Handbook of Combustion, Volume 5 (New technologies)

584 Pages·2010·74.026 MB·English

by  Maximilian Lackner,Franz Winter, Avinash Kumar Agarwal

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HCCI Combustion Chemistry, Reduced Kinetic Mechanisms
and Controlling Strategies
HatimMachrafi
1.1
Introduction
1.1.1
PresentSituation
In our daily life combustion engines are present everywhere. They are used for
purposes such as driving trucks, cars, and busses. This results in large-scale con-
sumptionoflimitedfossil-fuelreservesandtheproductionofexhaustgasesthatharm
boththeenvironmentandourhealth.Incontrast,italsoprovidesadvantages,suchas
mobility,thatarelinkedtothewidespreadavailabilityofpassengercars.Regardlessof
one’sperspective,itseemsreasonabletoexpectthatthenumberofvehicles(witha
combustionengine)willriseinthefuture,especiallygivenfactorssuchastherapid
economicdevelopmentinseveraldenselypopulatedareas.Furthermore,trendsfor
passengercarsindicatethatvehicleweightandsizearelikelytoincrease,togetherwith
thepowerrequirementsofthegrowingnumberofelectronicdevicesonboard.Allof
these factors will increase fuel consumption. Furthermore, legislation restricts the
amount of pollutants that are emitted by gasoline and diesel engines (Figure 1.1)
(http://www.itsnottingham.info/Commuter4_Pres3.htm).
Consequently, world-wide fuel consumption and exhaust emissions can only
realisticallybereducedifanalternativefortheIC(internalcombustion)engineis
developedwithcharacteristicssignificantlybetterthanthoseofpresentengines.
1.1.2
HCCIEngines,ANewAlternative
Ideally,anyalternativetoICenginesshouldnotbedependentonfossilfuels,emitno
harmfulproducts,andhaveabetterefficiency.Thewords“fuelcell”arepromising.
Unfortunately,large-scaleintroductionofthispropulsiontechnologyisexpectedto
takemanyyears,mainlyduetohighcosts,problemswithon-boardhydrogenstorage,
andthelackofhydrogeninfrastructure.Furthermore,thewell-to-wheelefficiency
offuelcellpowertrainswithallthenecessaryauxiliaryequipmentisestimatedtobe
HandbookofCombustionVol.5:NewTechnologies
EditedbyMaximilianLackner,FranzWinter,andAvinashK.Agarwal
Copyright(cid:1)2010WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim
ISBN:978-3-527-32449-1
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2 1 HCCICombustionChemistry,ReducedKineticMechanismsandControllingStrategies
Figure1.1 Europeanon-highwayemissionlimits–D0(1990)toD5(2010).
similar, or even lower, than that of optimized diesel engines [1, 2]. Production of
hydrogeninthevehiclewouldsolvesomeoftheaboveproblems,butaslongasfossil
fuelreformationisapplied,thiswouldnotbeasustainablesolution.Trulycleanfuel
celltechnologywouldbebasedonarenewablesource,suchaswindorsolarenergy.
However,withpresenttechnologicalstandardsintherenewableenergyfield,itisfar
fromrealistictobelievethatasubstantialportionofthefossilfuelusedforvehicle
propulsioncouldbereplacedbysustainablealternativesinthenearfuture.There-
fore,theuseofICengineswillmostlikelycontinueforawhileand,aslongasno
outstanding alternative is available, research focusing on improving the internal
combustionengineconceptisjustified.GiventhatICengineshaveexistedforover
acentury,whatimprovementscanbeachieved?Animpressivedevelopmentofthe
ICenginehastakenplace:fuelconsumptionandemissionshavebeenconsiderably
reduced,whileincreasingthepoweroutput.Thecombustionprocessesindieseland
sparkignitionenginesarediscussedinVol.3Ch.14andVol.3Ch.15[3,4].Other
systemshavebeenintroducedrecently:flexiblevalveoperation;cylinderde-activation
cycle-to-cycle, model-based engine control systems; and direct-injection of fuel.
Withoutquantifyingthepossiblefutureimprovements,itisconcludedthatdevel-
opmentofICengineswillprobablycontinueduringtheyearstocome.Logically,one
major possibility could now be to improve the conventional Otto and/or diesel
engines. Another major possibility that seems to hold much promise is homoge-
neouschargecombustionignition(HCCI).
1.2
HCCICombustion
1.2.1
Definition
HCCIcanbedefinedasapremixed,leanburncombustionprocess,precededbya
homogeneousair/fuelport-injection.Figure1.2showsthiscombustionmode.
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1.2 HCCICombustion 3
Figure1.2 SchemeoftheHCCIcombustionmode[5].
The HCCI engine generally runs on a lean, diluted mixture of fuel, air, and
combustion products, which is not ignited by a spark but by compression auto-
ignition instead. To speed up the kinetics, the temperature of the charge at the
beginningofthecompressionstrokecanbeincreased.Thiscanbedonebyheating
theintakeairorbykeepingpartofthewarmcombustionproductsinthecylinder
(integralgasrecirculation).Bothstrategiesresultinahighergastemperatureafter
compression, which in turn speeds up the chemical reactions that occur in the
(nearly) homogeneous mixture in the following cycle. In contrast with the spark
ignition(SI)engine,whereaspark-plugisusedtogenerateapropagatingflame,and
thecompressionignition(CI)engine,wheretheinjectioncausesadiffusionflame,
ignitionintheHCCIenginewilloccurwhenthetemperatureandpressurearehigh
enough for fuel oxidation chemistry to become very rapid. Figure 1.3 depicts the
differencesbetweentheHCCIengineandtheSI/CIengines.
Figure1.3 DifferencebetweentheHCCIengineanddiesel/sparkignitionengines[5].
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Figure1.4 DifferenceinpressurerisebetweentheHCCIengineanddieselengine[5].
Auto-ignitioninHCCIenginesissaidtooccursimultaneouslyatmanylocations,
calledhotspots,throughoutthecylinder.Thiscausesthepressurerisetotakeplace
inashortertimespanthaninadieselengine,whilstthetemperatureremainslower.
Figure1.4showsthispressurerise.
Furthermore, it is claimed that turbulence is of minor importance for the
combustionprocessinanHCCIengine,thoughmoreresearchisneededinthis
domain,andthatflamepropagationoccursinalesssignificantway.Itisgenerally
agreedthattheHCCIprocessismainlydrivenbychemistry.WhileOttoengines
arecharacterizedbyalowefficiencyatpartloadanddieselenginesexhausthigh
concentrations of particulate matter and NO , the HCCI engine has neither of
x
these problems: as the peak temperature in the cylinder is low for lean, diluted
mixtures, NO concentrations are reported [6, 7] to be two orders of magnitude
x
lower than for the Otto and diesel processes and particulate emissions are
negligible.Figure1.5showsthezoneofNOandsootformationasafunctionof
thetemperatureandequivalenceratioaswellastheoperatingzonesforengines
thatwouldrunintheSI,CI,andHCCImodes.Owingtoitslowemissionsandhigh
efficiency,theHCCIprocessmightinfuturereplacetheOttocycleduringpart-load
operation.
1.2.2
ProblemofImplementingtheHCCIMethod
Atpresent,theHCCIconceptcannotbeimplementedinautomotiveapplications
duetoseveralproblems:complexandexpensivesolutions(e.g.,variablevalvetrain
systems)maybenecessarytomakeHCCIcombustionpossibleandtheoperating
windowofsmoothHCCIoperationisstilloflimitedsize.Butthemajorproblemis
the difficulty in controlling the moment of auto-ignition and the energy release
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1.3 ChemicalKineticsinHCCICombustion 5
Figure1.5 NO andparticulatereductionatlowtemperatureswiththeHCCImethod[8].
x
rate. As the mixture in the cylinder is premixed and no spark plug is used, the
chemicalprocessesdeterminetheonsetofauto-ignition,aswellasthefuelburn
rate.IncontrasttotheSIandCIprocesses,HCCIcombustioncannotbecontrolled
directly;thespark-timingcanbemanipulatedinspark-ignitionengines,whilethe
moment and amount of fuel injected can be controlled in diesel engines. If we
assumethattheadditionoffuelinHCCIenginesiseitherbyport-injectionorearly
direct-injection, to obtain a near-homogeneous mixture, HCCI engines have no
directwayofcontrollingthecombustionprocess.Atpresent,HCCIresearchhas
cometothestageatwhichworkersseemtobeattemptingtosolvethisproblem.
Forthiscontrol,abetterunderstandingisneededofhowthechemicalreactions
interferewitheachotherandwhatthemostimportantaspectsarethatsetoffthe
ignition.Forthispurpose,ashortreviewfollowsfirst(Section1.3),explainingthe
typicalchemicalreactionpathwaysoflinear,branched,andaromaticalkanefuels
andtheroletheyplayintheauto-ignitionprocess.Section1.4thendiscussesthe
use of reduced kinetic mechanisms to study HCCI auto-ignition. Section 1.5
discusses how a kinetic mechanism can propose a controlling strategy for the
HCCIengine.
1.3
ChemicalKineticsinHCCICombustion
The fuels that are used in present conventional engines are mostly gasoline and
diesel. These fuels contain primarily linear alkanes, branched alkanes, and aro-
matics.Previousresearch[9–12]hasinvestigatedtheHCCIauto-ignitionprocessfor
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manycompounds.AgreatpartoftheHCCIandauto-ignitioninvestigationsuseso-
called Primary Reference Fuels such as iso-octane and n-heptane [10, 12, 13].
Therefore, the chemical reaction pathways of n-heptane and iso-octane will be
explainedindepth,forabetterunderstandingoftheHCCIcombustionphenomena.
Little research has been performed on the auto-ignition of aromatics [14, 15].
Nonetheless,themostinvestigatedaromaticcompoundseemstobetoluene[14–18].
Many studies concerning the features of auto-ignition have been performed. The
fundamental origins of these features have been discussed in detail in the litera-
ture [19–28]. Auto-ignition can be defined [29] as the ignition of a combustible
material,commonlywithair,asaresultofheatgenerationfromexothermicoxidation
reactionswithouttheinterventionofexternalenergysourcessuchasasparkora
flame.Asstatedabove,n-heptane,iso-octane,andtoluenearetakenasexamplesfor,
respectively,thelinearalkanes,branchedalkanes,andaromatics.Here,emphasisis
laid on the chemical reactions and the general pathways upon which most of the
literatureisagreed.
1.3.1
ChemicalCombustionMechanismofIso-Octane
The most important reaction branches through which the oxidation of n-heptane
proceedsarewellknown[13,30–33].Theinitiationreactionisdescribedby:
C H þO !C H .þHO . ð1:1Þ
7 16 2 7 15 2
Owingtoitshighendothermicity,thisreactionisnotanimportantroutetothe
formationofthen-heptylradical(C H .).Oncethereactionsystemhascreatedother
7 15
radicals,suchasOH.,.O.,andH.,hydrogenabstractionbysuchradicalsisfavored
morethantheabstractionbymolecularoxygen[34].Thesetypesofreactionsareoften
denotedastheconsumptionreactions:
C H þX.!C H .þXH ð1:2Þ
7 16 7 15
Here,X.isaradical.Thefastestrateofattackisbythehydroxylradical(OH.).The
Hatomcanbeabstractedfromdifferentsitesofthen-heptanemoleculeandtherate
constantisdependentonthetypeofC(cid:1)Hbondthatisbroken.n-Heptanehasonly
primaryandsecondarysites,whereasiso-octanealsohastertiarysites.
1.3.1.1 Low-TemperatureInterval
After the H-abstraction step, shown previously, the heptyl radical can react with
molecularoxygentoformanalkylperoxyradical:
C H .þO !C H OO. ð1:3Þ
7 15 2 7 15
According to Curran et al. [12], this is the most important reaction for low-
temperature oxidation. Furthermore, they state that the equilibrium constant of
this reaction is very strongly temperature dependent: at higher temperatures the
equilibriumofthereactionbeginstoshifttowardsdissociation,therebyindicating
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1.3 ChemicalKineticsinHCCICombustion 7
the switch from low- to high-temperature chemistry, via the intermediate-temper-
aturechemistry.
The low-temperature path continues with an isomerization step, in which the
alkylperoxyradicalistransformedintoahydroperoxyalkylradical:
C H OO.!.C H OOH ð1:4Þ
7 15 7 14
Above 800K, the ketohydroperoxide species will decompose into several frag-
ments,atleasttwoofwhichareradicals[33].Chainbranchingbecomesquiterapid
duetothelargenumberofradicalsformed[36].ThisaddsmoreenergyandOH.
radicalstothesystem.Hereby,olefins,aldehydes(whichsubsequentlydecompose,
forming CO), and OH. radicals are formed. The low-temperature reactions are
exothermic,havingreleasedheatduringthislow-temperatureinterval,raisingthe
system’s temperature [34].Additionally OH. radicals are produced that react with
thefuel.Thiscorrespondstothebeginningofatemperaturerisethatisreferredtoas
the“coolflame.”Formaldehydealsoisproducedinthecoolflame.Therefore,cool
flamescanbetracedbyformaldehydeanalysis.
1.3.1.2 Intermediate-TemperatureInterval
Whenthetemperaturebecomeshighenough,theintermediate-temperaturereac-
tions become more important than the low-temperature branch. These reactions
become dominant at intermediate temperatures, because the activation energy
barrier is more easily overcome at these elevated temperatures. Moreover, the
additionofthehydroperoxyalkylstomolecularoxygenstartstoshift.Theinterme-
diate-temperaturereactionsdescribetheconversionof.C H OOH.radicalradicals
7 14
into conjugate olefins, cyclic ethers, and beta-scission products, instead of the
reactionpathwaysthroughketohydroperoxidesformedatlowtemperatures[10,31].
Thebeta-scissionreactionsformnexttostableoxygenatedproducts,alsoOH.radical
radicals, eventually, and are, relatively, the most reactive pathway of the three
mentioned. The increasing importance of these propagation channels leads to a
lower reactivity of the system, which is observed as the NTC region [10]. NTC
(negativetemperaturecoefficient)referstothetemperatureintervalduringwhich
system reactivity decreases despite a rise in temperature. The reason that the
chemistry slows down is that the low-temperature chain-branching reactions are
replacedbypropagationreactionsatintermediatetemperatures,whichdonotlead
toincreasingnumbersofreactiveradicals.Atintermediatetemperatures,H O can
2 2
beformedasfollows[35]:
H.þO þM!HO .þM ð1:5Þ
2 2
AlkaneþHO .!AlkylradicalþH O ð1:6Þ
2 2 2
Inthefirstreaction,Misathirdbody.AccordingtoLietal.[37],H O isalsoformed
2 2
bythefollowingreaction:
HO .þHO .!H O þO ð1:7Þ
2 2 2 2 2
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8 1 HCCICombustionChemistry,ReducedKineticMechanismsandControllingStrategies
CoxandCole[38]alsodiscussthisreaction:
RCHOþHO .!H O þRCO ð1:8Þ
2 2 2
Inthisreaction,RCHOisanaldehyde.Thesereactionsdescribethebuildupof
H O attemperatureslowerthanapproximately1000Kuntilthetemperatureishigh
2 2
enoughforH O decompositiontobecomedominant.Thisprocessisverytypicalfor
2 2
hydrocarbonignitionandisthoroughlydiscussedbyWestbrook[35].
1.3.1.3 High-TemperatureInterval
Whenthetemperaturebecomes higherthan theintermediate-temperature range,
the high-temperature interval is reached. One might wonder how such a high
temperature is achieved when reactions become slower during the NTC period.
According to Amneus [39], a pre-requisite for the cool flames to induce full
combustion is that sufficient energy must be released by the low-temperature
reactions to raise the system temperature to the high-temperature region. More
importantly, the temperature can be raised sufficiently by external effects as well:
whenthecompressionratioishighenough,thegastemperatureinthecombustion
chamberofaninternalcombustionenginewillbeforcedintothehigh-temperature
regimebymeansofthecompressionworkduringthecompressionstroke.Athigher
temperatures(higherthanapproximately1000K),theconversionreactionsofheptyl
radicalsintobetadecompositionproductsandconjugateolefinstakeoverfromthe
additiononmolecularoxygen.
Anotherpossiblereactioninvolvingthealkylradicalisthefollowingisomerization
reaction:
C H .!isomer ð1:9Þ
7 15
AccordingtoCurranetal.[12],thereactionslikereaction(1.9)areonlyimportantat
temperatures above 850K due to the relatively high activation energy of these
endothermic beta-scission reactions. If the temperature rises above 1200K, the
relativelyhighactivationenergyofthereactionH. þ O ! .O. þ OH.isovercome
2
anditbecomesthedominantchainbranchingstep[35,40];thereactants,including
oneradical,leadtotwoproductradicals.Sincemolecularoxygenparticipatesinthis
reaction, lean fuel mixtures are more reactive in this high-temperature regime,
whereas rich fuel mixtures are oxidized quickly at low temperatures due to chain
branching, which depends on radical species formed directly from the parent
fuel[10].Whenthehigh-temperatureintervalisdiscussedbelow,tworeactionsare
considered[41]:
H O þM!OH.þOH.þM ð1:10Þ
2 2
H.þO !.O.þOH. ð1:11Þ
2
These radical-branching reactions are very important for the high-temperature
regime, because the products contain more radicals than the reactants. The first
chain-branchingreactiontakesplaceataround1000Kanditmarksthemomentof
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1.3 ChemicalKineticsinHCCICombustion 9
ignition [41]. Above 1200K, the reaction between the H. radicals and molecular
oxygennolongerformsHO butinsteadformsoxygenandhydroxylradicals.The
2
resultingOH.radicals,formedfromthereactionsdiscussedabove,rapidlyconsume
anyfuelandarapidincreaseintemperaturefollows.Thus,decompositionofH O
2 2
andconsumptionoffuelresultinignition[35].
1.3.2
ChemicalCombustionMechanismofIso-octane
The iso-octane oxidation scheme is very similar to that for n-heptane. The main
differenceliesinthedistributionofthemassflowsthroughthedifferentreaction
branches. The distribution of the products formed changes accordingly. This
differenceintheproductdistributionismainlydependentonthemolecularstructure
ofthefuel(Figure1.6).Thenumber“1”inthefigureindicatestheprimaryH-atoms,
“2” indicates secondary H-atoms, and “3” indicates tertiary H-atoms. One of the
differences between n-heptane oxidation and iso-octane oxidation is discussed by
Dagautetal.[30],whostatethatthesumofsecondaryandtertiaryHatoms(whichare
more reactive than primary H atoms, since primary C(cid:1)H bonds are stronger) is
muchlowerinthecaseofiso-octane.n-Heptanehastwoprimaryandfivesecondary
H-atoms. Iso-octane has five primary, one secondary and one tertiary H-atom.
However, while iso-octane has one tertiary H-atom, the much higher number of
secondaryH-atomsforn-heptaneoutweighthis,andn-heptanecanundergomuch
easierH-abstractions.
Consequently, H-abstractions and isomerization (including internal H abstrac-
tion)reactionsaresloweddown.Therefore,thepropagationsequencethroughalkyl
peroxyradicals,hydroperoxyalkylradicals,andcyclicethersisfavoredmoreforiso-
octanethanforn-heptane.Inturn,mostlystablespecies,suchasolefinsandcyclic
ethers,areproducedduringlow-temperatureoxidationofiso-octane,whiletherateof
oxygenatedspeciesformationislow.Togetherwiththechainbranchingpaththrough
ketohydroperoxides and their products, favored for n-heptane, this explains why
theheatreleaseduringthecoolflameperiodismuchmoresubstantialforn-heptane
than for iso-octane. Li et al. [37], who measured molar fractions of intermediate
species under motored engine conditions, also found that the formation of cyclic
etherswasmuchlargerforiso-octanethanforn-heptane.
Figure1.6 Molecularstructuresofn-heptaneandiso-octane.
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10 1 HCCICombustionChemistry,ReducedKineticMechanismsandControllingStrategies
Westbrooketal.[42]haveusedsensitivityanalysistostudytheproductionofOH.
radicalsforfuelswithvaryingRONnumbers.Thecitedauthorsconcludedthatthe
productionrateofOH.decreasesgloballywithincreasingoctanenumber.Sincethe
fastestreactionsconsumingfuelarethosewithOH.,thisisanimportantconclusion.
Theseobservationsaretrendsratherthanfacts.Theoctanenumberdoesnotindicate
preciselyandconsequentlythebehavioroftheauto-ignitiondelay.Asaresultofall
this, n-heptane is less resistant to auto-ignition than iso-octane. Consequently,
ignitiondelaytimesaretypicallyshorterforn-heptane.
1.3.3
ChemicalCombustionMechanismofToluene
1.3.3.1 InitiationReactions
The major difference between aromatics and alkanes can be explained by the
structureofaromatics.Figure1.7showsasexamplesbenzeneandtoluene.
Benzeneandtoluenepresentaspecialprobleminthat,toaccountforallthebonds,
theremustbealternatingcarbondoublebonds(Figure1.7a).UsingX-raydiffraction,
researchers discovered that all of the carbon–carbon bonds in benzene are of the
samelengthof140picometres(pm).TheC(cid:1)Cbondlengthsaregreaterthanadouble
bond(134pm)butshorterthanasinglebond(147pm).Thisintermediatedistanceis
explainedbyelectrondelocalization:theelectronsforC(cid:1)Cbondingaredistributed
equallybetweeneachofthesixcarbonatoms.Onerepresentationisthatthestructure
existsasasuperpositionofso-calledresonancestructures,ratherthaneitherform
individually. This delocalization of electrons is known as aromaticity, and gives
benzeneandtoluenegreatstability,oftencalledstabilitybyresonance.Thisenhanced
stabilityisthefundamentalpropertyofaromaticmoleculesthatdifferentiatesthem
frommoleculesthatarenon-aromatic.Thisalsocausesthegreaterdifficultyinauto-
ignitingaromatics.
Theinitiationreactionfortolueneis:
C H CH þO !C H CH .þHO . ð1:12Þ
6 5 3 2 6 5 2 2
C C C C
(a)
C C C C C
C C C C
benzene toluene
(b)
C
Figure1.7 Molecularchemicalstructuresofbenzeneandtoluene(a)andtheirresonance
structures(b).