the most efficient engine possible in thermodynamics?

The Carnot cycle is described as the most efficient engine possible in thermodynamics because it serves as a theoretical benchmark for the efficiency of all heat engines. The significance of the Carnot cycle lies in its foundation on two key principles: it operates between two temperature reservoirs, allowing for maximum energy extraction from the heat input while minimizing waste heat.

In the Carnot cycle, the working fluid undergoes reversible processes (isothermal and adiabatic) which ensures that there are no irreversibilities or losses associated with friction, turbulence, or heat transfer inefficiencies. This theoretical cycle achieves the maximum possible efficiency, defined by the ratio of work output to heat input, as given by the temperature difference between the hot and cold reservoirs. Specifically, its efficiency can be expressed mathematically as 1 minus the ratio of the absolute temperatures of the cold reservoir to the hot reservoir.

Though practical engines, such as those based on the Otto cycle, Miller cycle, or Brayton cycle, are used in various applications, they encounter inefficiencies due to real-world factors that do not allow them to achieve Carnot efficiency. Thus, the Carnot cycle remains a crucial reference point for understanding the limits of thermal efficiency in engines.