a stirling engine is a heat engine operating by cyclic compression and expansion of air

8/2/2019 A Stirling Engine is a Heat Engine Operating by Cyclic Compression and Expansion of Air

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A Stirling engine is aheat engineoperating by cycliccompression and expansion of air, another gas, or liquid, sometimes referred to as theworking fluid, at different temperature levels such that there is a net conversion ofheatenergy to mechanicalwork.[1][2]

Like the steam engine, the Stirling engine is traditionally classified as anexternal combustionengine, as all heat transfers to and from the working fluid take place through the engine wall.This contrasts with aninternal combustion enginewhere heat input is by combustion of afuelwithin the body of the working fluid. Unlike a steam engine's (or more generally aRankinecycleengine's) usage of a working fluid in both its liquid and gaseous phases, the Stirlingengine encloses a fixed quantity of permanently gaseous fluid such as air.

Typical of heat engines, the general cycle consists of compressing cool gas, heating the gas,expanding the hot gas, and finally cooling the gas before repeating the cycle. Theefficiencyof the process is narrowly restricted by the efficiency of theCarnot cycle, which depends onthe temperature difference between the hot and cold reservoir.

Originally conceived in 1816 as an industrial prime mover to rival thesteam engine, itspractical use was largely confined to low-power domestic applications for over a century.[3]

The Stirling engine is noted for its high efficiency compared to steam engines,[4]quietoperation, and the ease with which it can use almost any heat source. This compatibility withalternative and renewable energy sources has become increasingly significant as the price ofconventional fuels rises, and also in light of concerns such aspeak oilandclimate change.This engine is currently exciting interest as the core component ofmicro combined heat and

power(CHP) units, in which it is more efficient and safer than a comparable steamengine.[5][6]

Contents

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1 Name and definition 2 Functional description

o 2.1 Key components

2.1.1 Heat source 2.1.2 Heater / hot side heat exchanger
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2.1.3 Regenerator 2.1.4 Cooler / cold side heat exchanger 2.1.5 Heat sink 2.1.6 Displacer

o 2.2 Configurations 2.2.1 Alpha Stirling

2.2.1.1 Action of an alpha type Stirling engine 2.2.2 Beta Stirling

2.2.2.1 Action of a beta type Stirling engine 2.2.3 Gamma Stirling 2.2.4 Other types 2.2.5 Free piston Stirling engines

2.2.5.1 Thermoacoustic cycle 3 History

o 3.1 Later nineteenth centuryo 3.2 Twentieth century revival

4 Theoryo 4.1 Operationo 4.2 Pressurizationo 4.3 Lubricants and friction

5 Analysiso 5.1 Comparison with internal combustion engines

5.1.1 Advantages 5.1.2 Disadvantages

5.1.2.1 Size and cost issues 5.1.2.2 Power and torque issues

5.1.2.3 Gas choice issues 6 Applications 7 Alternatives 8 Photo gallery 9 See also 10 References 11 Bibliography 12 Further reading 13 External links

[edit] Name and definition

Robert Stirlingwas the Scottish inventor of the first practical example of a closed cycleairenginein 1816, and it was suggested byFleeming Jenkinas early as 1884 that all suchengines should therefore generically be called Stirling engines. This naming proposal foundlittle favour, and the various types on the market continued to be known by the name of theirindividual designers or manufacturers, e.g. Rider's, Robinson's, or Heinrici's (hot) air engine.In the 1940s, thePhilipscompany was seeking a suitable name for its own version of the 'airengine', which by that time had been tested with working fluids other than air, and decidedupon 'Stirling engine' in April 1945.[7]However, nearly thirty years later Graham Walker was

still bemoaning the fact such terms as 'hot air engine' continued to be used interchangeablywith 'Stirling engine', which itself was applied widely and indiscriminately.[8]The situation
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has now improved somewhat, at least in academic literature, and it is now generally accepted'Stirling engine' should refer exclusively to a closed-cycle regenerativeheat enginewith apermanentlygaseousworkingfluid, where closed-cycle is defined as athermodynamicsystemin which the working fluid is permanently contained within the system, andregenerative describes the use of a specific type of internalheat exchangerand thermal store,

known as the regenerator.

It follows from the closed cycle operation the Stirling engine is anexternal combustionenginethat isolates its working fluid from the energy input supplied by an external heatsource. There are many possible implementations of the Stirling engine most of which fallinto the category ofreciprocating piston engine.

[edit] Functional description

The engine is designed so that the working gas is generally compressed in the colder portion

of the engine and expanded in the hotter portion resulting in a net conversion of heat intowork.[2]An internalRegenerative heat exchangerincreases the Stirling engine's thermalefficiency compared to simplerhot air engineslacking this feature.

[edit] Key components

Cut-away diagram of arhombic drivebeta configuration

Stirling engine design:

1. PinkHot cylinder wall2. Dark greyCold cylinder wall3. Yellow - Coolant inlet and outlet pipes4. Dark greenThermal insulation separating the

two cylinder ends
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5. Light greenDisplacerpiston6. Dark bluePower piston7. Light blueLinkage crank and flywheels

Not shown: Heat source and heat sinks. In this design the

displacer piston is constructed without a purpose-built

regenerator.

As a consequence of closed cycle operation, the heat driving a Stirling engine must betransmitted from a heat source to the working fluid byheat exchangersand finally to a heatsink. A Stirling engine system has at least one heat source, one heat sink and up to five heatexchangers. Some types may combine or dispense with some of these.

[edit] Heat source

Point focus parabolic mirror with Stirling engine at its center and itssolar trackeratPlataforma Solar de Almera(PSA) in Spain

The heat source may be provided by thecombustionof a fuel and, since the combustionproducts do not mix with the working fluid and hence do not come into contact with theinternal parts of the engine, a Stirling engine can run on fuels that would damage other typesof engines' internals, such aslandfill gaswhich containssiloxane.

Other suitable heat sources are concentratedsolar energy,geothermal energy,nuclear energy,

waste heat, or evenbiological. If the heat source is solar power, regularsolar mirrorsandsolar dishes may be used. Also,fresnel lensesand mirrors have been advocated to be used(for example, for planetary surface exploration).[9]Solar powered Stirling engines arebecoming increasingly popular, as they are a very environmentally sound option forproducing power. Also, some designs are economically attractive in development projects .[10]

[edit] Heater / hot side heat exchanger

In small, low power engines this may simply consist of the walls of the hot space(s) butwhere larger powers are required a greater surface area is needed in order to transfersufficient heat. Typical implementations are internal and external fins or multiple small bore

tubes
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Designing Stirling engine heat exchangers is a balance between high heat transfer with lowviscouspumping lossesand low dead space (unswept internal volume). With enginesoperating at high powers and pressures, the heat exchangers on the hot side must be made ofalloys that retain considerable strength at temperature and that will also not corrode or creep.

[edit] Regenerator

Main article:Regenerative heat exchanger

In a Stirling engine, the regenerator is an internal heat exchanger and temporary heat storeplaced between the hot and cold spaces such that the working fluid passes through it first inone direction then the other. Its function is to retain within thesystemthat heat which wouldotherwise be exchanged with the environment at temperatures intermediate to the maximumand minimum cycle temperatures,[11]thus enabling the thermal efficiency of the cycle toapproach the limitingCarnotefficiency defined by those maxima and minima.

The primary effect of regeneration in a Stirling engine is to increase the thermal efficiency by'recycling' internal heat which would otherwise pass through the engineirreversibly. As asecondary effect, increased thermal efficiency yields a higher power output from a given setof hot and cold end heat exchangers. It is these which usually limit the engine's heatthroughput. In practice this additional power may not be fully realized as the additional "deadspace" (unswept volume) and pumping loss inherent in practical regenerators reduces thepotential efficiency gains from regeneration.

The design challenge for a Stirling engine regenerator is to provide sufficient heat transfercapacity without introducing too much additional internal volume ('dead space') or flowresistance. These inherent design conflicts are one of many factors which limit the efficiencyof practical Stirling engines. A typical design is a stack of fine metalwiremeshes, with lowporosityto reduce dead space, and with the wire axesperpendicularto the gas flow to reduceconduction in that direction and to maximize convective heat transfer.[12]

The regenerator is the key component invented byRobert Stirlingand its presencedistinguishes a true Stirling engine from any other closed cyclehot air engine. Many small'toy' Stirling engines, particularly low-temperature difference (LTD) types, do not have adistinct regenerator component and might be considered hot air engines, however a smallamount of regeneration is provided by the surface of displacer itself and the nearby cylinderwall, or similarly the passage connecting the hot and cold cylinders of an alpha configuration

engine.

[edit] Cooler / cold side heat exchanger

In small, low power engines this may simply consist of the walls of the cold space(s), butwhere larger powers are required a cooler using a liquid like water is needed in order totransfer sufficient heat.

[edit] Heat sink

The heat sink is typically the environment at ambient temperature. In the case of medium to

high power engines, aradiatoris required to transfer the heat from the engine to the ambient
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air. Marine engines can use the ambient water. In the case of combined heat and powersystems, the engine's cooling water is used directly or indirectly for heating purposes.

Alternatively, heat may be supplied at ambient temperature and the heat sink maintained at alower temperature by such means ascryogenic fluid(seeLiquid nitrogen economy) or iced

water.

[edit] Displacer

The displacer is a special-purposepiston, used in Beta and Gamma type Stirling engines, tomove the working gas back and forth between the hot and cold heat exchangers. Dependingon the type of engine design, the displacer may or may not be sealed to the cylinder, i.e. it is aloose fit within the cylinder and allows the working gas to pass around it as it moves tooccupy the part of the cylinder beyond.

[edit] Configurations

There are two major types of Stirling engines that are distinguished by the way they move theair between the hot and cold sides of the cylinder:

1. The two piston alpha type design has pistons in independent cylinders, and gas isdriven between the hot and cold spaces.

2. The displacement type Stirling engines, known as beta and gamma types, use aninsulated mechanical displacer to push the working gas between the hot and cold sidesof the cylinder. The displacer is large enough to insulate the hot and cold sides of thecylinder thermally and to displace a large quantity of gas. It must have enough of a

gap between the displacer and the cylinder wall to allow gas to flow around thedisplacer easily.

[edit] Alpha Stirling

An alpha Stirling contains two power pistons in separate cylinders, one hot and one cold.The hot cylinder is situated inside the high temperatureheat exchangerand the cold cylinderis situated inside the low temperature heat exchanger. This type of engine has a high power-to-volume ratio but has technical problems due to the usually high temperature of the hotpiston and the durability of its seals.[13]In practice, this piston usually carries a largeinsulating head to move the seals away from the hot zone at the expense of some additional

dead space.

[edit] Action of an alpha type Stirling engine

The following diagrams do not show internal heat exchangers in the compression andexpansion spaces, which are needed to produce power. Aregeneratorwould be placed in thepipe connecting the two cylinders. The crankshaft has also been omitted.
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1. Most of the working gas is in contact with the hotcylinder walls, it has been heated and expansion haspushed the hot piston to the bottom of its travel in thecylinder. The expansion continues in the cold cylinder,which is 90 behind the hot piston in its cycle,extracting more work from the hot gas.

2. The gas is now at its maximum volume. The hotcylinder piston begins to move most of the gas into thecold cylinder, where it cools and the pressure drops.

3. Almost all the gas is now in the cold cylinder andcooling continues. The cold piston, powered byflywheel momentum (or other piston pairs on the sameshaft) compresses the remaining part of the gas.

4. The gas reaches its minimum volume, and it willnow expand in the hot cylinder where it will be heatedonce more, driving the hot piston in its power stroke.

The complete alpha type Stirling cycle

[edit] Beta Stirling

A beta Stirling has a single power piston arranged within the same cylinder on the sameshaft as adisplacerpiston. The displacer piston is a loose fit and does not extract any powerfrom the expanding gas but only serves to shuttle the working gas from the hot heatexchanger to the cold heat exchanger. When the working gas is pushed to the hot end of the
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cylinder it expands and pushes the power piston. When it is pushed to the cold end of thecylinder it contracts and the momentum of the machine, usually enhanced by aflywheel,pushes the power piston the other way to compress the gas. Unlike the alpha type, the betatype avoids the technical problems of hot moving seals.[14]

[edit] Action of a beta type Stirling engine

Again, the following diagrams do not show internal heat exchangers or a regenerator, whichwould be placed in the gas path around the displacer.

1. Power piston (darkgrey) has compressedthe gas, the displacer

piston (light grey) hasmoved so that most ofthe gas is adjacent tothe hot heat exchanger.

2. The heated gasincreases in pressureand pushes the power

piston to the farthestlimit of the powerstroke.

3. The displacer pistonnow moves, shuntingthe gas to the cold end

of the cylinder.

4. The cooled gas isnow compressed by theflywheel momentum.

This takes less energy,since when it is cooledits pressure drops.

The complete beta type Stirling cycle

[edit] Gamma Stirling

A gamma Stirling is simply a beta Stirling in which the power piston is mounted in aseparate cylinder alongside the displacer piston cylinder, but is still connected to the same
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flywheel. The gas in the two cylinders can flow freely between them and remains a singlebody. This configuration produces a lowercompression ratiobut is mechanically simpler andoften used in multi-cylinder Stirling engines.

[edit] Other types

Other Stirling configurations continue to interest engineers and inventors.

The hybrid between piston and rotary configuration is a double acting engine. This designrotates the displacers on either side of the power piston

Top view of two rotating displacer powering the horizontal piston. Regenerators and radiatorremoved for clarity

There is also the rotary Stirling engine which seeks to convert power from the Stirling cycledirectly into torque, similar to therotary combustion engine. No practical engine has yet beenbuilt but a number of concepts, models and patents have been produced for example theQuasiturbine engine.[15]

Another alternative is the Fluidyne engine (Fluidyne heat pump), which use hydraulicpistons to implement theStirling cycle. The work produced by aFluidyne enginegoes intopumping the liquid. In its simplest form, the engine contains a working gas, a liquid and twonon-return valves.

The Ringbom engine concept published in 1907 has no rotary mechanism or linkage for thedisplacer. This is instead driven by a small auxiliary piston, usually a thick displacer rod, withthe movement limited by stops.[16][17]

The two-cylinder stirling with Ross yoke is a two-cylinder stirling engine (not positioned at90, but at 0) connected with a special yoke. The engine configuration/yoke setup wasinvented byAndy Ross (engineer)[disambiguation needed ].[18]

The Franchot engine is a double acting engine invented by Franchot in the nineteenthcentury. A double acting engine is one where both sides of the piston are acted upon by thepressure of the working fluid. One of the simplest forms of a double acting machine, theFranchot engine consists of two pistons and two cylinders and acts like two separate alphamachines. In the Franchot engine, each piston acts in two gas phases, which makes moreefficient use of the mechanical components than a single acting alpha machine. However, adisadvantage of this machine is that one connecting rod must have a sliding seal at the hot

side of the engine, which is a difficult task when dealing with high pressures and hightemperatures[citation needed].
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[edit] Free piston Stirling engines

Various Free-Piston Stirling Configurations... F."free cylinder", G. Fluidyne, H. "double-acting" Stirling (typically 4 cylinders)

"Free piston" Stirling engines include those withliquid pistonsand those with diaphragmsas pistons. In a "free piston" device, energy may be added or removed by an electricallinearalternator,pumpor other coaxial device. This avoids the need for a linkage, and reduces thenumber of moving parts. In some designs, friction and wear are nearly eliminated by the useof non-contactgas bearingsor very precise suspension through planarsprings.

Four basic steps in the cycle of a Free piston Stirling engine,

1. The power piston is pushed outwards by the expanding gas thus doing work. Gravityplays no role in the cycle.

2. The gas volume in the engine increases and therefore the pressure reduces, which willcause a pressure difference across the displacer rod to force the displacer towards thehot end. When the displacer moves the piston is almost stationary and therefore thegas volume is almost constant. This step results in the constant volume coolingprocess which reduces the pressure of the gas.

3. The reduced pressure now arrests the outward motion of the piston and it begins toaccelerate towards the hot end again and by its own inertia, compresses the now cold

gas which is mainly in the cold space.
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4. As the pressure increases, a point is reached where the pressure differential across thedisplacer rod becomes large enough to begin to push the displacer rod (and thereforealso the displacer) towards the piston and thereby collapsing the cold space andtransferring the cold, compressed gas towards the hot side in an almost constantvolume 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, W.T. Beale invented a free piston version of the Stirling engine in order toovercome the difficulty of lubricating the crank mechanism.[19]While the invention of thebasic free piston Stirling engine is generally attributed to Beale, independent inventions ofsimilar types of engines were made by E.H. Cooke-Yarborough and C. West at the HarwellLaboratories of the UKAERE.[20]G.M. Benson also made important early contributions andpatented many novel free-piston configurations.[21]

What appears to be the first mention of a Stirling cycle machine using freely movingcomponents is a British patent disclosure in 1876.[22]This machine was envisaged as a

refrigerator (i.e., the reversedStirling cycle). The first consumer product to utilize a freepiston Stirling device was a portable refrigerator manufactured byTwinbird CorporationofJapan and offered in the US byColemanin 2004.

[edit] Thermoacoustic cycle

Thermoacoustic devices are very different from Stirling devices, although the individual pathtravelled by each working gas molecule does follow a realStirling cycle. These devicesinclude thethermoacoustic engineandthermoacoustic refrigerator. High-amplitude acousticstanding wavescause compression and expansion analogous to a Stirling power piston, whileout-of-phase acoustictravelling wavescause displacement along a temperaturegradient,

analogous to a Stirling displacer piston. Thus a thermoacoustic device typically does not havea displacer, as found in a beta or gamma Stirling.

[edit] History

Illustration to Robert Stirling's 1816 patent application of the air engine design which latercame to be known as the Stirling Engine

The Stirling engine (or Stirling's air engine as it was known at the time) was invented andpatented byRobert Stirlingin 1816.[23]It followedearlier attempts at making an air enginebut was probably the first to be put to practical use when in 1818 an engine built by Stirling

was employed pumping water in aquarry.

[24]

The main subject of Stirling's original patentwas a heat exchanger which he called an "economiser" for its enhancement of fuel economy
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in a variety of applications. The patent also described in detail the employment of one formof the economiser in his unique closed-cycleair enginedesign[25]in which application it isnow generally known as a 'regenerator'. Subsequent development by Robert Stirling and hisbrotherJames, an engineer, resulted in patents for various improved configurations of theoriginal engine including pressurization which had by 1843 sufficiently increased power

output to drive all the machinery at aDundeeiron foundry.[26]

Though it has been disputed[27]it is widely supposed that as well as saving fuel the inventorswere motivated to create a safer alternative to thesteam enginesof the time,[28]whoseboilersfrequently exploded causing many injuries and fatalities.[29][30]The need for Stirling enginesto run at very high temperatures to maximize power and efficiency exposed limitations in thematerials of the day and the few engines that were built in those early years sufferedunacceptably frequent failures (albeit with far less disastrous consequences than a boilerexplosion[31])for example, the Dundee foundry engine was replaced by a steam engineafter three hot cylinder failures in four years.[32]

[edit] Later nineteenth century

A typical late nineteenth/early twentieth century water pumping engine by theRider-EricssonEngine Company

Subsequent to the failure of the Dundee foundry engine there is no record of the Stirlingbrothers having any further involvement with air engine development and the Stirling enginenever again competed with steam as an industrial scale power source (steam boilers werebecoming safer[33]and steam engines more efficient, thus presenting less of a target to rivalprime movers). However, from about 1860 smaller engines of the Stirling/hot air type wereproduced in substantial numbers finding applications wherever a reliable source of low tomedium power was required, such as raising water or providing air for church organs.[34]These generally operated at lower temperatures so as not to tax available materials, so wererelatively inefficient. But their selling point was that, unlike a steam engine, they could beoperated safely by anybody capable of managing a fire.[35]Several types remained in

production beyond the end of the century, but apart from a few minor mechanicalimprovements the design of the Stirling engine in general stagnated during this period.[36]
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[edit] Twentieth century revival

During the early part of the twentieth century the role of the Stirling engine as a "domesticmotor"[37]was gradually taken over by theelectric motorand smallinternal combustionengines. By the late 1930s it was largely forgotten, only produced for toys and a few small

ventilating fans.[38]

At this timePhilipswas seeking to expand sales of its radios into parts of the world wheremains electricity was unavailable and the supply of batteries uncertain. Philips' managementdecided that offering a low-power portable generator would facilitate such sales and tasked agroup of engineers at the company's research lab inEindhovento evaluate alternative ways ofachieving this aim. After a systematic comparison of variousprime movers, the team decidedto go forward with the Stirling engine, citing its quiet operation (both audibly and in terms ofradio interference) and ability to run on a variety of heat sources (common lamp oil"cheapand available everywhere"was favoured).[39]They were also aware that, unlike steam andinternal combustion engines, virtually no serious development work had been carried out onthe Stirling engine for many years and asserted that modern materials and know-how shouldenable great improvements.[40]

Philips MP1002CA Stirling generator of 1951

Encouraged by their first experimental engine, which produced 16 W of shaft power from abore and stroke of 30mm 25mm,[41]various development models were produced in aprogram which continued throughout World War II. By the late 1940s the 'Type 10' wasready to be handed over to Philips' subsidiary Johan de Witt in Dordrecht to beproductionised and incorporated into a generator set as originally planned. The result, rated at180/200 W electrical output from a bore and stroke of 55 mm x 27 mm, was designatedMP1002CA (known as the "Bungalow set"). Production of an initial batch of 250 began in

1951, but it became clear that they could not be made at a competitive price besides whichthe advent of transistor radios with their much lower power requirements meant that theoriginal rationale for the set was disappearing. Approximately 150 of these sets wereeventually produced.[42]Some found their way into university and college engineeringdepartments around the world[43]giving generations of students a valuable introduction to theStirling engine.

Philips went on to develop experimental Stirling engines for a wide variety of applicationsand continued to work in the field until the late 1970s, but only achieved commercial successwith the 'reversed Stirling engine'cryocooler. However, they filed a large number of patentsand amassed a wealth of information, which they licensed to other companies and which

formed the basis of much of the development work in the modern era.[44]
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Starting in 1986, Infinia Corporation began developing both highly reliable pulsed free-pistonStirling engines, and thermoacoustic coolers using related technology. The published designuses flexural bearings and hermetically sealed Helium gas cycles, to achieve testedreliabilities exceeding 20 years. As of 2010, the corporation had amassed more than 30patents, and developed a number of commercial products for both combined heat and power,

and solar power.[45]

More recently,NASAhas considerednuclear-decay heated StirlingEnginesfor extended missions to the outer solar system.[46]

[edit] Theory

Main article:Stirling cycle

Apressure/volume graphof the idealized Stirling cycle

The idealised Stirling cycle consists of fourthermodynamic processesacting on the workingfluid:

1. IsothermalExpansion. The expansion-space and associated heat exchanger aremaintained at a constant high temperature, and the gas undergoes near-isothermalexpansion absorbing heat from the hot source.

2. Constant-Volume (known asisovolumetricorisochoric) heat-removal. The gas ispassed through theregenerator, where it cools transferring heat to the regenerator foruse in the next cycle.

3. IsothermalCompression. The compression space and associated heat exchanger aremaintained at a constant low temperature so the gas undergoes near-isothermalcompression rejecting heat to the cold sink

4. Constant-Volume (known asisovolumetricorisochoric) heat-addition. The gas passesback through the regenerator where it recovers much of the heat transferred in 2,heating up on its way to the expansion space.

Theoreticalthermal efficiencyequals that of the hypotheticalCarnot cycle- i.e. the highestefficiency attainable by any heat engine. However, though it is useful for illustrating generalprinciples, the text book cycle is a long way from representing what is actually going oninside a practical Stirling engine and should only be regarded as a starting point for analysis.In fact 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.

[47][48]
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Other real-world issues reduce the efficiency of actual engines, due to limits ofconvectiveheat transfer, andviscous flow(friction). There are also practical mechanical considerations,for instance a simple kinematic linkage may be favoured over a more complex mechanismneeded to replicate the idealized cycle, and limitations imposed by available materials such asnon-idealproperties of the working gas,thermal conductivity,tensile strength,creep,rupture

strength, andmelting point. A question that often arises is whether the ideal cycle withisothermal expansion and compression is in fact the correct ideal cycle to apply to the Stirlingengine. Professor C. J. Rallis has pointed out that it is very difficult to imagine any conditionwhere the expansion and compression spaces may approach isothermal behavior and it is farmore realistic to imagine these spaces as adiabatic.[49]An ideal analysis where the expansionand compression spaces are taken to be adiabatic with isothermal heat exchangers and perfectregeneration was analyzed Rallis and presented as a better ideal yardstick for Stirlingmachinery. 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. Afurther conclusion of this ideal cycle is that maximum efficiencies are found at lowercompression ratios, a characteristic observed in real machines. In an independent work, T.

Finkelstein also assumed adiabatic expansion and compression spaces in his analysis ofStirling machinery[50]

[edit] Operation

Since the Stirling engine is a closed cycle, it contains a fixed mass of gas called the "workingfluid", most commonlyair,hydrogenorhelium. In normal operation, the engine is sealed andno gas enters or leaves the engine. No valves are required, unlike other types of pistonengines. The Stirling engine, like most heat engines, cycles through four main processes:cooling, compression, heating and expansion. This is accomplished by moving the gas back

and forth between hot and coldheat exchangers, often with a regenerator between the heaterand cooler. The hot heat exchanger is in thermal contact with an external heat source, such asa fuel burner, and the cold heat exchanger being in thermal contact with an external heat sink,such as air fins. A change in gas temperature will cause a corresponding change in gaspressure, while the motion of the piston causes the gas to be alternately expanded andcompressed.

The gas follows the behaviour described by thegas lawswhich describe how a gas'pressure,temperatureandvolumeare related. When the gas is heated, because it is in a sealedchamber, the pressure rises and this then acts on the powerpistonto produce a power stroke.When the gas is cooled the pressure drops and this means that less work needs to be done by

the piston to compress the gas on the return stroke, thus yielding a net power output.

When one side of the piston is open to the atmosphere, the operation is slightly different. Asthe sealed volume of working gas comes in contact with the hot side, it expands, doing workon both the piston and on the atmosphere. When the working gas contacts the cold side, itspressure drops below atmospheric pressure and the atmosphere pushes on the piston and doeswork on the gas.

To summarize, the Stirling engine uses the temperature difference between its hot end andcold end to establish a cycle of a fixed mass of gas, heated and expanded, and cooled andcompressed, thus converting thermalenergyinto mechanical energy. The greater the

temperature difference between the hot and cold sources, the greater the thermal efficiency.
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a stirling engine is a heat engine operating by cyclic compression and expansion of air

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