Bosch-Oxygen-Sensor

13ServiceTechMagazine/May2001Boschdelivered10millionoxygensensorstotheU.S.marketin1976andby1983thenumberhadrisento50million.Today,Boschproduces33millionoxygensensorsperyear.In1982Boschlaunchedtheheatedoxygensensorwhichreachesfulloperabilityin30secondsafteracoldengineisstarted.Thesensorisheatedto400oCandhasaservicelifeof160,000km,twiceaslongasthepreviousunheatedsensor.In1994Boschdevelopedanoxygensensorwithaplanarceramicstructurethatisfullyfunctioning10secondsafterthevehicleisstarted.Today’soxygensensorOxygensensors(seeFigure1)arerequiredtodayduetotheincreasinglytoughexhaustemissionsandgohand-in-handwiththecatalyticconverters.Oneoxygensensorisusedintheexhaustbranchrightbeforethecatalyticconverter.Sometimesasecondoxygensensorisplacedintheexhaustsystemafterthecatalyticconverterofaspark-ignitionenginetopermitopti-mumperformanceofthethree-waycatalyticconverters.Theinformationobtainedfromthesensorsindicateshowcompletethecombustionprocessisinthecombustioncham-ber.Theoptimumreadingsareobtainedwhentheairtofuelratiois14.7toone.Thestoichiometricair/fuelratioisthemassof14.7kgofairto1kgofgasolinetheoreticallynecessaryforcompletecombustion.Theexcessairfactororairratio(λ)indicatesthedeviationoftheactualair/fuelratiofromthetheoreticallyrequiredratio.λ=(actualinducedairmass)/(theoreticalairrequirement).ThefirstoxygensensordevelopedbyBoschwasinstalledinaVolvo240/260seriesvehicle25yearsago.Figure1.Boschoxygensensor.ServiceTechMagazine/May2001Figure2.Controlrangeandreductionsinexhaustunderthreescenarios.Number1iswithoutacatalyticconverter.Number2iswithacatalyticconverter.Number3istheλoxygensensorvoltagecurve.λcontrolrange(catalystwindow)123EngineemissionsNOxCOHCNOxCOHCEngineemissionsλ-sensorvoltage0.9751.01.0251.05richleanExcess-airfactorλ14ServiceTechMagazine/May2001Variationsfromthisoptimumratioresultinvariouslevelsofemissions.Excessfuelresultsintheformationofhydrocar-bons(HC)andcarbonmonoxide(CO).Excessaircancauseincreasedlevelsofnitrogenoxides(NOx).Theoxygensensororsensorscanidentifyanyvariationsfromtheidealair/fuelratioandsendasignaltotheenginemanagementsystemtoadjusttheignitionandinjectionprocesses.ThethreewaycatalyticconverterisabletoreducetheHC,CO,andNOxemissionsbymorethan98%providedtheengineoperateswithinaverynarrowscatterrange(1%)centeredaroundthestoichiometricair/fuelratio(seeFigure2).Aclosedloopcontrolsystemthatreliesonaclosedloopcontrolcircuittomaintaintheair/fuelmixtureconsistentlywithintheoptimalrangeknownasthecatalystwindowisthebeststrategy.SensordesignTheoxygensensor(seeFigure3)isagal-vanicoxygenconcentrationcellwithasolidstateelectrolyte.Thesolidstateelec-trolyteisanimpermeablezirconiumdiox-ideceramicunitstabilizedwithyttriumoxide.Itisopenononeendandclosedontheother.Mountedonboththeinnerandoutersurfacesaregaspermeableplatinumelectrodes.Theplatinumelectrodeontheoutsideactsasaminiaturecatalysttosupportreac-tionsintheincomingexhaustgasesandbringthemintoastateofstoichiometricbalance.Thesidethatisexposedtotheexhaustgasesalsohasaporousceramiclayer(Spinellcoating)toprotectagainstcontamination.Ametaltubewithnumer-ousslotsguardstheceramicbodyagainstimpactsandthermalshocks.Theinnercavityisopentotheatmospherewhichservesastheunit’sreferencegas.Thetwo-statesensoroperationisbasedintheNernstPrinciple.Thesensor’sce-ramicmaterialconductsoxygenionsattemperatures350oCandabove.Disparitiesinoxygenlevelsontherespectivesidesofthesensorwillresultinthegenerationof1.Ceramiccoating2.Electrodes3.Contacts4.Housingcontacts5.Exhaustpipe6.Ceramicsupportshield(porous)7.Exhaustgas8.Ambientair1.Guardtube2.Ceramicsealassembly3.Sensorhousing4.Ceramicsupporttube5.Planarsensorelement6.Protectivecap7.ConnectionwireFigure4.Heatedoxygensensor.1.Sensorhousing2.Ceramicsupporttube3.Connectionwire4.Guardtubewithslots5.Activeceramicsensorlayer6.Contact7.Protectivecap8.Heaterelement9.Crimpedconnectionsforheaterelement10.SpringwasherFigure5.PlanaroxygensensorFigure3.Oxygensensorinexhaustpipe.1.Porousprotectivelayer2.Externalelectrode3.Sensorlaminate4.Internalelectrode5.Referenceairlaminate6.Insulationlayer7.Heater8.Heaterlaminate9.ConnectioncontactsFigure6.Operationallayersinaplanaroxygensensor.123456781234567896123456789101234567V,voltage15ServiceTechMagazine/May2001electricalvoltagebetweenthetwosurfaces.Thisvoltageservesastheindexofhowmuchtheoxygenlevelsvaryonthetwosidesofthesensor.Theamountofresidualoxygenintheexhaustfluctuatessharplyinresponsetothevariationintheinductionmixture’sair/fuelratio.Oxygensensitivevoltagegenerationrangesfrom800to1000millivoltsforrichmixturestoaslowas100millivoltsforleanmixtures.Thetransitionfromrichtoleancorrespondsto450to500millivolts.HeatedoxygensensorAnelectricheaterelement(seeFigure4)isusedtowarmtheceramicmaterialwhentheengineisoperatingatlowloadfactors.Atthehigherloadfactorsthesensor’stemperatureisdeterminedbytheexhaustgas.Theheatedoxygensensorhelpsensurelowandstableemissionsduetotheconsistentmainte-nanceofoptimaloperatingtemperatures.PlanarλoxygensensorThebasicoperatingconcept(seeFigure5)isthesameastheheatedfinger-typese