Hydrogen-fuel Cell + IC Engines

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PERFORMANCE COMPARISON OF

HYDROGEN FUEL CELL AND HYDROGEN

INTERNAL COMBUSTION ENGINERACING CARS

G. Pearson1, M. Leary1, A. Subic 1, J. Wellnitz2

1SchoolofAerospaceMechanicalandManufacturingEngineering,RMITUniversity,

BundooraVictoria3083Australia; Email:[email protected]

2UniversityofAppliedSciencesIngolstadt,Esplanade10,IngolstadtGermany,

E-mail:[email protected]

Abstract: Students from RMIT University and the University of Applied Sci-

ences Ingolstadthave collaborated tobuild ahydrogen-powered racing car. As

partof the initialconceptualdesign,a lapsimulationwasdeveloped tocompare

performanceandfuelusageofhydrogeninternalcombustionengineandhydrogen

fuelcellvehicles. Forthevehicleandtrackspecificationsanalyzed,itwasfound

that fuelcellsrequireapowerdensityof5kg/kW tobecompetitivewith thehy-

drogen internalcombustionengine. Thestudyalsohighlighted thecomplexna-

tureofthealternativefuelsdebate.

1 Introduction

1.1 The case for hydrogen vehicles

Mankindsdependenceonpersonaltransportisincreasingaseconomiesgrowand

asemergingnationsmovetowardsamoreWesternlifestyle. Howeverthetrans-

portsectorreliesheavilyonpetroleumproductsasafuelsource,resulting in the

productionof large quantitiesofgreenhousegases,andexposureof the transport

sectortogreatuncertaintyinthefaceofdiminishingoilsupplies.

Apossiblealternativefuelishydrogen. Whenhydrogenreactswithoxygenin

theairtheonlyproductofreactioniswater,andthereforeitoffersthepotentialof

carbon-free transport. Two availablepowerunits forhydrogenvehicles are thefuelcell(H2FC)andtheinternalcombustionengine(H2ICE). Ofthetwooptions,

thefuelcelloffersadvantagesinpotentialefficiencyasthechemicalreactionen-

ergyisdirectlyharnessedasanelectriccurrenttodriveamotor. Incomparison,
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the H2ICE cycle includes an intermediary heat phase, restricting potential effi-

ciency to idealheatcycle limits. From theseefficiencyconsiderations, fuelcell

vehiclesareoftenseenastheideal end-goal forhydrogenpoweredtransport.

However fuelcellscanbeheavierandmoreexpensive than internalcombus-tionengines,andrequirehighpurityhydrogentopreventthecellfrombeingpoi-

soned. Hydrogen internalcombustionenginesarelessefficient,butofferweight

andcostadvantagesandcanberunonmultipleorimpurefuels.

Amotorvehicleisadynamicsystemandthereforeavehiclespowertrainmust

usesomeofitsavailablepowertoaccelerate itself.Thisleadstothe question:in

anautomotivesituation,howmuchisthepotentialefficiencyadvantageofafuel

celloffsetbytheadditionalmassofthefuelcellitself?

1.2 Formula H

RMITUniversity(Australia)andHochschuleIngolstadt(Germany)havecollabo-

rated tobuildahydrogenpoweredracingcar. Knownas FormulaH, theobjec-

tivesoftheprojectweretoprovethatalternativefuelvehiclesareachievablewith

existing technologies, and to expose the student team to a real-world multi-

nationaldesignproject. TheprojectdeliveredafullyfunctioningLeMansproto-

type style racing car,poweredby aBMW800ccmotorcycle engine runningon

hydrogenfuel,andfeaturinga200barcompressedgashydrogensupplysystem.

2 Vehicle Comparison Methodology

2.1 Lap Simulation

Earlyconceptualdesignprocessrequiredthe FormulaHteamtoassesstherelative

performanceof internal combustion engines and fuel cells for racetrackuse. A

preliminary study was undertaken in the form of a quasi-static lap simulation,

basedonthe3kmlongWintonracetrackinVictoriaAustralia. Avehiclemodel

representingabenchmarkH2ICEvehiclewas simulated,withpredicted lap time

andfuelconsumptionresultsrecorded. Theseresultswerethencomparedagainst

fuelcellvehiclemodelsofvaryingpowersandweights.

Thesimulationwasconstructedasfollows:

Thevehiclepathwasmodeledasastringofstraightlinesandconstantradiusarcs,brokeninto1metreincrements.

Eachvehiclewasmodeledasapointmasswith representative tyregripand

poweroutputcharacteristics.

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Vehicles travelled at constant speed through corners, at the tyre grip limit.

Corneringspeeddefinedentryandexitspeedsforeachstraight.

Forwardaccelerationwascalculatedasafunctionofenginepower(PENGINE),

drivelineefficiency( ),resistancepower(PRESISTANCE, thecombinedaerody-namicdragandrollingresistance),velocity(v)andvehiclemass(m):

(Gillespie,1992)

Velocityunderbrakingwas calculatedusing combinedwind resistance and

maximumtyregripforce.

Vehiclepowerwasassumedconstantandindependentofenginespeed.

Aerodynamicdragwas calculated as a functionofvelocity (v), frontal area

(A),dragcoefficient(CD

)andairdensity(

),asdescribedin [1]:

Vehicle energy requirements were calculated by summing kinetic energy

changes, air resistance and rolling resistance energies across each track in-

crement.

Relative fuelusedwascalculatedasa functionof the totalenergy required,

and overall thermodynamic efficiency. Thermodynamic efficiency of the

H2ICEwasestimatedat25%,andthefuelcellat40%.

Themodelingofdetailedvehicledynamics such as cornering transients,weighttransfer,orsuspensiondynamicswereconsideredunnecessaryforthispreliminary

study.

2.2 Tyre Grip

Acriticalmodelingrequirementwas tocapture theeffect thatvaryingvehicle

weightswouldhaveontyreperformance. Whilstatyresgripforceincreaseswithvertical load, the relationship isnot linear and thegrip coefficient actually falls

withincreasingnormalforce [2]. Thisisknownastyreloadsensitivity,andpub-

lishedtestdata [3] forGoodyeartyressimilartothoseusedonthe FormulaHve-

hicleconfirms thischaracteristic. Thegripcoefficient isseen tofallbyapproxi-

mately0.03% perNewtonofload(Fig.1).

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Fig. 1 Frictiondata,Goodyear20.0-7.0x13D2509 FSAEtyre([3]).

Note that the above data were taken in controlled laboratory tests on a rolling

drumtyretestingmachine,andarebelievedtooverestimateabsolutefrictioncoef-

ficientvalues. Amorerepresentativevalueoffrictioncoefficient isestimatedat

approximately1.4-1.5,basedonobservationsofthecorneringperformanceofthe

RMIT Formula SAE vehicle using similar tyres. Tyre load sensitivity is ex-

pressed in termsofpercentagevarianceperNewtonof load,so isunaffectedby

thisadjustmentofscale.

2.3 Vehicle Specifications: H2ICE Benchmark Vehicle

ThebenchmarkH2ICEvehiclewasmodeledon the FormulaHvehicle,with the

followingspecifications:

Table 1.H2ICEvehiclespecifications.

Specification Value

Totalvehiclemass(includingdriverandengine) 600kg

Enginesystemmass(notincludingH2supplysystem) 80kg

Tractivepower(rearwheels) 30kW

Tyregripcoefficient(alldirections) 1.4

Frontalarea 1.2m2

Dragcoefficient(CFDestimate) 0.62

Rollingresistance(assumedconstant) 200N

Thermodynamicefficiency 25%

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Theabovevehiclemodelcompletedonelapofthecircuitin105.1seconds,witha

maximumspeedof129.8kmh. Note that the topspeedof theactual FormulaH

carhasbeenmeasuredat133kmh,confirmingthatthesimulationisofreasonable

accuracy.

2.4 Vehicle Specifications: Fuel Cell Vehicle

Fuel cell vehicles were simulated by replacing the IC engine system mass and

outputinthebaselinevehiclemodelwitheachcombinationofthefollowingfuel

cellattributes:

Poweroutput:30-80kW,in10kWincrements. Fuelcellunitpowertoweightratio:5-10kg/kW,in1kg/kWincrements.

Itwasassumed that thecompressedgashydrogensupplysystemwouldbecom-

montofuelcellandH2ICEvehicles. Ineachcaseamodifiedtyrefrictioncoeffi-

cientwascalculated,usinga tyre load sensitivityof0.03% reduction in friction

coefficientperNewtonoftotalvehicleweight(relativetothebaselinevehicle).

3 Vehicle Comparison Results

Inspectionof the resultshighlights the importanceof fuelcellweight reduction.

TheonlyfuelcellvehiclestoachieveequivalentorfasterlaptimesthantheH2ICE

vehiclewerethosewithafuelcellmasstopowerratioof5kg/kW,andonlythose

withpoweroutputsofaround45kWorgreater(Fig.2). Theeffectofadditional

massof theheaviervehiclesslowed themsufficiently in thecorners thatanyad-

vantageonthestraightswasnegated. Thiswasdespitethefactthatinmanycases

thehigherpoweredfuelcellvehicleshadagreateroverallpower toweightratio

thanthebenchmarkH2ICEvehicle.

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Fig 2.Comparativelaptimes.

All fuel cell vehicles showed a similar trend of increased fuel usage at higher

poweroutputs, as lesser cornering speeds andhigher top speeds createdgreater

kinetic energy and aerodynamic drag energy demand (Fig. 3). Of the variants

tested,onlythe50kWand60kWfuelcelldesignoptionsof5kg/kWpowerdensitywerecalculatedtoofferfasterlaptimesandlowerfuelconsumption.

Fig. 3 Comparativefuelused

It is interesting to note that lap times show a distinct negative trend at higher

poweroutputs. Thevehicledesignerwouldneedtobeawarethatincreasingpow-

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ertrainoutputinanattempttogaincompetitivenesscaninfactresultinbothworse

lap times and greater fuel consumption. This is particularly dependant on the

tyresloadsensitivitycharacteristics.

4 Conclusions

Theaboveworkisapreliminarystudyonly,andthepresentedresultsarerelevant

onlytotheparticularvehicle,tyresandracetrackusedinthestudy. Moreover,the

resultspresentedarefor raceconditionsat the limitsofavehiclesperformance,

andwouldnotberepresentativeofcommonpassengervehicleusage.

Howeverthestudyhighlights thedangeroffocusingon individualparameters

whenassessingvehicleperformance. Amotorvehicle isacomplexsystem,and

decisionsmadeonthebasisofisolatedcriteriasuchasthermodynamicefficiency

orpowerdensitymaybepoorlyinformedwhenobservedatasystemlevel. Itis

recommended that further investigationbeundertaken into the relativemeritsof

hydrogenfuelcellandhydrogeninternalcombustionenginevehicles.

References

[1] Gillespie,T.D.(1992)Fundamentalsof Vehicle Dynamics,SAE International,Warrendale,

Pa

[2] Milliken W.F. and Milliken D.L. (1995)RaceCar Vehicle Dynamics. SAE International,

Warrendale,Pa

[3] Milliken&Associates(2008)Formula SAETyre TestingConsortium testdata. Available:

http://www.millikenresearch.com/fsaettc.html(Accessed:24 February,2009).

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Hydrogen-fuel Cell + IC Engines

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