Math 2.0

Classical thermodynamics, heat transfer Definition

Heat engine

Builds on Thermodynamic cycleHeatThermodynamic workFirst law of thermodynamicsKelvin–Planck statement of the second law

Definition

A heat engine is a system that repeatedly performs a thermodynamic cycle such that, in each cycle:

  1. it absorbs a quantity of heat QH>0Q_H > 0 from one or more bodies at higher temperature,
  2. it gives a quantity of heat QC≥0Q_C \ge 0 to one or more bodies at lower temperature, and
  3. the net work WW it does on its surroundings is positive.

Remarks

By the first law of thermodynamics applied to the cycle, W=QH−QCW = Q_H - Q_C. In the simplest case there are two reservoirs, a hot one at temperature THT_H and a cold one at TC<THT_C < T_H. Example: an engine that per cycle takes 1000 J1000\,\mathrm{J} from a boiler, gives 600 J600\,\mathrm{J} to a condenser and does 400 J400\,\mathrm{J} of work meets all three clauses. A non-example: a refrigerator also runs a cycle between two reservoirs, but work is done on it, W<0W < 0, so clause 3 fails. By the Kelvin–Planck statement of the second law, every heat engine has QC>0Q_C > 0.

Reading path

To understand this page from the ground up, read these 12 pages in order. The foundations come first.

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Heat, concept, no evidence levelHeatconceptHeat engine, concept, no evidence levelHeat engineconceptInternal energy, concept, no evidence levelInternal energyconceptState function, concept, no evidence levelState functionconceptTemperature, concept, no evidence levelTemperatureconceptThermodynamic cycle, concept, no evidence levelThermodynamic cycleconceptThermodynamic system, concept, no evidence levelThermodynamic systemconceptThermodynamic work, concept, no evidence levelThermodynamic workconceptFirst law of thermodynamics, statement, no evidence levelFirst law of thermody…statementClausius statement of the second law, statement, no evidence levelClausius statement of…statementKelvin–Planck statement of the second law, statement, no evidence levelKelvin–Planck stateme…statementHeat engine, definition, no evidence levelHeat enginedefinition
Each page uses the pages above it. The colour of the bar on a box shows how strong the evidence is, and the box also gives it as text. The list has the same pages.
  • no evidence level
  1. Heat concept
  2. Heat engine concept
  3. Internal energy concept
  4. State function concept
  5. Temperature concept
  6. Thermodynamic cycle concept
  7. Thermodynamic system concept
  8. Thermodynamic work concept
  9. First law of thermodynamics statement
  10. Clausius statement of the second law statement
  11. Kelvin–Planck statement of the second law statement
  12. Heat engine definition

Further reading

  1. Web https://en.wikipedia.org/wiki/Heat_engine
About this page
Tag1J2QVS4CK6
URIm2://1J2QVS4CK6
Kinddefinition
WritingW0 stub
MSC80A10, 80A05
DefinesHeat engine
UsesThermodynamic cycle, Heat, Thermodynamic work, First law of thermodynamics, Kelvin–Planck statement of the second law
Changed3 November 2026

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