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DOE/NASA!1060-4NASA TM-81658

Applicability of AdvancedAutomotive Feat Engines toSolar Thermal Power

(NASA-TM-81658) APPLICABILITY OF ADVANCEDAUTOMOTIVE HEAT LNGINES TC SOLAR THERMALPOWER (NASA) 26 p HC A03/AF A01 CSCL 10E

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Unclas63/44 29575

Donald G. Beremand. David G. Evans,and Donald L. AlgerNational Aeronautics and Space AdministrationLewis Research Center

Work performed forU.S. DEPARTMENT OF ENERGYConservation and Solar EnergyDivision of Solar Thermal Energy Systems

Prepared forSociety of Automotive Engineers International EngineeringCongress and ExpositionDetroit, Michigan, February 23-27, 1981

DOE/NASA/1060-4NASA TM-81658

Applicability of AdvancedAutomotive Head Engines toSolar Thermal Power

Donald G. Beremand, David G. Evans,and Donald L. AlgerNational Aeronautics and Space AdministrationLewis Research CenterCleveland, Ohio 44135

Performed forU.S. DEPARTMENT OF ENERGYConservation and Solar EnergyDivision of Solar Thermal Energy SystemsWashington, D.C. 20545Under Interagency Agreement EX-76-A- 29-1060

Prepared forSociety of Automotive Engineers International EngineeringCongress and ExpositionDetroit, Michigan, February 23-27, 1981

APPLICABILITY OF ADVANCED AUTOMOTIVE HEAT

ENGINES TO SOLAR THERMAL POWER

by Donald G. Beremand, David G. Evans,and Donald L. Alger

National Aeronautics and Space AdministrationLewis Research CenterCleveland, Ohio 44135

ABSTRACT

The NASA Lewis Research Center (LeRC)as Project Manager of the Department ofEnergy's Automotive Heat Engine Program, isdeveloping advanced gas turbine and Stirlingengines for automobiles. To accomplish theprogram goals of improved fuel economy,reduced emissions, and broad alternativefuel capability major development programshave been undertaken for both engines.These programs are described along with thepredicted characteristics of the enginesunder development and the key technologyproblems that must be resolved to achievethese objectives. NASA LeRC is also respon-sible for the development of the power con-version systems for the Parabolic Dish SolarThermal Power Systems being developed forthe Department of Energy by the NASA JetPropulsion Laboratory. The requirements ofthis solar thermal application are reviewedand compared with the predicted characteris-tics of the automobile engines underdevelopment. A good match was found interms of power level and efficiency when theautomobile engines, designed for maximumpowers of 65-100 kW (87 to 133 hp) wereoperated to the nominal 20-40 kW electricoutput requirement of the solar thermal ap-plication. At these reduced power levels itappears that the automotive gas turbine andStirling engines have the potential to del-iver the 40+ percent efficiency goal of thesolar thermal program. However, in-depthstudies are required to determine the extentof modifications required to adapt the en-gines for the solar application, and tofully assess the probable life, durabilityand resultant efficiency that can realisti-cally be achieved in this application.

- - - -m ,

THE NASA JET PROPULSION LABORATORY (JPL),California Institute of Technology, is con-ducting a program foe the U.S. Department ofEnergy to develop Parabolic Dish Solar Ther-mal Power Systems. In this effort the NASALewis Research Center (LeRC) is responsiblefor the selection and development of thePower Conversion Subsystem (PCS). The PCSconverts thermal energy to electricity andnormally includes an engine, an alternator,and associated auxiliaries.

Consideration is being given to adapt-ing the automotive Stirling and gas turbineengines, currently in development under theDepartment of Energy's Automotive HeatEngine Program, to this use. The LewisResearch Center also has project management

`

responsibility for the Heat Engine Program.In this program major development effortsare underway to develop improved gas turbineand Stirling engines which will provide atleast a 30 percent improvement in fueleconomy, broad alternative fuel capability,and reduced emissions. In this paper, theseengine development efforts are brieflydescribed, and the projected characteristicsof the engines are evaluated against therequirements of the engines for the PowerConversion Systems in the solar thermalapplication.

BACKGROUND - AUTOMOTIVE HEAT ENGINE PROGRAM

The Automotive Heat Engine Program wasbegun by the Environmental Protection Agency(EPA) in 1971 and had the initial objectiveof developing alternative automotive heatengines with significantly reduced exhaustemissions. In 1973, the objectives of im-proved fuel economy and multifuel capabilitywere added. In this program all known typesof heat engines were evaluated and moredetailed investigations of the more promis-ing candidates were carried out. Today, theHeat Engine Program has converged on the twomost promising alternative engine candi-dates, Gas Turbine and Stirling.

With the formation of the EnergyResearch and Development Agency (ERDA), theEPA automotive propulsion system activitieswere transferred to ERDA, and additionalemphasis was placed on developing alterna-tive propulsion systems with substantially 2improved fuel economy and adaptability tovarious fuels while at th, same time meeting

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the legislated emissiooi standards. In thisrevised program, project management respon-sibility for implementation of the Automo-tive Heat Engine Program was assigned to theNASA Lewis Research Center (LeRC). Thisrelationship was continued when the trans-portation conservation activities of ERDAwere transferred to the newly formed Oep4rt-ment of Energy (DOE), and continues today.

In February 1978, the President signedinto law Title III, P.L. %-238 entitled,"Automotive Propulsion Research and Develop-ment Act of 1978." This law specificallydirects the Department of Energy to "estab-lish.and conduct new projects and accelerateexisting projects which may contribute tothe development of advanced dutomobile pro-pulsion systems and give priority attentionto the development of advanced propulsionsystems, with appropriate attention to theseadvanced propulsion systems which are flexi-ble in the type of fuel used." Consistentwith these and other directives of P.L.95-2.i8 and with specific guidelines providedby the DOE Office of Transportation Pro-grains, the Advanced Automotive Heat EngineProgram now has the following goal:

Tr develop and demonstrate in 1984-1985advanced gas turbine and Stirling automobilepropulsion systems that meet the followingobjectives;

- At least 30 percent improvement infuel economy (mpg) over a 1984 productionvehicle of the same class and performance,powered by conventional spark ignitionengines (based on equal BTU content of fuelused) .

- Emissions levels that meet or exceedthe most stringent Federa l, :-asearcnstandards; 0.4/3.4/0.4 glmi, HCICU/NOx -(particulate levels will bei added whendefined).

- Ability to use a broad range of liq-uid fuels derived from crude oil as well assynthetic fuels from coal, oil shale, andother sources.

- Suitability for cost competitive massproduction.

Current development efforts for boththe gas turbine and Stirling engines arebeing carried out in accordance with theProgram Schedule Chart, Fig. 1. As shown,development efforts for both engines includetwo engine generations and will concludewith verification testing by EPA of theMUD 2 engines in vehicles.

TECHNOLOGY HISTORY - AUTOMOTIVESTIRLING ENGINE (ASE) - The Stirling engineis a relatively undeveloped engine with nocurrent base of production engine experiencefor any application. Further, the technol-ogy base for the Stirling engine residesprimarily in Europe with N. V. Philips,United Stirling of Sweden, andMaschinenf abriek Augsberg Nuremberg(M.A.N.). General Motors Corporation con-ducted an extensive Stirling engine effortfrom 1958 to 1970 under a license agreementwith N. V. Philips. They performed sub-stantial development work on the Stirlingengine during this period (1)*, butapparently never seriously pursued itsapplication to the conventional auttvnobile,The Ford Motor Company established alicensing agreement and undertook a jointeffort with N. V. Philips in 1971. Underthis agreement Ford set out to specificallyassess the Stirling engine for theautomobile. As part of this effortN. V. Philips designed and built the fourcylinder, 120 hp, swashplate drive, 4-215Stirling engine (Fig. 2) for installationand test by Ford in a 1975 Torino vehicle.Results of these tests, published in Ref. 2,yielded fuel economies from 10 to 20 percentbelow the baseline spark ignition poweredTorino vehicle, and 13 to 23 percent belowpredicted values. In spite of these rela-tively poor initial results, it was believedthat the Stirling engine did have signifi-cant fuel economy improvement potential.This view was supported by the AutomobilePower Systems Evaluation Study (3), com-pleted by the Jet Propulsion Laboratory in1975.

In September 1977 the Ford Motor Com-pany initiated work on a seven year, costshared Stirling Engine Development contractfunded by DOE and managed by NASA. Thefirst year of this effort was an intensivefuel economy assessment effort aimed atfirmly establishing the fuel economy poten-tial of the Stirling engine in theautomobile.

This activity utilized the Ford 4-215Stirling engine as a date base and estimatedfuel economy improvement potential of a pro-

*Num ers in parentheses designate References 4at end of paper.

jetted fourth generation (1984) engine basedon both analytical and experimental evalua-tions of potential improvements. The re-sults indicated an excellent fuel economypotential of from 38 to 81 percent beyondthat of the baseline spark ignition enginevehicle of the same class and perfonmance.In spite of these results, Ford Motor Com-pany chose to terminate their Stirling

` engine activities due to tl.?ir need to de-vote available resources to more near termproblems.

The primary engine development efforti in the Stirling Engine Program is now beingi conducted by a team consisting of Mechanical

Technology Inc. (MTI), United Stirling ofSweden (U.S.S.), and AM Gene ral (AMG) - a

wholly-owned subsidiary of American MotorsCorporation. This DOE funded, NASA con-tracted effort was initiated onMarch 22, 1978. This effort is directed tothe oevelopment of an advanced experimentalStirling engine for automotive applicationwhich will meet the program goal and achievethe transfer of Stirling engine technologyto the United States. MTI is responsiblefor overall program management, developmentof component and subsystem technology, andtransfer of Stirling engine technology toU.S. manufacturers. USS is primarily re-sponsible for engine development. AMG isresponsible for engine-vehicle integration,testing, and evaluation. In addition, it isintended to add to the project team duringthe course of development an American enginemanufacturer who will be licensed to producethe Stirling automotive engine.

TECHNOLOGY HISTORY - AUTOMOTIVE GASTURBINE (AGT) - In contrast to the Stirlingengine, gas turbine engines have a long his-tory of development and production for awide variety of military and commercialapplications. The Lewis Research Center hasbeen involved to various extents in theevolution of the gas turbine engine forautomotive application for approximately20 years. Initially. LeRC was consulted bythe Chrysler Corporation in the area ofturbomachinery aerodynamic design for theirearly generations of experimental autoc,.otivegas turbine engines. To our knowledge, thiswas the first consideration of aircraft oradvanced technology to this type of engine.

'` The performance and features of the last of

these engines, the 6th generation Chryslerengine, is shown as item e. Fig. 3 andTable 1.

Direct Government involvement in thedevelopment of the gas turbine engine as anapproach to reducing automotive emissionscame in 1972 with the award of a contract toChrysler. Under this contract, durabilitytests of the 6th generation engine for3500 hours of automotive type operation weresuccessfully performed. The engine was alsodown-sized and upgraded with componentsembodying early 1970's technology. Thisincluded an air bearing, a ceramic regenera-tor, an elec'kronic control system, a low-emissions combustor system and LeRC-designedcompressor and turbine aerodynamics. Theresults of this effort, the Upgraded Engineproject, item d, Fig. 3 and Table 1, showedthat the application of the improved tech-nology could improve the thermodynamic effi-ciency of the engine, particularly at part-powers where the engine operated over mostof its automotive duty cycle.

Part of this increase in thermodynamicperformance was due to a 24° C (75' F) in-crease in the allowable turbine-inlet-temperature (TIT) made possible by the useof more advanced metal alloys in the tur-bines. However, these materials were con-sidered too costly and strategic for wide-spread automotive use, and while the steadystate fuel economy approximated that of thespark ignition engine, it fell far short ofthe newly established Automotive Heat EngineProgram objectives. To meet these new ob-jectives, in-depth LeRC-directed and DOE-funded studies were conducted by automotiveand aerospace contractors (4-7) to generateautomotive gas turbine designs that wouldmeet or exceed the program objectives.Considered were regenerative and recupera-tive engines having one, two, or threeshafts with ceramic and, in some cases,metal turbines, and with a level of technol-ogy which was felt to be achievable by nolater than the early-to-mid 198U's. Theresults of the studies (4-7) were three-fold. They indicated that (1) structuralceramic materials could be developed as areplacement for the metal alloys in the com-busters and turbines; that (2) as a result,the TIT could be increased by as much as 6333° C (600° F); and hence, (3) thermo-dynamic efficiency could be improved by a

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factor of approximately two, making the gasturbine engine capable of meeting the fueleconomy objectives of the Automotive HeatEngine Program.

Currently, two major AGT developmentefforts are underway. The first, pursuing a

A single shaft AGT, is being carried out byAiResearch Manufacturing Company (AiR) withFo-d Motor Company providing the vehicleintegration and some ceramic componentdevelopment activities. The second effort,a two-shaft AGT, is being conducted byDetroit Diesel Allison Division of GeneralMotors (DDA) with the Pontiac Division pro-viding vehicle integration activities.A third contract effort with ChryslerCorporation with support from Williams

i

Research Corporation (WRC) is being con-sidered pending availability of funds.

SOLAR THERMAL PCS REQUIREMENTS

A description of the complete solartnermal power system and a detailed discus-sion of the power conversion subsystem re-quirements is contained in a companionpaper, "Heat Engine Requirements forAdvanced Solar Thermal Power Systems" byH. G. Phan ► and L. P. Taffe, Jet PropulsionLaporatory, California Institute of Technol-ogy, being prepared for this same 1981 SAECongress. The following is a summary of keypreliminary PCS system requirements and con-straints which were assumed as the basisagainst which the ari)licability of automo-tive heat engines r ee assessed in this paper.

OPERATION - The PCS with the solarreceiver subsystem (SRS) must be able tooperate with solar input only, fuel firedheat input only, or with a combination ofsolar input and fuel fired heat. Since thepower system must track the sun, the PCSmust be capable of operation from 0 to 90*from the horizontal.

POWER AND PERFORMANCE - Rated net elec-trical output of 60 Hertz power is antici-pated to be between 20 and 40 kW, with thefinal value dependent on mirror size, andthe efficiencies of the various subsystems.The PCS efficiency at rated power shall be40 percent or greater (including the alter-nator unit and any auxiliary losses), andshall be 37 percent or greater at 50 percentrated power. Average output power is esti-mated to be in the order of 80-100 percentof rated power.

RELIABILITY AND MAINTENANCE - Time be-tween major overhauls should be at least100,000 hours of operation in the solar-onlymode. Minor maintenance should not exceedfour times per year nor require more thanone man-hour each time.

COMPARISON WITH AUTOMOBILE APPLICATION -In contrast to the above, the automobile hasa life requirement of about 3500 hours with-out major overhaul, a maximum power require-ment in the order of 65-100 kW (87 to133 hp), and an iverage power of about10-20 percent of maximum power. Further,the automobile engine experiences frequentrapid speed and power changes, particularlyin urban driving. These solar thermal andautomobile requirements are compared brieflyin Table 2.

AGT APPLICATION TO THE SOLAR THERMAL PCS

This section discusses the potentia!application of the three AGT engines evolvedfrom the studies of Refs. 4 to 7, and now indevelopment, to the solar thermalapplication.

PERFORMANCE EVALUATION - The predictedperformance and design features of the threeAGT engines are shown as items a, b, and cin Figs. 3 and 4 and Table 1. The effi-•ciency curves of Fig. 3 are for a variablespeed mode of operation. The variable com-pressor inlet guide vanes (VIGV's) in-corporated in these engines are fixed in theposition to provide maximum power and highefficiency at all spreas. All three designsaeliver near their maximum efficiencies inthe power range required for the PCS appli-cation. Differences in efficiency levelsbetween the engines reflect differences inturbine inlet temperatures and engine con-figuration since all three engines in-corporate appoximately the same level ofcomponent technology. Applying an assumedefficiency of 94 percent for the alternatorunit to each of the designs at a nominal30 kW power level yields "base" PCS effi-ciencies of 44.1, 38.2, and 36.7 percent forthe AiR, ODA, and Chrsyler designs, respec-tively. However, actual application of anyof the three engines must consider theunique features of each, and the potentialeffects of necessary modifications and sys- gtem integration on the installed performanceof the PCS. The following discussion

addresses areas of potential performanceimpact as well as considerations for dur-ability.

MODIFICATIONS FOR RECEIVER - Becausethe PCS must be capable of hybrid operation(operation on solar energy, fossil fuel. orboth). the AGT engine must retain its owncombustor system, or it muss, be incorporatedwith the solar receiver. In either case,addition of external flow ducting is re-quired to stage the airflow through both thesolar receiver and the Lombustor. An exam-ple of these structural changes and ductingadditions required for theAiResearch /Ford-AGT, Fig. 4(a), is shown inFig. 6. These changes increase the systempressure loss, leakage and heat loss, andrequires physical modifications to the com-bustor and engine housing and will requiredetai l ed evaluation.

MODIFICATIONS FOR ALTERNATOR - Integra-tion of the alternator unit into the PCSpresents a number of technical questions and,ii:-sign options. These start with the very„ gh turbine output shaft speeds rangingfrom 48,000 to 87,000 rpm, in Fig. 5 and thenaed to vary speed to modulate power.Potential design solutions include directdrive high speed alternators, the use ofgearboxes, variable speed drives, and avariety of electrical conversion arrange-ments, including do to ac conversions aswell as ac to ac frequency conversions.

In order to simplify the requirementsfor the alternator unit, a constant-speedmode of throttling the engine may be desir-able. This mode of operation is illustratedin Fig. 7 on the performance map for theAiR-AGT engine. The engines' variable inletguide vanes (VIGV) would be utilized to varythe power at constant speed. Throttlingline (a) on the figure shows that this modewould decrease engine efficiencies somewhatfrom those shown in Fig. 3 (throttling linesb and c on Fig. 7). However, the possiblebenefits of this mode of operation wouldhave to be evaluated against the potentialimpact on engine reliability and durability,part-power operating range, and the effi-ciency and operation of the alternator unitto determine its suitability for the PCS.

MODIFICATION FOR DURABILITY - The AGTengine must operate at from three to five 9times the average power ievel and for thirtytimes the number of hours in the PCS appli-

cation as compared to an automobile applica-tion (Table 2). However, the number ofequivalent start-stop cycles andacceleration-deceleration c ctes ma DeY Ysomewhat reduced. The rotative speed in-formation in Fig. 5 indicates that theengines in the solar application would oper-ate at shaft speeds of 62-87 percent oftheir maximum automotive design speed (gasi-fier speed in the case of the DDA-AGT2-shaft engine). Finally, Fig. 8 which is aplot, of the operating temperatures for theAGT's, indicates that the AiResearch enginewould operate at generally less than itsmaximum design turbine inlet temperature(TIT) but at its maximum regenerator inlettemperature (RIT), over the rated powerrange for the PCS, while just the reversewould be true for the ODA engine. For theautomotive application, the average rotativespeed for the AGT's range from approximately55 to 60 percent of maximum speed, the aver-ageTlT's are from 90 to 150 C (160 and210 * 0 ) below their maximum value, and theRIT is at its maximum value. The totalamount of engine time spent at 100 percentrotative speed and maximum TIT to satisfythe requirements for the automotive applica-tion is approximately 100 hours.

Evaluation of the combined effect ofthese differences on engine life and re-quired modifications is beyond the scope ofthis paper. However, several areas areapparent as follows:

The regenerator cores and seals willnot have adequate life to meet the require-ments for the PCS. Two to four man-hoursare required to replace the regeneratorseals in the Chyrsler Upgraded engine com-pared to the desire to limit the down-timefor periodic maintenance on the PCS to oneman-hour. This, combined with the adverseeffect that seal leakage has on efficiency,suggests the desirability to develop a re-cuperator to replace the regenerator sys-tem. Balanced against this would be anextensive modification to the engine's flowpath and housing to change to a recuperativesystem. Also, the technology and experiencein ceramic recuperator design is not asadvanced as regenerators.

The B-1 life for the rolling contactbearings used on both shafts of the DDA-AGT,and the cold-end of the shaft for theAiResearch and Chrysler ACT's will be rough-

10

ly twice the 3500-hour life required for theengines when operating over their normalautomotive duty-cycle. Those bearings wouldrequire replacement with probably air-journal-bearings and air-thrust-bearings,for tre PCS application. This would requirea bearing development and engine modifica-tion effort. The life of the contact shaftseals would also be inadequate, and wouldrequire ether periodic replacement, whichwould require engine teardown, or replace-ment with noncontacting labyrinth type shaftseals, which would probably increase engineair leaxage and performance losses.

The inherent buildup of oxide films onthe surface of ceramic parts may presentproblems for the PCS application. Matingparts in sliding contact with each other,such as the variable-geometry parts of theAGT combustor, or various housing parts, maystick or fuse together. This is because oftheir exposure to higher operating tempera-turEs for longer periods of time betweenchanges in engine speed and operating tem-perature, and over a much longer life thanin the AGT. The moving sections of the com-bustor are exposed to approximately 260 to320" C (470' to 575 0 F) higher air tempera-tures in the PCS when operating in thesolar-thermal mode than in the fuel-onlymode. However, elimination of the variable-geometry feature may be feasible withoutincreasing emissions if the operating rangefor the combustor in the PCS application isless than the AGT. Other modifications tothe combustor may also be required to pre-vent fuel coking, or auto-ignition duringhybrid operation.

ASE APPLICATION TO THE SOLAR-THERMAL. PCS

A schematic drewing of the ASE design,currently being developed, is shown inFig. 9. Its potential for application tothe solar thermal requirements is discussedin this section.

The projected performance map of theASE, based on information supplied by MTIand U.S.S., is shown in Fig. 10. The topperformance curve is a plot of net shaftpower versus engine speed for a charge pres-sure of 15 MPa 2175 psis) and heater tubetemperature of 820" C (1508' F). An overlay 11plot of constant net efficiency is alsoshown. Net efficiency is defined as net

shaft power divided by the Q of the fuel.Note that at 1800 rpm a net shaft power of39 kW (52.3 hp) can be developed at anengine efficiency between 42 and 42.5 per-cent. At 1200 rpm, A net shaft power of27 kW (36.2 hp) can be developed at an effi-ciency better than 43 percent.

Varying engine pressure and selectingeither 1200 or 1800 rpm allows the engineefficiency to be kept at approximately42 percent or higher over an engine powerrange of 17-39 kW (23-52 hp). It alsoallows constant speed operation for fixed60 Hertz f re4uency with a direct drivealternator. if we assume that the ASEengine heater head can be used in the solarapplication ith the same void volume andheater tube heat distribution characteris-tics as the automotive application, and thatheat is rejected by a natural convectionheat exchanger, the engine and PCS systemcan be expected to perform as shown inTable 3. In this analysis, Helium has beensubstituted for Hydrogen as the workingfluid. Auxiliaries for the solar applica-tion include in oil pump, water pump, andhelium compressor. This analysis indicatesa maximum PCS output power of 35.1 kW(47 hp) at 1800 rpm assuming a 94 percentefficient alternator unit. A full 40 kW(53.6 hp) PCS output power could be achievedby increasing engine speed to approximately21UO rpm. However, additional efficiencypenalties would be encountered, includingabout one paint in engine efficiency andeither gearbox losses or electrical f re-que ►icy conversion losses encountered inproviding the 60 Hertz output. The re-sulting reduced efficiency and increasedcomplexity should provide considerableincentive in final system sizing trade-offsfor limiting PCS output power to 35 kW(47 hp) or less.

MODIFICATION FOR PERFORMANCE - When aStirling engine is integrated with a solarreceiver as a power conversion system theheat exchanger tubes that intercept thesolar radiation may connect directly to theengine cylinder heads which contain theworking fluid. In some designs, a heat pipeheat transport system may couple the solarinsolation to the engine heater tubes.Since the Stirling engine heater tube volume 12is "dead" volume that decreases the cylicpower output of the engine, an attempt

should be made during engine design to mini-mize heater head dead volume without com-promising the he,t transfer properties ofthe heater head. Design of the Stirlingengine heater head for a solar applicationwill depend upon the type of solar receiverthat is attached to the engine. For exam-ple, a solar receiver that uses a sodiumheat pipe heat transport system, between thesolar heated evaporator end of the heatpipes and the engine heater tubes, willrequire less heat exchanger surface area(less dead volume) than a combustor ordirect solar radiation heated heater head.Such a heat transport system offers veryuniform heating of the heater tubes thuspermitting engine operation at higher aver-age neater tube temperatures for the samelimiting maximum heater tube temperature.This should yield a net improvement inengine efficiency.

The efficiency predictions of Table 3were based on Helium working fluid. Theutilization of hydrogen as the workingfluid, as in the ASE, would yield approxi-mately a two point efficiency improvementand should be carefully evaluated againstthe considerations for hydrogen handling,hydrogen permeability, and materialsstrength and compatability (see below).

MODIFICATION FOR DURABILITY - The oper-ating lifetime goal for the solar PCS is100,000 hours. Critical components of theASE engine that must be considered relativeto such a lifetime goal are the engine hotsection parts consisting of the heatertubes, cylinder housings, and regeneratorhousings.

There are several design approachesthat may be taken to extend the lifetime ofthese critical ASE engine components beyondthe present 3+00 hours. A significant in-crease in creep rupture lifetime will occurif engine hot section temperature is de-creased from near 800 * C (1472 F) toward700 * C (1292 F). Decreasing engine gaspressure also results in a substantial in-crease in creep rupture lifetime, especiallyat the lower temperature near 700 * C(1292 F ) . Of course, reducing temperaturesand pressures will also reduce engineefficiencies.

Stirling engines in the past have 13

achieved reasonable engine lifetime by usingcobalt based alloys for the hot section com-

>a

A-

ponents. N-155 alloy (20 percent cobalt)has been used f or the heater tubes, and NS31(54 percent cobalt) for the cylinder andregenerator housings. N. V. Philips companyhas tested an engine with heater tubes madeof N-155 alloy for more than 10,000 hourswithout any sign of degradation of the mate-rial (8). However, because cobalt is a rareard costly metal, a mass production enginecould not economically or strategically con-tain this araourt of cobalt.

Iron based alloys are being pursued asthe primary c&ndidates for use in the ASEengine in the automotive application becausenickel-based alloys are generally un-satisfactory for use with the hydroggen work-ing fluid. Typical alloys are 194L forthe heater tubes and XF818 for the cylinderand regenerator housings. Since helium isplanned as the working fluid for the solarengine application, nickel-based alloys mayalso be used. The creep properties ofInconel 617 and 625 are attractive forheater head fabrication. Nickel-chromium713LC alloy is an excellent cylinder andregenerator housing material. It has goodcastability and could be cast in the samemold that is used to cast the automobile ASEengine housings.

It is customary in cylinder and re-generator designs to define allowable creeplifetime of these components in terms ofpercent deformation. Typically, this fail-ure criterion is 1 percent creep deforma-tion. During design modifications of theASE engine for solar application, a studyshould first be performed to determine theextent of cylinder creep deformation thatcould be allowed in the design, short ofrupture, without degrading the ASE engineperfomance below an acceptable level.

For this application, piston rod andpiston ring seals may have to be consideredas maintenance replacement elements.Operation of the ASE engine at the lower(1200 rpm) speed should extend the lifetimeof these seals beyond the current 3500 hourobjective. However, achieving 100,000 hourswithout seal replacement woul^ appear veryunlikely. The rod seal that is used in theASE engine is a proprietary PumpingLeningrader seal that is under developmentby United Stirling Corporation. Enginetesting of the seal has exceeding 3000 hoursduration at a speed of 2000 rpm. The seal

14

has been tested in a ins and seal rigs fora total of more than 1600 hours (9).

Another type of seal that could 'be con=sidere4 as an alternate rod seal is thePhi 1 ips ,rollsock seal. This seal has beensuccessfully tested in a Philip"s 1-98engine for 10,000 hours at a speed of3000 rpm (10). Philips is also developing adual pumping ring seal as.an alternate tothe rollsock seal. This seal has accumu-lated more than 7000 hours of testing in aseal test rig. In consideration of theextremely long life requirement and the sus-ceptibility of the engine to degradationfrom oil contamination, some form of posi-tive rod seal such as the rollsock, a dia-phragm, or bellows should be given strongconsideration for this application.

Based on the above reported seal test-ing work, it is reasonable to assume that bythe year 1985 a reliable seal with a life-time of at least 10,000 hours can be devel-oped, provided that an intensive seal devel-opment effort is undertaken. Further deesignmodifications may be required to facilitatethe desired short seal replacement times.

SUMMARY AND CONCLUSIONS

This preliminary evaluation indicatesthat both the AGT and ASE engines have thepotential for meeting the efficiency objec-tives of the solar thermal program. How-ever, in both cases there is significantquestion about the potential for achievingthe life and durability goals of the pro-gram. Some reduction in efficiency mayresult during the actual process of adaptingthe engines to the application and in-corporating the necessary modifications indesign and/or operating conditions to meetthe life and durability requirements. Inboth cases, in-depth design studies arerequired to fully assess the probable effi-ciency, life, and durability that can beachieved and the amount of modificationrequired to adapt these automotive enginesto the solar thermal application. Assumingthat much of the existing basic engine de-signs could be retained, substantial devel-opment cost savings should be rea'lized withthis approach. However, it would be desir-able to evaluate the efficiency, life, dur-ability, production cost, and life cyclecost differences between this approach and

15

'A'A

that of "clean sheet" gas turbine and Stirl-inn engines designed specifically for thesolar thermal application, before enteringinto an active program to adapt one of theseengines to the solar thermal application.

REFERENCES

1. "A Collection of Stirling EngineReports from General Motors' Research - 1958to 1970," GMR-2690-Pts. 1-7, General MotorsResearch Labs., Warren, MI, April 1978.

2. "Stirling Engine Feasibility Studyof an 80-100 hp Engine and of ImprovementPotential for Emissions and Fuel Economy,"000/263142, U.S. Dept. of Energy,Washington, D.C., November 1977.

3. "Should We Have a New EngineW AnAutomobile Power Systems Evaluation, Vol. 1Summary, Vol. 2 - Technical Reports,"California Institute of Techonology, JetPropulsion Labdrat.ory, JPL-SP-43-17, 1975.Also Society of Automotive Engineers,New York, SAE-SP-J99, Vol. I and SAE-SP-400,Vol. 2, 1975.

4. Conceptual Design Study of ImprovedAutomotive Gas Turbine Powertrain. FordMotor Co., Dearborn, MI; A iResearch Mfg.Co., Phoenix, Al, NASA CR-159580, May 1979;

5. R. A. Johnson, Detroit DieselAllison, Indianapolis, IN, NASA CR-159604,June 1979;

6. C. E. Wagner and R. C. Pampreen,Chrysler Corp., Detroit, MI, NASA CR-159672,June 1979;

7. W. I. Chapman, Williams ResearchCorp., Walled Lake, MI, NASA CR-159852,March 1980.

8. H. C. Van 8eukering and H. F rokker,"Present State of the Art of the Philip:Stirling Engine," Combined Commercial Vehi-cle Engineering and Power Plant Meetings,Chicago, Ili., 1973.

9. M. Allen, "Automotive StirlingEngine Development Program,"DOE/NASA/0032-80/7, Mechanical Technology,Inc., Latham, NY, June 1980, pp. 3-58.

10. H. C. Van 8eukering and H. Fokker,"Present State of the Art of the PhilipsStirling Ei ,igire," SAE Paper No. 730646, 1973.

^^ s

16

Table 1. - Design Characteristics of Automotive Gas Turbine Engines

Engine designation Max. Max. Max. No. of Max. shaft Speed RemarksTIT, RIT, power, shafts gasifier, power,

JLFJ ^.rpm

(a) AiResearch/Ford AGT 1371 1094 97.5* 1 100,000 100,000 Ceramic regenerator,(2500) (2000) (130) burner, turbine

(b) ODA/Pontiac AGT 1288 1055 64.5 2 86,200 68,000 Ceramic regenerator,(2350) (1930) (86) burner, turbines

(c) Chrysler/WRC AGT 1260 983 65.3 1 99,500 99,500 Ceramic regenerators,(2300) (18DO) (87) burner, turbine

(d) Chrysler Upgraded 1052 748 78 2 58,500 50,000 Ceramic regenerator,(1925) (1378) (140) advanced controls,

compressor, turbines,and bearing technology

(e) Chrysler 6th generation 1010 727 113 2 44,600 44,500 Demonstrated 3500 hr.(1850) (1340) (150) endurance under EPA

part of program

*Gearbox limited to 75 kW (100 hp).

Table 2. - Requirements Comparison

Solar Thermal PCS vs. Automobile

Solar thermal PCS Automobile

Life, hours 100,000 3500Maximum power, kW 20-40 65-100Average power, percent of max. 80-100 10-20Transient operation infrequent Frequent

Table 3. - Solar ASE/Alternator PCS

[Natural convection heat rejection, helium working fluid]

1200 rpm 1800 rpmPower output* Power output*

Full Half Full Half

Engine speed, rpm 1200 1200 1800 1800Heater head temp., 'C ('F) 820 (1508) 820 (1508) 820 (1508) 820 (1508)Charge pressure, MPa (psis) 15 (2175) ** 15 (2175) **Brake power, kW (hp) 26.2 (35.1) 13.1 (17.6) 37.3 (50.0) 18.7 (25.1)Brake efficiency, percent 45.0 42.7 43.5 41.7PCS efficiency, percent 42.3 40.1 40.9 39.2

(alternator eff. - 94 percentPCS output power, kW (hp) 24.6 (33.0) 12.3 (16.5) 35.1 (47.1) 17.6 (23.6)

*Based on computed ASE performance curve (Fig. 10), but using estimated powerconsumption values of auxiliaries that are required for solar stationaryapplication.

**Pressure reduced sufficiently to produce half output power for same enginespeed and heater head temperature conditions.

AIR INLETREGENERATOR

COOLER TUBES,OUTPUT SHAFT

__CROSSHEADAND SLIDERS

'-ROLL SOCK

\PISTON ROD

DOUBLE ACTING

PISTON

HEATER TUBESCS-79-790

IGNITOR

ATOMIZER

EXHAUSI

BUR

FISCAL YEAR

77 78 79 ID 81 82 83 04 IS Ii

ADVANCED GASTURBINE

PROPULSION SYSTEM

ADVANCED STI ZLINGPROPULSION SYSTEM

1 4 2 S— —

1 4 2 S b•

--

^-

CODE

L 1ST GENERATION IMOD II POWERTRAIN DESIGN REVIEW2. MOD I POWERTRAIN CHARACTERIZATION COMPLETE

*EPA TEST 3. MOD I POWERTRAIN IN VEHICLEl MOD 11 POWERTRAIN DESIGN REVIEWS. MOD II POWERTRAIN CHARACTERIZATION6. MOD II POWERTRAIN IN VEHICLE

Figure L - Automotive hot engine program schedule.

PREHEATER

Figure Z - Ford/Philips stirling engine.

t ►XIG AL PAGE 1SOF P(K)R QUAUTY

s

t

ENGINE DESIGNATION

a AIR ACT, TIT • 131111 C 12W n, 1-SHAFT.p ODA ACT, TIT • 12W C 12W F), 2-SHAFT.C CHRYSLER AD% TIT - 126d a C f23000 F). 1-SHAFT.0 CHRYSLER UPGRADED, TIT - 10320 C 11925 n,

2-SHAFT.e CHRYSLER 6TH CQIFRATION, TIT - 10100 C 11W n.

2-SHAFT.• PROACTED FOR FULLY DEVELOPED ENGINE.

MO C 1850 n DAY152 m 1500 fU ALTITUDE

50

b.

35 ^^

W 30

w25 ((

20 /1

'%ce

RAZED POWER RANGE FORENGINE IN PCS APPLICATION

94% EFFICIENT ALTERNATOR UNIT

_e

10

S t I I I I I J

0 20 10 60 80 100 120 140 160ENGINE POWER, hp

0 15 30 45 60 75 90 105 120

ENGINE POWER, M

Figure 3. - Engine performance characteristicsfor variable speed mode of aeration.

x

i.k •" rSEARCHiFq ft

IDi DETROIT DIESEL ALLISONIPONTIAC.

ici CHRYSIFRIWILLIAMS RESEARCH CO.

Figure l - Schematic of AGT engines.

103 14D • DDA ACT (2-SHAFTI0 Alt ACT 11-SHAFT)

t10 UO o CHRYSLER AGT 11-SHAifI1 1 GASIFIER rpex10-3

100 RATED POWER

RANGE FOR AGT (f!<110

0 THE KSAPPLICATION (711AS

60 ---T-

^o

-- --- -- -

^ ^ 1641

--1

- - *-6&-F)F) DAY1S0 L

201 1 L IST►

`SZ L13M ftL 1 ALTITUDE

L 30 10 SO 60 70 /D 10DENGINE OUTPUT SPEED, rpex10'

Figure S. - AGT operating sp«ds for varisW sp@W madeof engine operation.

REGENERATOR

TURBINE

lr

FROMRECEIVER

r '^COMBUSTOR

TO

RECEIVER

/ \ PERMANENT

MAGNET

COMPRESSOR ALTERNATOR

Figure 6. - Sketch of example modifications to AIr-AGT for PCSapplication.

t

s&

w w

MrCI 11

I w a v ^ ^ ^ Gi

10 '3an1Va3dWU SVO

9

o ^'

.d b

Q'

30 '3an1Va3dW31 SVJ

^i I

c h- fi ri o

SZN j 9 >

gia Ix

S N^pt ^M

s ^0 'a3MOd 3NION3

^ t t i ! __L

AM '83MOd 3NI JN3

_

in o

8 9

S!-

OF

OB

i.9

0i

I^ATElt

-CYI

'" ^^—PISTONµ^ ASSEMBLY.GENERATOR

r ---PISTON RODJ_CROSS HEAD

DRIVE SHAFT-- —CONNECTING ROD

'CRANKSHAFT

SUMP--..^ x ^`CRANKCASE

ENGINE CRD.g S SECTION

Figure 9 - ASE engine schematic.

EO 15 MPS12175 psla)MEAN

HYDROGEN 12 M aPRESSURE , 11740 psis)

50

Weo (1305 psis) .

40a

ET EFFICIENCY

3t 420

` 6 MPa(870 psla)

'10

0 0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000ASE ENGINE SPEED. rpm

Figure 10. - Performance map of ASE engine.

_. 47^