Editing Carnot heat engine (section)

== As a macroscopic construct ==
The Carnot heat engine is, ultimately, a theoretical construct based on an ''idealized'' [[thermodynamic system]].  On a practical human-scale level the [[Carnot cycle]] has proven a valuable model, as in advancing the development of the [[diesel engine]].  However, on a macroscopic scale limitations placed by the model's assumptions prove it impractical, and, ultimately, incapable of doing any [[work (physics)|work]].<ref name=hang>{{Cite journal |last=Liu |first=Hang |last2=Meng |first2=Xin-He |date=2017-08-18 |title=Effects of dark energy on the efficiency of charged AdS black holes as heat engines |url=https://doi.org/10.1140/epjc/s10052-017-5134-9 |journal=The European Physical Journal C |language=en |volume=77 |issue=8 |pages=556 |doi=10.1140/epjc/s10052-017-5134-9 |issn=1434-6052|quote=...since the Carnot heat engine, setting an upper bound on the efficiency of a heat engine is an ideal, reversible engine of which a single cycle must be performed in infinite time which is impractical and so the Carnot engine has zero power.|arxiv=1704.04363 }}</ref> As such, per [[Carnot's theorem (thermodynamics)|Carnot's theorem]], the Carnot engine may be thought as the theoretical limit of macroscopic scale heat engines rather than any practical device that could ever be built.<ref>{{Cite journal |last=Benenti |first=Giuliano |last2=Casati |first2=Giulio |last3=Wang |first3=Jiao |date=2020 |title=Power, efficiency, and fluctuations in steady-state heat engines |url=https://arxiv.org/pdf/2007.08573.pdf |journal=Physical Review E |volume=102 |issue=4 |quote=However, fluctuations [in reservoir temperature] make impractical such engines.}}</ref>

For example, for the [[Isothermal process|isothermal expansion]] part of the Carnot cycle, the following 
''infinitesimal'' conditions must be satisfied simultaneously at every step in the expansion:<ref>{{Cite web|last=D|first=Bob|date=2020-01-15|title=In the isothermal expansion phase of a Carnot cycle, why does the gas expand?|url=https://physics.stackexchange.com/q/525217|access-date=2022-01-02|website=StackExchange}}</ref>

* The hot reservoir temperature ''T<sub>H</sub>'' is infinitesimally higher than the system gas temperature ''T'' so heat flow (energy transfer) from the hot reservoir to the gas is made without increasing ''T'' (via infinitesimal work on the surroundings by the gas as another energy transfer); if ''T<sub>H</sub>'' is significantly higher than ''T'', then ''T'' may be not uniform through the gas so the system would deviate from thermal equilibrium as well as not being a reversible process (i.e. not a Carnot cycle) or ''T'' might increase noticeably so it would not be an isothermal process.
* The force externally applied on the piston (opposite to the internal force on the piston by the gas) needs to be infinitesimally reduced externally. Without this assistance, it would not be possible to follow a gas PV (Pressure-Volume) curve downward at a constant ''T'' since following this curve means that the gas-to-piston force decreases (''P'' decreases) as the volume expands (the piston moves outward). If this assistance is so strong that the volume expansion is significant, the system may deviate from [[thermal equilibrium]], and the process fail to be reversible (and thus not a Carnot cycle).

Such "infinitesimal" requirements as these (and others) cause the Carnot cycle to take an ''infinite amount of time'', rendering the production of work impossible.<ref name=hang/> 

Other practical requirements that make the Carnot cycle impractical to realize include fine control of the gas, and perfect thermal contact with the surroundings (including high and low temperature reservoirs).{{cn|date=March 2024}}