Editing Stirling engine (section)

== Other types ==
[[File:Solar Miller Style Stirling layout.JPG|thumb|Top view of two rotating displacers powering the horizontal piston. Regenerators and radiator removed for clarity]]

Other Stirling configurations continue to interest engineers and inventors.{{citation needed|date=July 2020}}

* The [[rotary Stirling engine]] seeks to convert power from the Stirling cycle directly into torque, similar to the [[rotary combustion engine]]. No practical engine has yet been built but a number of concepts, models and patents have been produced, such as the Quasiturbine engine.<ref name="Quasiturbine" />
* A hybrid between piston and rotary configuration is a double-acting engine. This design rotates the displacers on either side of the power piston. In addition to giving great design variability in the heat transfer area, this layout eliminates all but one external seal on the output shaft and one internal seal on the piston. Also, both sides can be highly pressurized as they balance against each other.{{citation needed|date=July 2020}}
* Another alternative is the [[Fluidyne engine]] (or Fluidyne heat pump), which uses hydraulic pistons to implement the [[Stirling cycle]]. The work produced by a [[Fluidyne engine]] goes into pumping the liquid. In its simplest form, the engine contains a working gas, a liquid, and two non-return valves.{{citation needed|date=July 2020}}
* The [[Ringbom engine]] concept published in 1907 has no rotary mechanism or linkage for the displacer. This is instead driven by a small auxiliary piston, usually a thick displacer rod, with the movement limited by stops.<ref name="Senft-1993" /><ref name="patent-00856102" />
* The engineer [[Andy Ross (engineer)|Andy Ross]] invented a two-cylinder Stirling engine (positioned at 0°, not 90°) connected using a special yoke.<ref name="animatedengines" />{{promotion inline|date=May 2021}}
* The [[Franchot engine]] is a double-acting engine invented by [[Charles-Louis-Félix Franchot]] in the nineteenth century. In a double-acting engine, the pressure of the working fluid acts on both sides of the piston. One of the simplest forms of a double-acting machine, the Franchot engine consists of two pistons and two cylinders, and acts like two separate alpha machines. In the Franchot engine, each piston acts in two gas phases, which makes more efficient use of the mechanical components than a single-acting alpha machine. However, a disadvantage of this machine is that one connecting rod must have a sliding seal at the hot side of the engine, which is difficult when dealing with high pressures and temperatures.<ref name="Raballand" />

=== Free-piston engines ===
[[File:Free-Piston Configurations.jpg|thumb|Various free-piston Stirling configurations... F. "free cylinder", G. Fluidyne, H. "double-acting" Stirling (typically 4 cylinders).]]

Free-piston Stirling engines include those with [[Fluidyne engine|liquid pistons]] and those with diaphragms as pistons. In a free-piston device, energy may be added or removed by an electrical [[linear alternator]], [[pump]] or other coaxial device. This avoids the need for a linkage, and reduces the number of moving parts. In some designs, friction and wear are nearly eliminated by the use of non-contact [[gas bearing]]s or very precise suspension through planar [[spring (device)|springs]].{{citation needed|date=July 2020}}

Four basic steps in the cycle of a free-piston Stirling engine are:{{citation needed|date=July 2020}}

# The power piston is pushed outwards by the expanding gas thus doing work. Gravity plays no role in the cycle.
# The gas volume in the engine increases and therefore the pressure reduces, which causes a pressure difference across the displacer rod to force the displacer towards the hot end. When the displacer moves, the piston is almost stationary and therefore the gas volume is almost constant. This step results in the constant volume cooling process, which reduces the pressure of the gas.
# The reduced pressure now arrests the outward motion of the piston and it begins to accelerate towards the hot end again and by its own inertia, compresses the now cold gas, which is mainly in the cold space.
# As the pressure increases, a point is reached where the pressure differential across the displacer rod becomes large enough to begin to push the displacer rod (and therefore also the displacer) towards the piston and thereby collapsing the cold space and transferring the cold, compressed gas towards the hot side in an almost constant volume process. As the gas arrives in the hot side the pressure increases and begins to move the piston outwards to initiate the expansion step as explained in (1).

In the early 1960s, [[William T. Beale]] of [[Ohio University]] located in Athens, Ohio, invented a free piston version of the Stirling engine to overcome the difficulty of lubricating the crank mechanism.<ref name="Walker-Springer-1985" /> While the invention of the basic free piston Stirling engine is generally attributed to Beale, independent inventions of similar types of engines were made by [[Ted Cooke-Yarborough|E.H. Cooke-Yarborough]] and C. West at the Harwell Laboratories of the [[Atomic Energy Research Establishment|UK AERE]].<ref name="Cooke-Yarborough-IEE" /> G.M. Benson also made important early contributions and patented many novel free-piston configurations.<ref name="Benson-1973" /><ref name="Benson-1977" />

The first known mention of a Stirling cycle machine using freely moving components is a British patent disclosure in 1876.<ref name="Postle-1873" /> This machine was envisaged as a refrigerator (i.e., the ''reversed'' Stirling cycle). The first consumer product to utilize a free piston Stirling device was a portable refrigerator manufactured by [[Twinbird Corporation]] of Japan and offered in the US by [[Coleman Company|Coleman]] in 2004.{{citation needed|date=July 2020}}

=== Flat engines ===
[[File:FlatStirlingEngine800x242.gif|thumb|400px|Cutaway of the flat Stirling engine:
10: Hot cylinder.
11: A volume of hot cylinder.
12: B volume of hot cylinder.
17: Warm piston diaphragm.
18: Heating medium.
19: Piston rod.
20: Cold cylinder.
21: A Volume of cold cylinder.
22: B Volume of cold cylinder.
27: Cold piston diaphragm.
28: Coolant medium.
30: Working cylinder.
31: A volume of working cylinder.
32: B volume of working cylinder.
37: Working piston diaphragm.
41: Regenerator mass of A volume.
42: Regenerator mass of B volume.
48: Heat accumulator.
50: Thermal insulation.
60: Generator.
63: Magnetic circuit.
64: Electrical winding.
70: Channel connecting warm and working cylinders.
]]

Design of the flat double-acting Stirling engine solves the drive of a displacer with the help of the fact that areas of the hot and cold pistons of the displacer are different.{{citation needed|date=July 2020}}

The drive does so without any mechanical transmission.{{citation needed|date=July 2020}} Using diaphragms eliminates friction and need for lubricants.{{citation needed|date=July 2020}}

When the displacer is in motion, the generator holds the working piston in the limit position, which brings the engine working cycle close to an ideal Stirling cycle.{{citation needed|date=July 2020}} The ratio of the area of the heat exchangers to the volume of the machine increases by the implementation of a flat design.{{citation needed|date=July 2020}}

Flat design of the working cylinder approximates thermal process of the expansion and compression closer to the isothermal one.{{citation needed|date=July 2020}}

The disadvantage is a large area of the thermal insulation between the hot and cold space.<ref name="WO2012062231" />

=== Thermoacoustic cycle ===
Thermoacoustic devices are very different from Stirling devices, although the individual path travelled by each working gas molecule does follow a real [[Stirling cycle]]. These devices include the [[Thermoacoustic hot air engine|thermoacoustic engine]] and [[Thermoacoustic refrigeration|thermoacoustic refrigerator]]. High-amplitude acoustic [[standing wave]]s cause compression and expansion analogous to a Stirling power piston, while out-of-phase acoustic [[travelling wave]]s cause displacement along a temperature [[gradient]], analogous to a Stirling displacer piston. Thus a thermoacoustic device typically does not have a displacer, as found in a beta or gamma Stirling.{{citation needed|date=July 2020}}

=== Other developments ===
[[NASA]] has considered [[Stirling radioisotope generator|nuclear-decay heated Stirling Engines]] for extended missions to the outer solar system.<ref name="Schimdt-2003" /> In 2018, NASA and the United States Department of Energy announced that they had successfully tested a new type of nuclear reactor called [[KRUSTY]], which stands for "Kilopower Reactor Using Stirling TechnologY", and which is designed to be able to power deep space vehicles and probes as well as exoplanetary encampments.<ref name="NASA-NPR" />

At the 2012 Cable-Tec Expo put on by the Society of Cable Telecommunications Engineers, Dean Kamen took the stage with Time Warner Cable Chief Technology Officer Mike LaJoie to announce a new initiative between his company Deka Research and the SCTE. Kamen refers to it as a Stirling engine.<ref name="Silbey" /><ref name="dekaresearch" />

The smallest Stirling engine was built by two German scientists at the [[University of Stuttgart]]. It operates on the micron-length scale.<ref>{{cite news |title=World's 'smallest steam engine' built in Germany |url=https://www.bbc.com/news/technology-16147965 |access-date=24 May 2025 |work=BBC News |date=12 December 2011}}</ref><ref>{{cite news |last1=Noack |first1=Rick |title=Scientists develop world's smallest steam engine |url=https://edition.cnn.com/2011/12/13/world/europe/germany-smallest-steam-engine/index.html |access-date=24 May 2025 |work=CNN |date=13 December 2011 |language=en}}</ref>