prechamber spark plug.docx
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prechamber spark plug.docx
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prechambersparkplug
1.Reasonsforleanburnengines
LegalregulationslimittheemissionofpollutantssuchasNOx,COandHCwhichareformed,amongothers,bythecombustionofnaturalgasininternalcombustionengines.Thelimitscanbemetifsuitabletechnicalmeasuresaretaken.
Themeasuresconsistofeithercleaningtheexhaustgasesdownstreambyacatalyticconverterorrearrangingtheinternalcombustionprocessinsuchawaythatthepollutingconstituentscannotbeformedtoanappreciableamount.Thislowemissionenergyconversionprocesscanberealizedinaleanburngasengine.
AcharacteristicfeaturetherebyisacertainamountofexcessairinthenaturalgasairmixturewhichlowersthecombustiontemperatureandtherebyfreezesthechemicalreactionfortheformationofNOx.Thepenalityforthisisthatthecombustionisaffected,tooandtheunburnedconstituentssuchasCOandHCareincreased.
Finally,theproblemsofleanburnenginescanbemasteredbytheuseofexhaustturbochargers,highpowerignitionsystems,specialsparkplugdesign,favourablecombustionchamberdesignandairfuelratiocontrolling,tonamebutafew.Inthiswayareliableoperationoftheenginewithemissionssignificantlylowerthanlegalstandardsisachievedwithoutanylossofengineefficiency.
Thus,leanburnenginesofferthesimplestwaytorealizetheobjectivesofecologyandeconomyfortheconversionofenergyunderfavourableexpendítures.
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2.Problemsofignition
Thehighvoltagecreatedbytheignitionsystemwhichcandelivervaluesmorethan30KVatamaximumcausesthesparktojumpacrossthesparkgapandignitesthegasairmixture.
Therequirementofignitionvoltageislowest(around10KV)atasmallgapofabout0,35mmandairfuelratios(AFR)=1,0---1,3.Itincreaseswithleanermixturestoabout15----20KVatAFR=1,6whennewstandardsparkplugsareused.Awearoftheelectrodeswhichincreasesthesparkgapforonly0,2mmelevatesthevoltagerequirementclearlyabove20KV.Iftheignitionsystemdoesnotprovidethisrequirement,thenmisfiringoccursandfinallyadowntimeoftheenginewillhappen.
Withstandardsparkplugsthispointisalreadyreachedafter700---1500hoursofservice.Thenofcoursethereisthepossibilityofreducingthegapdistancebybendingtheelectrodesbackagainandresumingoperation.
Thehighrequirementofignitionvoltageinleanburnenginescallsforthedevelopmentofsparkplugconfigurationswithmoderatevoltagerequirements.
Inthisregardthefollowingparametersofthesparkplugareofmajorinfluence:
-dimensionofsparkgap
-formofelectrodes
-materialofelectrodes
Onthehydrodynamicsidethefollowingparametershaveamajoreffectonvoltagerequirements:
-fieldwithmacroturbulentstructure
-fieldwithmicroturbulentstructure
Theignitionvoltagerequirementisaffectedbythesingleparametersinthefollowingway:
sparkgapdimension:
withincreasingdistanceoftheelectrodestherequirementofignitionvoltagedevelopsrapidly
formofelectrodes:
smallelectrodediametersincreasetheelectricalfieldstrengthandreducetherequirementofignitionvoltage
materialofelectrodes:
materialwithlowelectrontransitionworkpropertyasforexampleplatinuminconnectionwithelevatedmaterialtemperaturefacilitatetheescapeofelectronsandtheformationofaspark
macroturbulence:
theso-calledhydrodynamicflowwithlargescalevortexesisofdisadvantageforthesparkformationbecausetheescapingelectronsarecarriedawaybeforetheyareabletoformaspark
microturbulence:
theso-calledhydrodynamicflowwithsmall-scalevortexesconcentratetheescapingelectronsinasinglevortexandsupportsparkformation.Additionally,themicroturbulencebuildsupahomogeneousflowfieldandpromotesarapidinflammationofthegasairmixture.
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3.Sparkplugconfigurations
Withthedevelopmentofleanburnenginesforeignsparkplugconfigurationswerealsotested.
Sparkplugsforcarsarewidelyknownandexistinalargenumberofvarieties.Differencesaretobeseeninformandarrangementofelectrodes,heatrange,threadsizesandplugdimensions.Commontoallistheopenpositioningoftheelectrodesatthelowerendofthethread.Sotheelectrodesprojectintothecombustionchamberandareexposedtodifferentkindsofinfluencesofthecombustionprocess.
Thefollowinginfluencesareofspecialimportance:
Hydrodynamics:
effectsthesparkformationandspeedofinflammation
Heattransfer:
effectstheoperatingtemperatureofthesparkpluganditsreliabilityofoperation
Foulingprocess:
effectsthepossibilityofsparkformation
ThedevelopmentoftheMULTITORCHprechambersparkplugconstitutesanewapproachfortheimprovementofleanburnengineoperation.Figure1illustratesitsmainconfiguration.
Theignitionelectrodeisarrangedinsidetheprechamberandthereforenotexposedtotheinfluenceswhichproceedfromthemaincombustionchamber.
Theconnectionbetweenprechamberandmaincombustionchamberisestablishedby5orifices(1centraland4tangential).
Duetothisseparationoftheelectrodefromthemaincombustionchamberitbecomespossibletocreatefavourableconditionsfortheinflammationintheprechamber.
Thetestresultswhichwilbeshownlaterrevealconsiderableoperationadvantageswiththistypeofsparkplug.
FIG.1:
Prechambersparkplug
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4.Prechamberprocess
Asshowninfigure1athinplatinumwireextendsfromthemiddleelectrodetothewallandcreatesthesparkgapbetweenthetipandthewall.Therethewholecombustionprocessisinitiatedbythespark.
Theignitionprocessinthemainchamberisreleasedbyanadvancedprechamberprocessconsistingoftwostageswhichareillustratedinfigure2.
FIG.2:
Prechamberprocess
Duringthechargingstagetheprechamberisfilledwithfreshgasairmixturebythecompressionstroke.Residualgasfromthepreviousstageispushedintothebackwardarea.Averyrapidinflammationoftheprechamberloadbythesparkwillbeachievedbytheinfluenceofthefollowingparameters
1.swirlofload
2.microturbulence
3.platinumelectrode
4.elevatedtemperatureofelectrode
5.separationoftheelectrode
Aftertheinflammationthevaluesforpressureandtemperaturerisesteeplyintheprechambersothatthecombustionproductsarepressedthroughtheorificestherebycorrespondingtotheimageofafiringdragon.
Thehighimpulserateofthetorchjetsandtheirhemisphericaldistributioninsureaintensiveinflammationoftheloadinthemainchamber.Theignitionenergytherebytakingeffectis100---1000timeshigherthanwithastandardsparkplug.Inthemaincombustionchamberarapidanduniformenergyconversionoftheleanmixtureisobserved.
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5.Testresults
TheMULTITORCHprechambersparkplugsweretestedinvarioustypesofenginesfrommorethan10differentmanufacturers.Theresultsshownbelowhavebeenobservedatalltestswithmoreorelessthesamequality.Thespecificquantityhereofwascorrelatedtospecialfeaturesoftheengines.
Ithasbeenestablishedthattheresultsbetweenprechamberandstandardsparkplugdifferslightlyatsmallcylindersizesandconsiderablywithbiggersizessuchascylinderdisplacementsmorethan4liters.Themostdistinctdifferencescanbeobservedwithcylinderdiametersaround200mmandmore.
5.1Variationsofcombustionpressure
Thetestresultsareshowninfigure3.TheywereworkedoutatanAFRof1,68andcylinderdiameterof170mm.TheNOxemissiondatawassetaccordingTA-Luftstandards.
Thescatteringoftheindicatedmeaneffectivepressureinfigure3isclearlysmallerthanwithastandardplugasshowninfigure4.
Thestatisticaldistributionoftheindicatedmeaneffectivepressurevariationsshowninfigure3and4differforabout50%inthevariationmagnitudeandforabout70%infrequencylatitude.Thispointsoutasignificantlysmallerengineroughnesswiththeprechambersparkplug.
Thesignificantdecreaseinengineroughnessreducesthedynamicloadofpartsandcomponentsoftheengineandextendstheirservicelife.
Theseimprovementsareofspecialinterestforenginegeneratorservicesincethefluctuationsofelectricalpowerduringproductionarealsoreducedperceptibly.
FIG.3:
IndicatedmeaneffectivepressurerecordwithMULTITORCHsparkplug
FIG.4:
Indicatedmeaneffectivepressurerecordwithstandardsparkplug
5.2NOx-Emissions
ThecorrelationofAFRandNOxemissionsisillustratedinfigure5.OperationofabovementionedenginewithprechambersparkplugsallowsthereductionofNOxemissionfarbelowtheTA-LuftlimitbyincreasingtheAFRwithoutattainingmisfire.AdverselyinthiscasetheTA-LuftlimitcannotbereachedbyanincreaseoftheAFRwithstandardsparkplugsbecauseofmisfire.
FIG.5:
NOx-Emissions
5.3Ignitionvoltagerequirementsandsparkpluglife
Ignitionsystemsprovideupto30KV.Thedifferencebetweenthisvalueandtheactualrequirementoftheplugrepresentsareserveforthecompensationofthewearatthesparkgap.Thelifetimeofthesparkplugisdeterminedbythisreserve.
Theignitionvoltagerequirementoftheprechambersparkplugisrelativelylowbecausetheeffectingparametersofchapter4areapplied.
ForexampletherequirementatanAFRof1,8fornewplugsamountsto17KVfortheprechamberplugcomparedtomorethan20KVforthestandardplug.
Experienceswiththe
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