The Stirling engine is a type of hot air engine, invented in 1816 by the Rev. Robert Stirling who sought to create a safer alternative to the steam engines of the time, whose boilers often exploded due to the high pressure of the steam and the primitive materials of the time. Stirling engines convert any temperature differential directly to movement: they use a displacer piston to move enclosed air back and forth between cold and hot reservoirs. At the hot reservoir, the air expands and pushes a power piston, producing work and displacing the air to the cold reservoir. There the air contracts and pulls the power piston, closing the cycle.

In Stirling engines a "regenerator," typically a mesh of wire, is located between the reservoirs. As the air cycles between the hot and cold sides, its heat is transferred to and from the regenerator. In some designs, the displacer piston is itself the regenerator.

Stirling engines operate as a Carnot heat engine and have higher thermodynamic efficiency than steam engines (or even some modern internal combustion and Diesel engines).

Because a heat exchanger separates the working fluid from the heat source, a wide range of fuels can be used, or the engine can be adapted to run on waste heat from other processes. Since the combustion is separated from the internal components of the engine, the Stirling engine can run on landfill gas containing siloxanes without the accumulation of silica that damages internal combustion engines running on this fuel.

Stirling engines can also work in reverse: when applying motion, a temperature differential appears between the reservoirs. Incidentally, one of their modern uses is in cryogenics.

Problems with Stirling engines

• Stirling engines are difficult to construct and require precise machining, thus making them both more expensive and much slower to produce.
• Stirling engines often have much less power output than other types of engines of similar size.
• Stirling engines, especially the type that run on small temperature differentials, are quite large for the amount of power that they produce.
• A "pure" Stirling engine cannot start instantly; it literally needs to "warm up.”

Stirling engine types

Stirling Engines come in three distinct types:

• An alpha Stirling contains two separate power pistons, one "hot" piston and one “cold" piston. The hot piston is situated after the higher temperature heat exchanger and the cold piston is situated after the low temperature heat exchanger. This type of engine has a very high power-to-volume ratio but has technical problems due to the usually high temperature of the "hot" piston and its seals.

• A beta Stirling has a single power piston arranged coaxially with a displacer piston. The displacer piston does not extract any power from the expanding gas but only serves to shuttle the working gas from the hot heat exchanger to the cold heat exchanger. This engine does not require moving seals in the hot portion of the engine and can achieve high compression ratios due to pistons being able to overlap in their motions.

• A gamma Stirling is simply a beta Stirling in which the power piston is not mounted coaxially to its displacer piston. This configuration produces a lower compression ratio but is often mechanically simpler and often used in multi-cylinder Stirling engines.

A Stirling engine and generator set 55 kW electrical output, for combined heat and power applications

External links

Indexes

• Open Directory Project: Stirling Engines (http://www.dmoz.org/Science/Technology/Energy/Devices/Stirling_Engines/)
• Web archive: Sun Power vejviser (http://web.archive.org/web/20011109125718/http://sunpower.com/Hobbyists/stirinfo.htm)
• Red Rock Energy Solar Heliostats: This is the start of Will's Stirling engine links (http://www.redrok.com/engine.htm#stirling)
• Robert Sier: Stirling Engine and Hot Air Engine resources (http://stirlingengines.org.uk/)
• Combined heat and power Stirling engine-generator sets (http://www.stmpower.com)
• Webring: Home > Hobbies & Crafts > Models, Stirling and Hot Air Engine Ring (http://M.webring.com/hub?ring=stirling)

How it works

• How Stuff Works: Stirling-engine (http://www.howstuffworks.com/stirling-engine.htm)
• David Haywood University of Canterbury: "Introduction to Stirling-Cycle Analysis" (PDF) (http://www.mech.canterbury.ac.nz/documents/sc_intro.pdf), Stirling-Cycle Research Group (http://www.mech.canterbury.ac.nz/research/stirling/stirling.htm)
• About The Stirling Heat Engine (http://www.stirlinghotairengine.com/about.htm)
• (Good information for builders) Why Aviation Needs the Stirling Engine by Darryl Phillips (http://www.qrmc.com/fourpartstirling.html), main address (http://www.qrmc.com/), mirror (http://www.airsport-corp.com/fourpartstirling.html) Quote: "...This 4-part series appeared in the March 1993 through March 1994 issues of Stirling Machine World...Common four cylinder engines such as Lycoming and Continental show torque that varies from a negative 100% to a positive 350% of the nominal torque...A Stirling with the same number of cylinders and identical power has a torque variation of +/- 5%!..."
• Stirling fly motor animation (http://www.qrmc.com/hydralinkanimation.gif)
• Animations: Ross type (http://www.keveney.com/Ross.html), Beta type (http://www.keveney.com/Stirling.html), Alpha type (http://www.keveney.com/Vstirling.html), Flash Animation (http://www.stirlingengine.com/mm5flash/mm5flash.html)
• Israel Urieli: Stirling Engine Simple Analysis (http://www.ent.ohiou.edu/~urieli/stirling/simple/simple.html), main address (http://www.ent.ohiou.edu/~urieli/), Alpha Stirlings (http://www.ent.ohiou.edu/~urieli/stirling/engines/engines.html), Beta Stirlings (http://www.ent.ohiou.edu/~urieli/stirling/engines/beta.html), Gamma Stirlings (http://www.ent.ohiou.edu/~urieli/stirling/engines/gamma.html)
• Peter Fette: Stirling Engine Research and Computer (simulation) Program Development (http://home.germany.net/101-276996/fette.htm), animation (http://home.germany.net/101-276996/animation.htm), prozess (http://home.germany.net/101-276996/prozess.htm), mirror (http://home.arcor.de/peterfette/)
• Quote: "...One possibility of equalizing the regenerators loss in double acting engines is to design it as a counterblow heat exchanger as described in [1 (http://home.germany.net/101-276996/etatherm.htm)..."]
• Quote: "...This Stirling Engine with 8 cylinders is twice double acting. (http://home.germany.net/101-276996/english.htm) Its special highlight is the facility for the heat transfer from a liquid [e.g. water] to the working fluid [e.g. air] which results in extremely low temperature losses....Because of the nearly isothermalized heat transfer the efficiency is near Carnot's ..."
• Amitabha Mukerjee: Stirling Engine (http://www.cse.iitk.ac.in/~amit/courses/371/abhishe/), usage (http://www.cse.iitk.ac.in/~amit/courses/371/abhishe/usage.html), How does it work? (http://www.cse.iitk.ac.in/~amit/courses/371/abhishe/main2.html) Quote: "...As a final note a solar powered Stirling engine coupled with a generator achieved a record solar-to-electric efficiency of 30%!"
• [1] (http://www.keveney.com/Engines.html) A fine collection of animations of how different engines work.

Information media

• Stirling Machine World SMW (http://stirmach.hypermart.net/)
• Stirling News is published quarterly in the UK by The Stirling Engine Society (http://stirlingengines.org.uk/files/soci2.html)
• More Stirling News papers, Search for "Stirling News" on this page (http://www.geocities.com/kenboak/stirling1.html)
• 10th Stirling Engine Conference 2001 ISEC (http://www.stirlingmotor.com/buecher/VDI_ISEC_01.htm)

Do-It-Yourself model Stirling/Hot-Air machines

• Simple Do-It-Yourself Stirling motor (http://www.emachineshop.com/engine/index.htm), links (http://www.emachineshop.com/engine/links.htm) Only requires a temperature difference of 8°C to run. A hot hand and/or an ice cube is enough to keep it running.
• Koichi Hirata: Welcome to Stirling Engine Home Page (http://www.bekkoame.ne.jp/~khirata/)
• Adresse til: Stirling Engine Home Page at National Maritime Research Institute (http://www.srimot.go.jp/eng/khirata/stirling/index_e.html)
• Test-Tube Stirling Engine (http://www.webcom.com/sknkwrks/stirling.htm)
• Web archive mirror (http://web.archive.org/web/20011102054940/http://www.steamengine.com.au/stirling/models/buildme/welcome.html), Melbourne Society of Model & Experimental Engineers Journal: A Novel Stirling Cycle Hot Air Engine To Build (http://www.steamengine.com.au/stirling/models/buildme/welcome.html)

Applications

• Home-scale generators, Micro Combined Heat and Power MCHP (http://www.whispertech.co.nz/)
• Power Producing Engines - American Stirling Company (http://www.stirlingengine.com/FullPower.adp)
• Stirling Technology, Inc. -- The leader in energy recovery ventilator technology (http://www.stirling-tech.com/)
• Stirling Cryogenics & Refrigeration BV (http://www.stirling.nl/), SPC: Stirling Process Cryogenerator (http://www.stirling.nl/sp/sp1.html), SGL: The Stirling gas liquefier (http://www.stirling.nl/sp/sp2.html), StirLIN: Stirling liquid nitrogen production plants (http://www.stirling.nl/sp/sp3.html), StirLOX: Stirling liquid oxygen production plants (http://www.stirling.nl/sp/sp5.html), Power Coolers (http://www.stirling.nl/sp/sp7.html)
• From ABC news (http://abcnews.go.com/Technology/CuttingEdge/story?id=98177&page=1) Quote: "A regular engine might be able to extract only about a quarter of the potential energy stored in the fuel used to run it. In a Stirling engine, ... By some estimates, as much as 50 percent of the potential energy stored in the fuel is converted to use in the Stirling engine's mechanical motion."