Editing Heat engine (section)

{{Short description|System that converts heat or thermal energy to mechanical work}}
{{See also|Thermodynamic cycle}}
[[File:heat engine.png|thumb|upright=1.0|Figure 1: Heat engine diagram]]
{{Use dmy dates|date=April 2022}}
{{Thermodynamics sidebar|cTopic=[[Thermodynamic system|Systems]]}}

A '''heat engine''' is a system that transfers [[thermal energy]] to do [[work (physics)|mechanical]] or [[voltage#Definition|electrical work]].<ref>''Fundamentals of Classical Thermodynamics'', 3rd ed. p. 159, (1985) by G. J. Van Wylen and R. E. Sonntag: "A heat engine may be defined as a device that operates in thermodynamic cycle and does a certain amount of net positive work as a result of heat transfer from a high-[[temperature]] body to a low-temperature body. Often the term heat engine is used in a broader sense to include all devices that produce work, either through heat transfer or combustion, even though the device does not operate in a thermodynamic cycle. The internal-combustion engine and the gas turbine are examples of such devices, and calling these heat engines is an acceptable use of the term."</ref><ref>''Mechanical efficiency of heat engines'', p. 1 (2007) by James R. Senf: "Heat engines are made to provide mechanical energy from thermal energy."</ref> While originally conceived in the context of mechanical energy, the concept of the heat [[engine]] has been applied to various other kinds of energy, particularly [[electrical energy|electrical]], since at least the late 19th century.<ref>{{cite journal |last1=Kenelly |first1=A.E. |author1-link=Arthur E. Kennelly |title=Discussion of 'Thermo-electric and galvanic actions compared' |journal=Journal of the Franklin Society |date=Dec 1898 |volume=CXLVI |page=442 |url=https://books.google.com/books?id=nXooAQAAIAAJ&pg=PA442}}</ref><ref>{{cite journal |last1=Laurie |first1=Arthur Pillans |author1-link=Arthur Pillans Laurie |title=Faraday society |journal=The Electrical Review |date=17 Jan 1914 |volume=72 |issue=1834 |page=90 |url=https://books.google.com/books?id=fko1AQAAMAAJ&pg=PA90 |access-date=11 February 2023}}</ref> The heat engine does this by bringing a [[working substance]] from a higher state temperature to a lower state temperature. A heat source generates thermal energy that brings the working substance to the higher temperature state. The working substance generates work in the [[thermodynamic system|working body]] of the engine while [[heat transfer|transferring heat]] to the colder [[thermal reservoir|sink]] until it reaches a lower temperature state. During this process some of the thermal energy is converted into [[energy|work]] by exploiting the properties of the working substance. The working substance can be any system with a non-zero [[heat capacity]], but it usually is a gas or liquid. During this process, some heat is normally lost to the surroundings and is not converted to work. Also, some energy is unusable because of [[friction]] and [[drag (physics)|drag]].

In general, an engine is any [[machine]] that converts [[energy]] to mechanical [[work (physics)|work]]. Heat engines distinguish themselves from other types of engines by the fact that their efficiency is fundamentally limited by [[Carnot's theorem (thermodynamics)|Carnot's theorem]] of [[thermodynamics]].<ref>''Thermal physics: entropy and free energies'', by Joon Chang Lee (2002), Appendix A, p. 183: "A heat engine absorbs energy from a heat source and then converts it into work for us.... When the engine absorbs heat energy, the absorbed heat energy comes with entropy." (heat energy <math>\Delta Q=T \Delta S</math>), "When the engine performs work, on the other hand, no entropy leaves the engine. This is problematic. We would like the engine to repeat the process again and again to provide us with a steady work source. ... to do so, the working substance inside the engine must return to its initial thermodynamic condition after a cycle, which requires to remove the remaining entropy. The engine can do this only in one way. It must let part of the absorbed heat energy leave without converting it into work. Therefore the engine cannot convert all of the input energy into work!"</ref> Although this efficiency limitation can be a drawback, an advantage of heat engines is that most forms of energy can be easily converted to heat by processes like [[exothermic reaction]]s (such as combustion), [[nuclear power|nuclear fission]], [[absorption (electromagnetic radiation)|absorption]] of light or energetic particles, [[friction]], [[dissipation]] and [[electrical resistance|resistance]]. Since the heat source that supplies thermal energy to the engine can thus be powered by virtually any kind of energy, heat engines cover a wide range of applications.

Heat engines are often confused with the cycles they attempt to implement. Typically, the term "engine" is used for a physical device and "cycle" for the models.