Innovative Rotary Displacer Stirling Engine: Sustainable Power

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AbstractThe Stirling cycle and related terminology

is defined. Selected contemporary researchinvolving the cycle in environmentally friendlyapplications is cited. Three conventional engineconfigurations that utilize a reciprocating powerpiston and crank shaft (the Alpha, Beta, andGamma) are characterized, in terms of minimumnumber of components, advantages, and disad-vantages. A new type of Stirling engine thatemploys a segmented rotary displacer and otherrelated design improvements is presented alongwith preliminary findings from engine test runs.

Keywords: Stirling engine, electric power

generation, renewable energy

Robert Stirling, in 1816, developed andpatented the air engine that bears his name. Thesteam engines of the early 19th century had areputation for unreliability, particularly regard-ing boiler explosion, and the Stirling cycleengine was thought by some as a possible sub-stitute. Fascination regarding the potential forthe Stirling cycle can be seen in the prolificdevelopment of Stirling cycle engines. Althoughthis cycle provides several distinct advantagesover other engine cycles currently in widespreaduse, there are some noteworthy disadvantagesthat technology has been hard pressed to elimi-nate. Thus, most innovation in Stirling enginedesign has not been adopted. Despite its short-comings, the Stirling cycle engine is once againunder study, but for entirely different reasons.

Worldwide concern over global warmingand depletion of nonrenewable energy sourceshas renewed interest in the Stirling cycle beingused for generating green or sustainable energy.Hsu, Lin, and Chiou (2003) reported on what isbelieved to be the first study of the Stirlingcycle, fueled by waste energy, to generate electricity. The heat source was an incinerator.Experimental hardware included a free-pistonStirling engine coupled to a linear alternator.The researchers selected the free-piston Stirlingbecause crank-driven Stirling engines “… presented some challenging design problems,including power modulation, leakage of workingfluid, isolation of lubricants, etc.” (Hsu et al.,

p. 61). Their findings demonstrated the difficul-ty in simulating actual engine performance, buton a larger scale, these authors pointed the wayfor other researchers to pursue the topic. In2008, Snyman, Harms, and Strauss investigatedapplying the Stirling engine to energy genera-tion through waste heat recovery. Their workfocused on the utilization of three design analy-sis methods for simulating the optimization of aBeta configuration Stirling engine utilizingwaste heat. Also included in their research wasan experimental setup, using an instrumentedBeta Stirling, which was powered by exhaustgases from the combustion of propane. Theexperimental setup replicated the simulationconditions. The authors’ findings indicated thatactual engine performance could be predicted bythe simulation analysis. Chang and Ko (2009)also studied waste heat recovery for electricitygeneration utilizing the Stirling cycle. Therenewable heat source used in their researchoriginated from a waste incinerator. Leu (2010)studied the viability of biomass as a heat sourcefor small-scale electrical generation using theStirling engine. In this application, solid bio-mass fueled a fixed-bed gasifier with a combus-tor. Flue gas from the combustor provided theenergy input for the Stirling engine-generatorset.

The Stirling Cycle and RelatedTerminologyStirling Cycle

All modern engines encountered in day-to-day use operate on a well-known cycle charac-teristic of their operation. For example, four-stroke reciprocating internal combustion enginesutilize spark ignition. These function accordingto the Otto cycle. Engines that utilize compres-sion ignition adhere to the Diesel cycle. Theseare mechanical cycles in which the workingfluid, a fuel-air mixture, does not undergo athermodynamic cycle involving cooling to theinitial state. They are not reversible. Rather, aftercombustion imparts work to the mechanism, theremains of the working fluid are expelled fromthe engine and replaced with fresh mixture. Acomplete cycle may require one or two 360°revolutions (Cengel & Boles, 1998; Howell &Buckius, 1992; Wood, 1991).

Innovative Rotary Displacer Stirling Engine:Sustainable Power Generation for Private and FleetVehicle Applications Phillip R. Foster

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The Stirling cycle is a reversible thermody-namic cycle consisting of four phases: heat addi-tion (isovolumetric heating), expansion (isother-mal expansion), heat rejection (isovolumetriccooling), and compression (isothermal compres-sion). The four phases, which constitute onecomplete cycle, are completed in a single 360°revolution. A displacer alternately shuttles theworking fluid from the cold to the hot work-spaces of the engine in synchronization with thepower piston. When in the hot workspace, theworking fluid is heated, its pressure increasesand it expands, thus moving a power piston anddoing work. When the displacer shuttles theworking fluid into the cold workspace, it iscooled, its pressure is reduced, and the powerpiston compresses the working fluid back to itsoriginal volume (Beale, 1984; Biwa, Tashiro, &Yazaki, 2008; Gras, 2011; WoodbankCommunications, Ltd., 2011).

The positive attributes of the cycle havebeen well known since its inception. Theyinclude quiet operation, high thermal efficiency,safe operation, ease of operation, and the abilityto function on any form of thermal energy(including both traditional combustion and non-polluting sources, such as biomass, solar energy,and geothermal energy). Shortcomings of theStirling cycle have hindered its wider applica-tion in competition with steam, electric, andinternal combustion. These shortcomingsinclude the complexity of design and a relativelylow power output per size and weight (Beale,1984; Der Minassians & Sanders, 2009;Karabulut, Yucesu, & Koca, 2000; Snyman etal., 2008).

Selected Stirling Engine Terminology

A basic knowledge of selected Stirlingengine terminology will help the reader under-stand the narrative pertaining to contemporaryengine configurations. Further, it is essential toan understanding of the major objectives of thisarticle, that is, the discussion of an innovativerotary displacer Stirling engine. The followingterms are defined: phase angle, dead space,regenerator, working fluid, workspace, displacer,and volume compression ratio.

Phase angle (a). The displacer piston alwaysmoves in advance of the power piston. Both pis-tons are mechanically connected to a commonflywheel and crankshaft. Typically, the phaseangle is 90° (Senft, 2002; Snyman et al., 2008).

Dead space (dead volume). Conduits, pas-sageways, internal heat exchangers, and similarfeatures for conveying the working fluid that isnot directly shuttled by the displacer make upthe dead space. Some dead space is inevitable,but it must be minimized because it is detrimen-tal to the indicated work of the cycle (Senft,2002).

Regenerator. A feature consisting of layersor coils of heat-absorbent material located onthe internal surfaces of working fluid conduitsor passageways. This feature is used to increasethe efficiency of the cycle by (a) accumulatingexcess heat from the expanding working fluid,which can then be transferred to the fluid duringsubsequent cycles, and (b) reducing the amountof heat that must be accommodated by the exter-nal heat sink through the cold workspace(Snyman et al., 2008).

Working fluid (working gas). The design ofStirling engines is such that the internal spacescontain a gas that is alternately heated andcooled during the cycle but is unable to escapefrom the mechanism. This gas, referred to as theworking fluid or working gas, is commonly air.However, gases with lighter molecular weight(i.e., helium or hydrogen) provide thermody-namic advantages over air. The working fluid isnot consumed in the cycle (Snyman et al.,2008).

Workspace. The interior volume of the dis-placer housing, excluding the displacer itselfand any engine dead space, constitutes the work-space. This area contains the bulk of the work-ing fluid and has provisions for the addition andrejection of heat. These provisions subdivide theworkspace into the hot workspace and the coldworkspace, areas that are thermally insulatedfrom each other.

Displacer. The displacer is a reciprocatingpiston that moves along the axis of the displacerhousing, thus alternately communicating theworking fluid to the hot and cold workspaces ofthe engine (Beale, 1984; Der Minassians &Sanders, 2009).

Volume compression ratio. Any givenengine has a maximum volume and a minimumvolume. The former is the sum of the displacervolume plus the power piston cylinder volume.The latter is the displacer volume alone. Theratio of the maximum engine volume to the

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minimum engine volume is referred to as thevolume compression ratio (Senft, 2007).

Conventional Commercial Stirling EngineConfigurations

Three related engine configurations havepersisted over the years and these are most com-monly used in contemporary commercial appli-cations: the Alpha, the Beta, and the Gamma.All of these configurations utilize the conven-tional piston, crankshaft, and cylinder arrange-ment, but two of the three (i.e., Beta andGamma) use them in conjunction with a unique-ly Stirling cycle addition, the displacer. The fol-lowing sections characterize the three prevalentreciprocating piston-type Stirling cycle engines.Descriptions and figures are generic interpreta-tions of these configurations. No attempt hasbeen made to describe or render the complexi-ties associated with actual working engines.

Characteristics of the Alpha Stirling Engine

The Alpha Stirling engine consists of twopower pistons, each with a separate cylinder andconnecting rod. One power piston and cylinderrepresents hot workspace, the other cold work-space. The connecting rods join a common jour-nal on a single flywheel/crankshaft. Thisarrangement is shown in Figure 1. As the figuredepicts, the hot and cold workspaces are physi-cally separated from each other. This featureprovides excellent thermal isolation for the twoworkspaces. The conduit that joins the twoworkspaces, however, adds to the dead spaceassociated with this design.

The Alpha then, in its simplest form, uti-lizes four reciprocating parts and one rotarypart. This configuration requires a close toler-ance fit between each power piston and itsrespective cylinder. This is not an issue for thosecomponents operating within the cold work-space. The hot workspace piston and cylinder dorepresent a problem with regard to maintaining a reliable seal in an environment with high heat

coupled with sliding friction. Seals on this piston can be subject to early failure due tothese operating conditions. Techniques that alle-viate piston seal failure may also increaseengine dead space. The Alpha is known for itshigh power-to-volume ratio.

Characteristics of the Beta Stirling Engine

The Beta Stirling engine included designfeatures that eliminated the hot seal failureissues of the Alpha. The engine utilizes a powerpiston with a connecting rod, similar to the“cold” power piston of the Alpha, but the “hot”power piston is replaced by a displacer with aconnecting rod. The displacer represents a majorimprovement. The power piston and the displac-er both share a common cylinder and a commonflywheel/crankshaft. This arrangement is shownin Figure 2. Unlike a piston, the displacer doesnot require a tight tolerance seal along its sur-faces. Its function is to simply shuttle workingfluid within the hot and cold workspaces. Thisinsulates the power piston from the high temper-atures that exist in the hot workspace. Also, deadspace is minimal.

A design in which there is sharing of acommon cylinder presents thermal conductionissues not encountered in the Alpha. The junc-tion of Beta hot and cold workspaces mustinclude an additional thermal barrier to reduceconduction and maintain efficiency. The Beta, inits simplest form, consists of four reciprocatingparts and one rotary part.

Characteristics of the Gamma Stirling Engine

The Gamma Stirling engine is similar to theBeta it that it utilizes the same type of movingparts. It has one major difference. The Gammapower piston does not share a common cylinderwith the displacer. Its design employs two dis-tinct cylinders, a feature evident in Figure 3.However, the hot and cold workspaces of thedisplacer cylinder require the addition of a ther-mal barrier. Therefore, in its simplest form, the

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Figure 2: Beta Configuration ofStirling Engine

Figure 1: Alpha Configuration ofStirling Engine

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Gamma configuration also consists of fourreciprocating parts and one rotary part. TheGamma shares the same advantages as the Betaand also holds the potential for being mechani-cally simpler. Gammas are particularly suited tomulticylinder applications.

In the preceding explanation, the reciprocat-ing and rotating part count was always prefacedby the phrase “in simplest form.” The reality ofconventional commercial Stirling design seldomif ever adheres to the simplest form.Contemporary engines display a range of mech-anisms, some fairly complex, to change linearmotion into rotary.

Summary of Conventional Stirling Configurations

Certain generalizations can be made fromthe preceding sections. There is a renewed inter-est in the Stirling cycle for sustainable and/orenvironmentally friendly electrical generation.Reciprocating piston-type Stirling engines, par-ticularly the Alpha, the Beta, and the Gamma,have been harnessed in these applications andhave been reported to be effective. These engineconfigurations, in their simplest form, utilizefour reciprocating parts and one rotary part (perpower cylinder). Actual commercial engines aretypically more complex (i.e., have more movingparts per power cylinder).

Genesis of the Stirling With Attitude (SWATT)Engine

The author was convinced that the cyclecould be effectively achieved with fewer thanfive moving parts per power cylinder. A reduc-tion in parts, particularly reciprocating parts,would contribute to mechanism longevity whilereducing complexity. It was proposed that thecycle be represented by three moving parts, tworeciprocating and one rotary. The reciprocatingparts included the power piston and connectingrod assembly. The rotary part was the displacer.Integrated into the displacer were the crank-shaft/flywheel and a valve mechanism. The

crankshaft/flywheel also incorporated provisionsfor teaming engines to provide greatly simpli-fied, multipower cylinder configurations.

The following parameters were selected as astarting point for initial engine design: liquidcooling, power cylinder bore and stroke =1.000” X .625” (volume = .491 in3), phaseangle 90°, and volume compression ratio 1.244.The power cylinder specifications and the vol-ume compression ratio established the volumeof the rotary displacer at 2.009 in3. Materialsselected included stainless steel for all majorstructural components, graphite for the piston,titanium for the connecting rod assembly, andpolymer for the rotary displacer. The followingsections address design features of major com-ponents, specifically, the individual displacersegments, the rotary displacer assembly, the dis-placer housing, and a rotary valve mechanism.

Individual Displacer Segments

The displacer of the subject engine is not ofone-piece construction. Rather, it is made up of16 polymer segments, 14 of which are virtuallyidentical. Individually balanced, each has astepped profile along about 180° of the outercircumference (see Figure 4), which is evidentalong the lower half of the segment in the fig-ure. The same profile is mirror imaged in theinternal surfaces of the two-piece stainless steeldisplacer housing. This design feature increasesthe surface area of the hot and cold workspace,enabling a more rapid isovolumetric heating(expansion) and cooling (compression) of a larg-er volume of working fluid. The six holes adja-cent to the stepped area balance the segment byremoving a mass equivalent to that removed inthe creation of the stepped profile.

The balancing holes, in the fully assembleddisplacer, are filled with an expanded foaminsert that has a very low mass coupled with ahigh temperature resistance. If left unfilled, balancing holes would add substantially to the

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Figure 3: Gamma Configuration ofStirling Engine

Figure 4: Typical Rotary DisplacerSegment

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engine’s dead space. The axial hole with keywaylocates the segment along the assembled dis-placer’s long axis while preventing rotationabout it. Individual segments are kept in positionthrough two retaining rings, one on either sideof each segment. One of these rings is shown inFigure 5.

Using a segmented displacer that had astepped profile that could enhance the heattransfer area was possible because the displacerwas rotary. This permitted the use of one of sev-eral relatively new engineered polymers.PolyEtherEtherKetone (PEEK) has mechanicaland thermal properties that make it ideal for thisapplication. These include: tensile strength of 16ksi, compressive strength of 20 ksi, maximumoperating temperature of 480°F, thermal conduc-tivity of 1.75 BTU-in./ft.2-hr.-°F, and a coeffi-cient of thermal expansion, 2.6 X 10-5in./in./°F(Boedeker Plastics, Inc., 2011, pp. 2-3). Thus,PEEK is a very good insulator that will neitherabsorb nor transmit too much thermal energy. Itis also strong for a polymer and has a high oper-ating temperature ceiling. Like most polymers,however, it “grows” when heated, and thisgrowth is more than that encountered with mostmetallics. The type of stainless steel used in thewater-cooled displacer housing is AISI 304,which has a much lower coefficient of thermalexpansion, 9.6 µ in./in..-°F (Oberg, Jones,Horton, & Ryffel 2004, p. 472). Because thedisplacer segments are individually locatedalong the axis of the housing using retainingrings, their relatively large coefficient of expan-sion is of consequence for a relatively short lat-eral dimension, their individual thickness. Thisexpansion is easily accommodated in the sizingof the corresponding workspace for a given seg-ment, even though the stainless “grows” very lit-tle by comparison.

The Rotary Displacer Assembly

Adoption of a rotary displacer over a recip-rocating one provided two significant advan-tages. These are reduced cycle power needs andincreased design flexibility. Each will be brieflydiscussed.

Reduced cycle power needs. Reciprocatingdisplacers waste energy. In the 360° rotation rep-resenting one complete cycle, the displacerchanges direction twice. The displacer is acceler-ated from rest at 0°. It achieves maximum veloc-ity at 90°, followed by deceleration and restagain at 180°. It is again accelerated achieving

maximum velocity at 270°, followed by deceler-ation as it approaches 360° in its return to start-ing point. Energy is consumed in both accelera-tion and deceleration. A rotary displacer, howev-er, never changes direction. Therefore, no engineenergy is consumed in the constant acceleration-deceleration associated with the reciprocatingarrangement. Further, its mass contributes tothat required by the flywheel/crankshaft assem-bly that is essential to the Stirling cycle.

Increased design flexibility. Reciprocatingdisplacers limit the design options available foroptimizing engine operation. This is becauseminimizing reciprocating mass must take prece-dence over other factors. Minimizing mass influ-ences material selection, which can result incompromises in thermal and mechanical proper-ties. For example, high stiffness, low weight,low thermal conductivity, and high thermalresistance are also very desirable displacer mate-rial characteristics. Unfortunately, it is difficultto identify a cost-effective material selection thatoffers such diverse characteristics and is, at thesame time, low mass. Also affected are the phys-ical design, the axial orientation (vertical vs.horizontal), and displacer surface topology (i.e.,its axial cross-section). Surface topologyincludes design features intended to increase thedisplacer surface area that is available for heattransfer. Minimal surface area for effective heattransfer has been cited as an issue. Senft (2007,p. 64) wrote “[i]n many real engines the expan-sion and compression processes for the mostpart occur in engine spaces that have relativelylittle heat transfer area.” Mechanism wear mustbe controlled to maintain an adequate servicelife. For the same reasons that they waste ener-gy, reciprocating displacers load other compo-nents (e.g., bearings and linkages), requiringmore robust design of these components.

A rotary displacer, at operational speed,exerts radial and thrust loads only. These are thenormal loads placed on bearings, and they donot adversely influence engine design in otherareas. A rotary displacer also opens up designoptions in other ways. Because mass is not anissue, there are novel design opportunities forthe displacer. The rotary displacer can be long,can incorporate features for increased surfacearea, and can still function from ambient tooperating temperature within the close confinesof the stainless housing.

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Figure 5 shows the fully assembled, seg-mented rotary displacer. The top surface of thedisplacer creates the hot and cold workspaces ofthe engine; this is in concert with the hot andcold sections of the displacer housing. The linesappearing along the circumference, indicatingthe sides of each segment, also represent thephysical space between segments. This spaceallows for the thermal “growth” that is

inevitable as the segment warms from closeproximity with the hot workspace. Each segmentis restricted in its lateral movement associatedwith this “growth,” by two retaining rings. Themaximum extent of this movement, either left orright, always coincides with the width of thecorresponding radial groove in the housing.

The Displacer Housing

The displacer housing consists of fourmajor components, all of type 304 stainlesssteel. The hot and cold workspaces are achievedthrough two partial cylinders that are joinedalong their edges to form a complete cylinder. APEEK seal is located along their linear joints toprevent thermal conduction between the hot andcold workspaces. Their internal configuration, aspreviously noted, is a mirror image of the rotarydisplacer. The ends of the housing contain thebearings and provisions for liquid cooling.Additionally, the right-hand end contains fea-tures that contribute to the rotary valve mecha-nism as well as the means for the hot and coldworkspaces to communicate with the power pis-ton. The internal ends of the housing are facedwith PEEK seals to prevent thermal conduction.Clearances between the rotary displacer and thehousing are tight to minimize dead space butsufficient to ensure no contact. Figure 6 shows asingle displacer segment in a partial assemblywith half of the displacer housing. In this figure,the housing shown is that of the cold workspace(note the coolant manifold connections at thelower left). A single displacer segment is shown

here to clarify the internal configuration of thehousing, the cold workspace. One segmentretaining ring also appears clearly.

Each of the radial grooves along the innersurface of the cold workspace corresponds toone of the 15 segments not yet in the displacerassembly. The internal configuration of the hotside of the displacer housing is identical to thatof the cold side.

Rotary Valve Mechanism

Another design feature that is possiblebecause of the rotary displacer is a rotary valveintegral with the first displacer segment and theadjacent portion of the housing. This arrange-ment does not add additional moving parts, butit effectively directs the working fluid in the fol-lowing way. The power cylinder has two ports,one in communication with the hot workspacethrough the hot port (see Figure 6) and one incommunication with the cold workspace throughthe cold port. The cold port is not visible inFigure 6; it is obscured by the segment.Regardless, it is situated 180° from the hot portand lies on the same circular centerline in theside of the displacer housing. These ports arepositioned immediately adjacent to the steppedprofile groove occupied by the first displacersegment. Recall that the stepped profile of thesegments exists for about 180° only (see Figure3). Hot and cold ports connect to the powercylinder through passageways. The cold port andits passageway represent a heat sink in that theyare kept cold by the engine’s liquid cooling sys-tem. These areas are also insulated from the hotworkspace. The hot port and its passageway rep-resent thermal energy input, being in communi-cation with the hot workspace. When the dis-placer rotates through its cycle, it alternatelyblocks and opens the hot and cold ports. Duringisovolumetric heating (expansion), therefore, thehot working fluid is inhibited from moving

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Stepped Profile

Surfaces of the Displacer WhichCreate Hot and Cold Workspaces

Retaining Ring

Figure 5: Complete Segmented RotaryDisplacer

First Displacer Segment

Coolant Connections

Hot Port

Retaining Ring Grooves

Balancing Holes

Figure 6: Open Displacer HousingShowing Cold Workspace and PartiallyAssembled Rotary Displacer

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through the cold passageway, even though pressure throughout the engine is the same.Similarly, during isovolumetric cooling (com-pression), the cold working fluid is inhibitedfrom moving through the hot passageway.

Preliminary Findings From Initial Running ofSWATT Engine

This engine has been under development forapproximately 10 years, and much of this timehas been spent on design and fabrication. Figure7 shows the assembled engine from the perspec-tive of the cold workspace. Major subassemblieshave been identified.

In order to run the engine, various auxiliaryunits must be connected. These include apropane gas burner, connections for the liquidcooling, and various instrumentation compo-nents. To facilitate collection of engine perform-ance data, a test bed was constructed to hold theengine and auxiliary units. The complete setuphas been operable for about two years and hasaccrued data from about 120 hours of run time.It is instrumented with five externally mountedsurface contact type K thermocouple probes.These are located at various positions to collecttemperature distribution data. Two probes moni-tor hot workspace (thermocouple locations 1 and3), two monitor cold workspace, and one moni-tors power cylinder temperature, also represent-ing cold workspace (thermocouple locations 2, 4and 5, respectively). Thermocouple probes areconnected to two Fluke 52 II and one Fluke 51II digital thermometers. This instrumentationdisplays data that provides the mean hot work-space temperature, the mean cold workspacetemperature, and the resulting difference in tem-perature (∆T). A noncontact laser phototachometer, an Extech Model 1PX61, is alsomounted on the test bed. This instrument displays the engine’s RPM. Because there is apatent pending on the engine, it is now possibleto release some design and performance information.

Although its appearance is radically differ-ent from Alpha, Beta, or Gamma Stirling engines,the SWATT engine definitely functions on theStirling cycle (i.e., the nonconsumed workingfluid is alternately heated and cooled but alwaysreturns to its initial state with each revolution).Although work on the design parameters it stillunder way, some unexpected findings regardingone, the phase angle, have come to light. The idealphase angle was found to be remarkably differentfrom that typically attributed to either Beta orGamma engines, which operate with a phaseangle of approximately 90°. As previously noted,the SWATT engine was initially designed andbuilt with this angle. Initial engine runs, however,indicated ceiling RPM of about 400, which wasless than anticipated. By varying the phase angleon the flywheel/crankshaft assembly, it was possi-ble to experimentally determine an ideal phaseangle through observation of tachometer readings.

Determination of the Ideal PhaseAngleEccentrics

The flywheel/crankshaft assembly consists ofseveral components, one of which serves as aneccentric. A cylindrical piece, the eccentric isbored through its longitudinal centerline to con-nect to the rotary displacer axle. When assem-bled, only one orientation is possible. A crankpin, which engages the power piston connectingrod, screws into the eccentric. The distance fromthe eccentric centerline to that of the crank pinestablishes the stroke of the engine. The radialdisplacement of the crank pin, in relation to thatof the rotary displacer, establishes the phaseangle. Eccentrics were constructed to enableengine operation with phase angles of 75, 80, 85,115, 120, 125, 130, and 135 degrees. Phaseangles on either side of 90° were selected becauseit was not known whether an ideal angle was lessthan or greater than 90°.

Engine Performance Log

An engine performance log was developed tostandardize data collection during phase angletesting. The log provided for manual recording ofthe time of an observation, the temperatures atthe five thermocouples, and the RPM. Alsorecorded were the mean hot workspace tempera-ture, the mean cold workspace temperature, andthe temperature difference (∆T) between each.The last three quantities are calculated andrecorded post test. A representation of this formis depicted in Table 1. This contains test data forthe 90° phase angle.

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Power Cylinder

Flywheel/CrankshaftAssemblyCoolant Manifold - In

Coolant Manifold - Out

Displacer Housing,Cold Workspace Side

Figure 7: Assembled SWATT Engine

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Note that the first data row, highlighted inred, shows only time and thermocouple tempera-tures at the start of the test. Calculation of meanhot and cold workspace temperatures as well asthe engine workspace temperature difference(∆T) are irrelevant at this point. At the bottom ofthe table, means are provided for temperaturestaken at all thermocouple locations, grandmeans for the hot and cold workspaces, meantemperature difference, and mean RPM for theduration of the test run.

Phase Angle Testing ProcedureExperience running the engine has demon-

strated that it needs approximately 20-23 min-utes of initial run time to stabilize operation.The term stabilize implies that engine RPM is ator very near maximum, and minimal burneradjustment is necessary to maintain that RPM.This condition is termed steady state, and onceachieved, heat input and RPM output remain rel-atively consistent over the duration of the enginetest run.

During the initial 20 minutes of operation,however, constant adjustment of the burner isrequired. Thus, temperature and RPM data col-lected during this time display fluctuations thatnecessarily differ from test run to test run. Thisis evident in first 11 rows of temperature datafor thermocouples monitoring hot workspace(T1 and T3) and the respective RPM figuresshown in Table 1. The magnitude of fluctuationsis dependent on how aggressively heat isapplied. Both excessive heat and insufficientheat are detrimental to maximal RPM. Thoughhigh temperatures and RPMs can occur duringthis time, they do not characterize sustainableoperation. These initial temperature and RPMfigures are useful, even essential, in stabilizingoperation, but that is the extent of their utility.For this reason, it was decided that all test runsin the phase angle study would be 31 minutes induration with a 23-minute stabilization period,during which collected data would not be uti-lized to assess the effectiveness of a given angle.Only the last 8 minutes of the test would be used

102 Thermocouple Locations (1-5) andRecorded Temperatures (°F)

Time(h:m) T1 T2 T3 T4 T5

MeanHot:

(T1 + T 3) ÷ 2

(°F)

MeanCold:

(T2 + T 4 + T 5)÷ 3

(°F)

∆T:(Mean H ot –Mean C old)

(°F)RPM

4:00 77 77 77 77 75 N/A N/A N/A N/A

4:05 182 80 181 79 76 182 78 104 576

4:07 197 81 185 80 77 191 79 112 577

4:09 197 82 169 81 78 183 80 103 420

4:11 198 82 169 81 79 184 81 103 484

4:13 198 82 168 81 79 183 81 102 428

4:15 190 82 156 81 80 173 81 92 368

4:17 198 82 174 81 80 186 81 105 511

4:19 204 82 179 81 81 192 81 111 407

4:21 198 83 164 81 81 181 82 99 382

4:23 193 83 158 81 81 176 82 94 364

4:25 193 82 159 81 81 176 81 95 392

4:27 194 83 159 81 82 177 82 95 408

4:29 194 82 160 81 82 177 82 95 418

4:31 194 82 161 81 82 178 82 96 426

µ 195 82 167 81 80 181 81 100 440

Table 1 90° Phase Angle Performance Log Showing RPM and Temperature in °F

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to collect data representing the performance of agiven phase angle. The temperature and RPMsampling interval for a complete test run was setat 2 minutes with the exception of the initialreading, which was 5 minutes from the com-mencement of the test (the engine requires anapproximate 3-minute warm up prior to run-ning). All temperatures were recorded in degreesFahrenheit (°F). Temperatures and RPMs wererecorded to the nearest integer and standardrounding practice was used.

Findings: Phase Angles 75°, 80°, and 85°

The 75° phase angle was tested first. It wasnot possible to start the engine. The same resultsoccurred with the 80° phase angle. At the 85°phase angle, the engine started, but it provedvery difficult to stabilize, and the test run wasaborted. Therefore, there is no performance datafor the first three tests. Lack of run data, howev-er, did provide valuable insight. Because the ini-tial engine configuration successfully employeda 90° phase angle, the difficulties encountered

with angles of 75°, 80°, and 85° demonstratedthat if there were potential for improved engineRPM performance through phase angle manipu-lation, that manipulation would involve phaseangles of 90° or more.

Findings: Phase Angle 90°

An eccentric having the 90° phase anglesuggested by the literature was initially builtwith the engine. It was effective when theengine was first run, but empirical evidencecharacterizing the extent of its effectiveness hadyet to be collected. Therefore, the fourth phaseangle test was based on 90°, and, as previouslynoted, findings are shown in Table 1. This table,however, can be condensed through the elimina-tion of the run data accrued during the 23-minute stabilization period. This reformatting(see Table 2), which will be used in the discus-sion of remaining phase angles as well, makes itpossible to compile means for temperatures andRPMs that are not influenced by startup fluctuations.

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Thermocouple Locations (1-5) andRecorded Temperatures (°F)

Time(Minutesinto Ru n)

T1 T2 T3 T4 T5

MeanHot:

(T1 + T 3) ÷ 2

(°F)

MeanCold:

(T2 + T 4 + T 5)÷ 3

(°F)

∆T: (Mean H ot –Mean C old)

(°F)RPM

25 193 82 159 81 81 176 81 95 392

27 194 83 159 81 82 177 82 95 408

29 194 82 160 81 82 177 82 95 418

31 194 82 161 81 82 178 82 96 426

µ 194 82 160 81 82 177 82 95 411

Table 2 90° Phase Angle Performance Log Showing RPM and Temperature in °F after Engine Stabilization



T1 T2 T3 T4 T5

MeanHot:

(T1 + T 3) ÷ 2

(°F)

MeanCold:

(T2 + T 4 + T 5)÷ 3

(°F)


(°F)RPM

25 194 80 163 77 81 179 79 100 713

27 196 80 164 78 82 180 80 100 709

29 197 80 163 78 82 180 80 100 696

31 197 80 162 78 82 180 80 100 685

µ 196 80 163 78 82 180 80 100 701


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Findings indicate a high level of consisten-cy for virtually all temperature readings. Theconsistency of cold workspace temperatures(taken from locations T2, T4, and T5,) indicatessustainable, effective cooling (grand mean of82°F). This is characteristic of steady state oper-ation. A similar statement can be said of hotworkspace temperatures taken from locations T1and T3 (grand mean of 177°F). The 90° phaseangle also rendered a mean ∆T of 95°F and anRPM of 411.


Contained in Table 3 are findings from the115° phase angle test. Again, temperatures display consistency. Cold workspace tempera-tures had a grand mean of 80° F, whereas hotworkspace temperatures indicate a grand meanof 180°F. This, in conjunction with consistencyin RPM, indicates the engine is functioning nearsteady state. The mean ∆T was 100°F and themean RPM was 701. The RPM statistic repre-sents a major improvement over that obtainedwith the 90° angle suggested by the literature.


Table 4 contains the findings for the 120°phase angle test. Temperatures remained consis-tent at a given thermocouple over time. Coldworkspace temperatures had a grand mean of82° F, whereas hot workspace temperatures indi-cate a grand mean of 179°F. These values aresimilar to those derived from the 115° phaseangle data. The ∆T, at 97°F, dropped slightlyfrom that encountered with the previous phaseangle, but the average RPM recorded during thetest increased to 712.


Table 5 contains the findings for the 125°phase angle test. Individual hot workspace ther-mocouples (T1 and T3) indicated temperaturesthat remained consistent, but to a somewhatlesser degree than the previous test. Individualcold workspace temperatures, as measured bythermocouples at locations T2, T4, and T5, dis-played the same high level of consistency asseen in previous tests.

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T1 T2 T3 T4 T5

MeanHot:

(T1 + T 3) ÷ 2

(°F)

MeanCold:

(T2 + T 4 + T 5)÷ 3

(°F)


(°F)RPM

25 194 82 163 81 82 179 82 97 729

27 195 83 163 81 82 179 82 97 720

29 195 83 162 81 82 179 82 97 708

31 195 83 162 81 82 179 82 97 691

µ 195 83 163 81 82 179 82 97 712




T1 T2 T3 T4 T5

MeanHot:

(T1 + T 3) ÷ 2

(°F)

MeanCold:

(T2 + T 4 + T 5)÷ 3

(°F)


(°F)RPM

25 194 75 160 71 81 177 76 101 776

27 192 74 157 71 81 175 75 100 800

29 191 74 156 71 81 174 75 99 798

31 191 74 157 71 81 174 75 99 782

µ 192 74 158 71 81 175 75 100 789


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Cold workspace temperatures had a grandmean of 75° F, whereas hot workspace tempera-tures indicate a grand mean of 175°F. The aver-age ∆T returned to 100°F. The average RPMrecorded during the test increased to 789, thehighest yet obtained.


Contained in Table 6 are findings from the130° phase angle test. Again, temperaturesremained consistent. The mean ∆T was 100°F,but the RPM dropped to 738.


This was the last phase angle tested. Findingsindicate consistency in all cold workspace tem-perature readings (grand mean of 82°F). Datafrom thermocouples monitoring hot workspacetemperature (T1 and T3), however, display somevariability that was not evident in previous tests.The hot workspace temperatures indicate a grandmean of 176°F. The average ∆T was 94°F, and the

RPM dropped to 700 (see Table 7). This is thesecond consecutive drop in RPM, and it repre-sents a substantial loss from the peak RPM valueobtained with a 125° phase angle.

Summary of Findings Regarding the Ideal PhaseAngle

The SWATT engine is a Stirling, however, itfunctions best with a phase angle that is signifi-cantly different from other Stirling engine con-figurations. It is not possible, at this time, tospeculate as to why. What can be said is theideal phase angle for SWATT is near 125°.

The mean ∆T has been reported in the dis-cussions of phase angle tests. This has been pro-vided for information purposes only. Although itseems to cluster around or at 100°F, it is not spec-ulated that this has any relationship whatsoever toa maximized RPM for a given phase angle. Hadthe test procedure specified temperature measure-ments in °C, the derived values for mean ∆Twould not have appeared so “calculated.”

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T1 T2 T3 T4 T5

MeanHot:

(T1 + T 3) ÷ 2

(°F)

MeanCold:

(T2 + T 4 + T 5)÷ 3

(°F)


(°F)RPM

25 193 81 163 79 80 178 80 98 794

27 193 81 164 79 80 179 80 99 795

29 194 81 165 79 81 180 80 100 751

31 195 81 166 79 81 181 80 101 610

µ 194 81 165 79 81 180 80 100 738




T1 T2 T3 T4 T5

MeanHot:

(T1 + T 3) ÷ 2

(°F)

MeanCold:

(T2 + T 4 + T 5)÷ 3

(°F)

∆T:(Mean H ot –Mean C old)

(°F)RPM

25 192 83 159 81 82 176 82 94 704

27 187 83 154 81 82 171 82 89 700

29 191 83 161 81 82 176 82 94 739

31 193 83 164 81 82 179 82 97 658

µ 191 83 160 81 82 176 82 94 700


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Contained in Table 8 is a brief summary offindings that support a phase angle of 125°.Other performance characteristics have beenobserved during the initial running of theengine. The mean ∆T from ambient at whichinitial running occurs is 70°F with a correspon-ding RPM of about 550. From this point, theengine gradually heats to a mean ∆T of 99°Fand the sustained 800-850 RPM. This repre-sents steady state and the performance remainsbasically unchanged for the length of an individ-ual test run (maximum to date of about sixhours). This performance is based on the previ-ous indicated parameters while using air as the

working fluid, atmospheric buffering frombelow, and no regenerator. The mean ∆T at initial start is noteworthy. It has been document-ed on all engine tests utilizing the 125° phaseangle.

The Direction of Future Research Regarding thisEngine

Commercial Stirling engines utilize noblegasses, such as hydrogen or helium, as the work-ing fluid. These have superior thermodynamicproperties. The SWATT engine was alsodesigned for pressurized helium, which requiresa gas-tight enclosure surrounding theflywheel/crankshaft and power piston/cylinderassemblies. The fabrication of this enclosure isincomplete at this writing. Once the enclosure iscomplete, the research will commence with opti-mization of selected traditional Stirling designparameters (e.g., volume compression ratio).The really significant optimization, however,will focus on the configuration of the rotary dis-placer itself. This is believed to be the key fea-ture in terms of simplicity, reliability, and, hope-fully, increased power output. Of particular con-sequence is the configuration of the first

displacer segment. This controls the opening andclosing of the hot and cold ports and also theduration of same as well as any overlap, duringwhich times both ports are simultaneously open.

Implications of this Technology for an EnergyConscious World

The idea of using the Stirling cycle inhybrid electric applications is not new. Recentresearch provides ample evidence. Obtainingreliable, sustained engine operation coupledwith reasonable power output, however, consti-tutes the real problem. When this becomes reality, there will be myriad applications.

The initial application of this technology, as faras the author was concerned, was in associationwith the small electric-powered vehicles thatwere recharged from the grid during non-opera-tional periods. The storage batteries used bythese vehicles provided sufficient charge forintra-city commuting for up to four hours, afterwhich recharging was necessary. The vehiclescould operate somewhat longer, however, failureto recharge at this point would shortly cause acondition known as deep discharge. Deep dis-charge always required excessive grid timerecharging and could also be potentially damag-ing to the batteries. An on-board Stirling engine,operating on environmentally friendly fuels,could act as an auxiliary recharger in the field,thus extending the vehicle’s range and lesseningrequired charging time on the grid.

A similar concept involves fleet use. In thisapplication, a larger Stirling-powered generatorin a stationary configuration, would be har-nessed to recharge batteries of small, electric-operated fleet vehicles. This would eliminate theadded cost of individual on-board Stirlingengines and would enable the use of exclusively

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Phase Angle ∆T (°F) RPM

90° 95 411

115° 100 701

120° 97 712

125° 100 789

130° 100 738

135° 94 700

Table 8 Summary of Phase Angle Study Findings

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electric propulsion without extra demands on the power grid.

On a broader scope, this engine could findapplication in electric generation through wasteheat recovery. Early research in this area, ascited in the introductory sections of this paper,indicates a potential contribution.

Dr. Phillip R. Foster is an Associate

Professor in the Department of Engineering

Technology in the College of Engineering at the

University of North Texas. He is a Member-at-

large of Epsilon Pi Tau.

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Innovative Rotary Displacer Stirling Engine: Sustainable Power