When intended to transform thermal energy in mechanical energy, the thermal efficiency is the percentage of energy that is transformed (in work). For example, when 1000 J of thermal energy is transformed in 300 J of mechanical energy (remaining 700 J as heat), the thermal efficiency will be 30%. Its maximum possible value is known as thermodynamic efficiency. The thermal efficiency of a heat engine is the efficiency in which the chemical energy of a fuel is turned into useful work. It can be determined by dividing the useful work by the higher heating value of the fuel. It is a number smaller than 100%.

Thermodynamics | Energy conversion