EngineEfficiencyandPowerDensity:DistinguishingLimitsfromLimitations

ChrisF.Edwards

AdvancedEnergySystemsLaboratoryDepartmentofMechanicalEngineering

StanfordUniversity

ExergytoEngines

Limits areimposedbytheresource,environment,andphysicsgoverningtransfers

andtransformations.

Limitations areintroducedbythechoiceofdevices andprocessesi.e.,bythearchitecture ofanengine.

ChemicalResource

RestrainedReaction

ElectrostaticWork

BatchExpansion

FlowWork

UnrestrainedReaction

BatchExpansion

FlowingExpansion

LorentzWork(MHD)

Exergy Classification Architecture Engines

Efficiency,EffectiveCompressionRatio,andIdealArchitectures

7080%IdealWork,6075%PeakPres.,7590%PeakTemp.

100

101

1020

10

20

30

40

50

60

70

80

90

100

Effective Compression Ratio (/0)

Firs

t Law

Effi

cien

cy (%

)

Fuel-air Atkinson cycleFuel-air Otto cycle70-80% of Otto cycleJaguar AV133, 5.0 LDISI gasoline, ULEV2

Volvo TG103/G10A, 11.8 LSI natural gasCummins 6BT5.9-G6, 5.9 LDiesel, turbocharged, Tier 1

Cummins QSM11-G4, 10.8 LDiesel, turbocharged, Tier 3Volvo Penta TAD734GE, 7.2 LDiesel, turbocharged, Tier 2

RCCI gasoline + Diesel, Gross-IndicatedPCI gasoline, Gross-IndicatedFPEC Diesel, Gross-Indicated

EquivalenceandCompressionRatios

Efficiencyandpeakpressurerequireuseofsignificantcompressionwithadilutemixture.

UseofLowTemperatureCombustion

UseofLTCto controlNOxemissionslimitsworkoutputto67barIMEP(56barBMEP).

Atkinson?NotLTC?

Efficient, pressurelimited,highoutputoperationmight beachievablewithoptimalexpansionandnondilutemixtures.

SpanningExergytoEngines

Limits areimposedbytheresource,environment,andphysicsgoverningtransfers

andtransformations.

Limitations areintroducedbythechoiceofdevices andprocessesi.e.,bythearchitecture ofanengine.

ChemicalResource

RestrainedReaction

ElectrostaticWork

BatchExpansion

FlowWork

UnrestrainedReaction

BatchExpansion

FlowingExpansion

LorentzWork(MHD)

Exergy Classification Architecture Engines

VanBlarigan/AichlmayrFreePistonEngineConcept

9

FreePistonArchitectureforHighCR

Balancedforces,nobearingloads Longstroketoboreratioforlow

surfacetovolumeratio Shortresidencetimeatmin.V

Canuselinearalternatorforworkextraction(vanBlarigan/Aichlmayr)

Gasdriver Gasdriver

Freepistons

Combustionchamber

0 20 40 60 800

0.2

0.4

0.6

0.8

1

Time (ms)

Vol

ume

(V/V

0)

Free-pistonexperimentSlider-crank

10

SamplebagCondenser

Vacuumpump

HCanalyzer

Heatedsampleline

Bandheaters

StanfordFreePiston,ExtremeCompressionApparatus

DieselCombustionatHighCompression

CR = 30:1, 1050 K CR = 100:1, 1550 K

#2 Diesel, 1 ms injection, EOI at TDC

InitialDieselEfficiencyResults

0 20 40 60 80 10020

30

40

50

60

70

80

Compression Ratio

Effi

cien

cy(%

)

Ideal 1st-law efficiencyIdeal cycle minus air experiment lossesExperimental indicated efficiency

Diesel #2 = 0.27 - 0.30

LimitedtoDieselStyleCombustion?

Premixedcombustionissootless. Premixedleancombustionisveryefficient. Premixedstoich combustionhashighpowerdensity. Premixedstoich combustionpermitsuseofaTWC,andthereforeverylowNOx emissions.

ToaccomplishthisathighCR,autoignitionmustbeheldoffuntiltheminimumvolume.

Mightbeabletoholdoffautoignitionby: Choiceoffuel(e.g.,methane/NG,methanol) Activecoolingofthecharge

TemperatureControlofAutoignition

Loweringtheinitialgastemperatureby50Klowersthetemperatureat100:1by210K.

Ignitionoccursatthedesiredvolume.

0 20 40 60 80 100200

400

600

800

1000

1200

1400

1600

/0

T (K

)

Tstart = 250 K

Tstart = 298 K

Model:

Adiabaticcompression Homogeneous,stoichiometric

methaneaircharge

Volumetimeprofilefromexperimentaldata

GRI3.0chemicalkinetics

TwoMethodsofCooling

Compressor

JTvalve

Coolingair

Refrigerant

ReactantsatT0,P0

12

Engine

Products

Compressor

Coolingair

ReactantsatT0,P0

1

2

Engine

3

Products

1 2 3 4

250

300

350

400

/0

T (K

)

S

S

2

P = 1 atm

1

1 2 3 4

250

300

350

400

/0

T (K

)

S

S

3

2

1

P = 2.17 atm

Thisisacommonturbocharger/intercooler,with2.17atmmanifoldpressure.

Experimentsw/Intercooling

Experimentalmethod: Chargecompressedpartway,remainingatwallT Usualrapidcompressionstartsfromthatpoint IntercoolingPchosenforignitionjustafterTDC

10-2 10-1 100100

101

102

103

Volume / Full Cyl. Volume

Pre

ssur

e (b

ar)

Fuel:premixedmethaneair

EffectiveCR:35to90:1

Equivalenceratio:0.96to1.04

PeakEfficiency:57%(Includescomp.work.)

MeasuredCombustionEfficiency

18

NOx,HC,andCOPremixedEmissionsw/Intercooling

0.96 0.98 1 1.02 1.04 1.060

10

20

30

40

Equivalence Ratio

Spe

cific

Em

issi

on (g

/kW

-hr)

~ 35:1 CR~ 60:1~ 80:1

0.96 0.98 1 1.02 1.04 1.060

0.5

1

1.5

2

Equivalence Ratio

Spe

cific

Em

issi

on (g

/kW

-hr)

~ 35:1 CR~ 60:1~ 80:1

0.96 0.98 1 1.02 1.04 1.060

2

4

6

8

10

12

14

Equivalence Ratio

Spe

cific

Em

issi

on (g

/kW

-hr)

~ 35:1 CR~ 60:1~ 80:1

NOx

HCCO

NOxEmissionsvs.GRI3.0

60:1CR,methane

~1%LossinCombustionEfficiency

EmissionsinContextofTWC

1J.Chiu,J.Wegrzyn,andK.Murphy,SAEPaper20040129822I.Saanum,M.Bysveen,P.Tunestal,andB.Johansson,SAEPaperNo.2007010015.

60:1CR,1.028

21

EvaporativeCooling?

Model:

Sameasbefore,butwithwatervaporization

Vaporizationratematchesinjectionrateofrealinjector

Startofinjectionchosentoavoidgassaturation

Injectliquidduringcompression(waterhasgoodproperties). Vaporizationdrawssensibleenergyfromthegas,thusloweringthe

temperature.

10-2 10-1 100100

101

102

103

V/V0

P (b

ar)

No coolingWater injection

3% mass fractiontotal water injected

Start waterinjection

10-1 100100

101

102

103

V/V0

Pres

sure

(bar

)

Water injectionIntercoolingWater model forequal cooling

SOIModel EOI

Experiment EOI

Experimentsw/EvaporativeCooling

22

AchievedTDCphasingupto60:1CR

Morewaterneeded(8%vs.1%)

10%decreaseinefficiency(53%)

Limitedbytheinjectorsetup:

Stratification Slowvaporization

ReductioninRateofRise,Ringing

~5000bar/CADforintercoolingapproach

~80bar/CADforwaterinjectionapproach

61 62 63 64 650

200

400

600

800

1000

Time (ms)

Pre

ssur

e (b

ar)

IntercoolingWater injection

Maximumrateofpressurerise,translatedtoslidercrankat1800RPM:

TakeAwayMessages

Exergysetsanabsolutelimitfortheworkfromaresourceinaspecifiedsetofsurroundings.Ifyouarenotaspiringtoapproachthislimit,pleaseadjustyourthinking.(Suspensionofdisbelief!)

Thephysicsofthevariousenergytransferandtransformationprocessesthatcanbeinvokedsetsadditionallimits.Taketheseseriouslyandchangetheprocessesusedifnecessary.

Thearchitectureyouchooseforyourengineintroduceslimitationsbasedonboththeprocessesinvolvedandthedevicesusedtoimplementthem.Tracktheexergydestructionthroughthesedevicestoknowhowwellyouaredoing.

Ifyouarenotdoingwell(exergyefficiencybelow50%),considerchangingthesetofprocesses,aswellasimprovingthedevices.Alsoconsiderusingnontraditionaldevicestoimplementtheprocesses.

Thekeytoimprovementistoknowwhereyoustand.(Absolutely!)

Engine Efficiency and Power Density:Distinguishing Limits from LimitationsExergy to EnginesEfficiency, Effective Compression Ratio, and Ideal ArchitecturesEquivalence and Compression RatiosUse of Low-Temperature CombustionAtkinson? Not LTC?Spanning Exergy to EnginesVan Blarigan/Aichlmayr Free-Piston Engine ConceptFree-Piston Architecture for High CRStanford Free-Piston, Extreme Compression ApparatusDiesel Combustion at High CompressionInitial Diesel Efficiency ResultsLimited to Diesel-Style Combustion?Temperature Control of AutoignitionTwo Methods of CoolingExperiments w/IntercoolingMeasured Combustion EfficiencyNOx, HC, and CO Premixed Emissions w/IntercoolingNOx Emissions vs. GRI3.0Emissions in Context of TWCEvaporative Cooling?Experiments w/Evaporative CoolingReduction in Rate-of-Rise, RingingTake-Away MessagesSlide Number 25Peak Conversion Efficiency of EnginesStoichiometric with EGROptical imaging systemExtinction, Schlieren, and LuminosityEmissions from Diesel CombustionIndicated Efficiency w/IntercoolingSoot measurement systemJunkers Free-Piston Compressor