Editing Stirling engine (section)

== Efficiency ==
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Theoretical [[thermal efficiency]] equals that of the ideal [[Carnot cycle]], i.e. the highest efficiency attainable by any heat engine. However, though it is useful for illustrating general principles, practical Stirling engines deviate substantially from the ideal.<ref name="Romanelli-2017" /><ref name="Romanelli-2024" /> It has been argued that its indiscriminate use in many standard books on engineering thermodynamics has done a disservice to the study of Stirling engines in general.<ref name="Finkelstein-2001-66-229" /><ref name="Organ-1992-3.1-3.2" />

Stirling engines cannot achieve total efficiencies typical of an [[internal combustion engine]], the main constraint being thermal efficiency. During internal combustion, temperatures achieve around 1500&nbsp;°C–1600&nbsp;°C for a short period of time, resulting in greater mean heat supply temperature of the thermodynamic cycle than any Stirling engine could achieve. It is not possible to supply heat at temperatures that high by conduction, as it is done in Stirling engines because no material could conduct heat from combustion in that high temperature without huge heat losses and problems related to heat deformation of materials.{{citation needed|date=January 2022}}

Stirling engines are capable of quiet operation and can use almost any heat source. The heat energy source is generated external to the Stirling engine rather than by internal combustion as with the [[Otto cycle]] or [[Diesel cycle]] engines. This type of engine is currently generating interest as the core component of [[micro combined heat and power]] (CHP) units, in which it is more efficient and safer than a comparable steam engine.<ref name="Organ-2007" /><ref name="Starr-2001" /> However, it has a low [[power-to-weight ratio]],<ref name="mpower" /> rendering it more suitable for use in static installations where space and weight are not at a premium.

Other real-world issues reduce the efficiency of actual engines, due to the limits of [[Convection (heat transfer)|convective heat transfer]] and [[Fluid dynamics#Viscous vs inviscid flow|viscous flow]] (friction). There are also practical, mechanical considerations: for instance, a simple kinematic linkage may be favoured over a more complex mechanism needed to replicate the idealized cycle, and limitations imposed by available materials such as [[ideal gas|non-ideal]] properties of the working gas, [[thermal conductivity]], [[tensile strength]], [[creep (deformation)|creep]], [[Flexural strength|rupture strength]], and [[melting point]]. A question that often arises is whether the ideal cycle with isothermal expansion and compression is in fact the correct ideal cycle to apply to the Stirling engine. Professor C. J. Rallis has pointed out that it is very difficult to imagine any condition where the expansion and compression spaces may approach [[isothermal]] behavior and it is far more realistic to imagine these spaces as [[adiabatic]].<ref name="Rallis-IECEC" /> An ideal analysis where the expansion and compression spaces are taken to be [[adiabatic]] with [[isothermal]] heat exchangers and perfect regeneration was analyzed by Rallis and presented as a better ideal yardstick for Stirling machinery. He called this cycle the 'pseudo-Stirling cycle' or 'ideal adiabatic Stirling cycle'. An important consequence of this ideal cycle is that it does not predict Carnot efficiency. A further conclusion of this ideal cycle is that maximum efficiencies are found at lower compression ratios, a characteristic observed in real machines. In an independent work, T. Finkelstein also assumed adiabatic expansion and compression spaces in his analysis of Stirling machinery.<ref name="Finkelstein-118B" />

The ideal Stirling cycle is unattainable in the real world, as with any heat engine. The efficiency of Stirling machines is also linked to the environmental temperature: higher efficiency is obtained when the weather is cooler, thus making this type of engine less attractive in places with warmer climates. As with other external combustion engines, Stirling engines can use heat sources other than the combustion of fuels. For example, various designs for [[solar-powered Stirling engine]]s have been developed.