Thermodynamic Efficiency of a Heat Engine

The thermodynamic efficiency (η) of a heat engine is the fraction of heat converted to work, where Tc is the temperature of the cold reservoir, and Th is the temperature of the hot reservoir. This efficiency is derived from the Kelvin-Planck statement of the second law of thermodynamics, which asserts that it is impossible for a cyclic process to convert all heat from a single source into work. This formula assumes that temperatures are measured in the same absolute units (e.g., Kelvin).

A practical application of this formula is calculating the theoretical maximum efficiency of thermal power plants or internal combustion engines. The temperatures should be considered absolute temperatures, meaning that they must be in Kelvin or Rankine, as the formula relies on the temperature difference ratio.