On Earth , the cold side of any heat engine is limited to the air temperature of the place where the engine is . On Earth , the cold side of any heat engine is limited to being close to the ambient temperature of the environment , or not much lower than 300 Kelvin , so most efforts to improve the thermodynamic efficiencies of various heat engines focus on increasing the temperature of the source , within material limits . The maximum theoretical efficiency of a heat engine ( which no engine ever obtains ) is equal to the temperature difference between the hot and cold ends divided by the temperature at the hot end , all expressed in absolute temperature or kelvins .