Heat Engine

A device for converting heat into mechanical work. Heat engines have the following characteristics:
  • They receive heat from a high-temperature source.
  • They convert part of this heat to work.
  • They reject the remaining waste heat to a lowtemperature sink such as the atmosphere or a body of water.
  • They operate on a cycle.
  • They usually involve a fluid used in the transfer of heat; this is called the working fluid.

The working heat engine is shown by the circle in the centre,

the hot reservoir is shown at the top and the cold reservoir is shown at the bottom.

If the first law of thermodynamics is applied to one complete cycle and that there is no net change of internal energy,

The thermal efficiency of the engine is defined as:

η = thermal efficiency
|QC| = the total number of units of heat rejected by the system [J]
|QH| = the total number of units of absorbed by the system [J]
|W| = the total number of work done by the system [J]

From this it can be seen that η will approach 1 (100 percent efficiency) when there is no outflow of heat (QC=0). This means that all of the absorbed heat is turned into work done. However, from the second law of thermodynamics "No process is possible whose sole result is the absorption of heat from a reservoir and the conversion of all of this heat into work" - Kelvin-Planck statement.

Turning heat into work is usually accomplished by two types of engine:

  1. External combustion engine

  2. Internal combustion engine
    • gasoline engine
    • diesel engine


William Inchley, "The Theory of Heat Engines", Longmans, Green and Co., 1923

See also: Carnot Limit, Energy Balance, Heat Balance, Heat Pump, Thermal Efficiency, Working Fluid.

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Subjects: Engines Thermodynamics