The efficiency of an engine is defined as the ratio of the work done by the engine to the heat extracted from the hot reservoir, so Efficiency = 1 – Q_cold/Q_hot,It can be shown that the maximum possible efficiency for an engine operating between two temperatures is for the Carnot engine which utilizes the Carnot cycle (to be covered more
 detailed in Physics 2) and this efficiency is given by Eff_MAX =1 – Q_cold/Q_hot , but no real engine operates at this ideal maximum efficiency.

Efficiency=1- Q_Cold /Q_Hot
 Where,

• Q_Hot = Heat input at high temperature
•  Q_Cold= Heat rejected at low temperature

Notes

In the Carnot engine, the heat that comes from the heat source is supplied at a constant temperature T_h. Meanwhile, the rejected heat goes into the heat sink, which is at a constant temperature T_c. Because the heat source and the heat sink are always at the same temperature, you can say that the ratio of the heat provided and rejected is the same as the ratio of those temperatures (expressed in kelvins)

epsilon =1−Q_Cold/QHot×100
 The above equation is multiplied by 100 to express the efficiency as percent.