Notice that it depends only on the ratio of the lower and upper temperatures, expressed in absolute or Kelvin form. The smaller this ratio, the larger the thermodynamic efficiency. Heat engines normally work via repeated cycling around some loop such as described above. The particular cycle we have discussed is called the Carnot cycle after the 19th century French scientist Sadi Carnot. Heat is accepted from a high temperature heat source, created, for example, by burning coal in a power plant.
Excess heat is disposed of in the atmosphere or in some source of running water such as a river.
Notice that the ability to get rid of excess heat at low temperature is as important to a heat engine as the supply of heat at a high temperature. This allows the expression of the efficiency simply in terms of the two temperatures. Second, the Carnot cycle is reversible , which means that no net entropy is generated. A Carnot engine running backwards acts as a refrigerator. Heat is extracted at temperature from the box being cooled with the aid of externally supplied work .