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Heat Engine Efficiency Formula

Under Thermodynamics, Heat engine is a system that converts heat energy and chemical energy into mechanical energy. This occurs by bringing a working substance from higher temperature state to a lower temperature state.  

The heat engine efficiency is the measure of energy which can be converted to work as a fraction of the total energy. The mathematical representation of an equation for calculating the efficiency is expressed by

The heat engine efficiency is a measure of the energy that can be converted to work as a fraction of the total energy. The concept can be expressed mathematically in an equation for calculating efficiency. Accordingly the heat engine efficiency formula is given by

η = (Th − Tl / Th )


η = Heat Engine Efficiency

Th = Higher temperature

Tl = Lower temperature state

The numerical value of efficiency is a fraction having a value between 0 and 1. Sometimes it is more convenient to express efficiency as a percentage rather than a fraction. This is done by applying the comparable formula.

%η = 100 (Th−Tl / Th)

Here, the efficiency is a value between 0% and 100%. Both the equations are equivalent and represent the same information. The ratio of the available energy to the total energy describes the maximum efficiency of a heat engine.

Example 1

Water near the surface of a tropical ocean has a temperature of 298.5 K, whereas the temperature of the water 700m underground is 290 K. Determine the efficiency of such an engine.


Given parameters are,

Th = 298.5 K

TI = 290 K

The maximum efficiency is the efficiency that an engine would possess operating between these two temperatures.

Substitute the value in the given formula, we get

η = (Th − Tl / Th)

   = (298.5 − 290 / 298.5)

Therefore, maximum efficiency, η = 0.028 (2.8%)

Example 2

The minimum power to drive a heat pump which maintains a house at 30oC is 5KW. If the temperature outside is 5oC, calculate the amount of heat the house loses per minute.


The given parameters are

Th= 303.15 K

TI= 278.15 K

The heat efficiency formula is given by,

η = (Th − Tl / Th)

  = (303.15 − 278.15 / 303.15)

Therefore, η = 0.082 (8.2%)