Case 621G - Page 235

7-MAINTENANCE
SelectiveCatalyticReduction(SCR)systemdescription
NHIL13ENG1393AA9
(1)Intakeairhumidityandtemperaturesensor(10)Exhaustap
(2)Airlter(11)EngineControlUnit(ECU)
(3)Supplymodule(12)SCRupstreamtemperaturesensor
(4)DEF/ADBLUE®levelandtemperaturesensor(13)SelectiveCatalystReduction(SCR)catalyst
(5)DEF/ADBLUE®qualitysensor(14)SCRdownstreamtemperaturesensor
(6)Dosingmodule(15)NH
3
sensor
(7)DOCandmixer(16)DownstreamNO
x
sensor
(8)DOCupstreamtemperaturesensor(17)Engine
(9)UpstreamNO
x
sensor
Thehumidityintheintaketotheengine(17)affectsthe
emissionsintheexhauststream.Theintakeairhumidity
sensor(1)islocatedintheintakeairpipingtotheengine
betweentheairlter(2)andtheintakemanifold.The
informationfromthissensorissentdirectlytotheEngine
ControlUnit(ECU)(11).
Engineexhaustsystemtemperatureiscriticaltothecat-
alyticreactionsthattakeplaceintheemissionscontrol
components.T oquicklyincreasethecatalysttempera-
tureatinitialstart-upandlightoperatingloads,thereis
anexhaustap(10)locatedintheexhauststreamnear
theturbocharger.Themaximumtemperatureoftheex-
haustapiscritical,thereforeenginecoolantiscirculated
throughthecoreofthisvalve.LimitingNO
x
emissionsin
theexhauststreamisoneofthemajorgoalsoftheemis-
sioncontrolsystemontheseengines.TheupstreamNO
x
sensor(9)islocatedintheexhauststreamattheinletto
theDieselOxidationCatalyst(DOC)(7).TheNO
x
sensor
transmitsdatatotheECUviatheengineCANdatabus.
Atemperaturesensor(8)islocatedattheinlettotheDOC
tocloselymonitortemperature.
DEF/ADBLUE®uidfreezesat-11°C(12°F).A
DEF/ADBLUE®uidlevelandtemperaturesensor(4)is
installedinthetankandtheinstrumentclusterdisplays
thoselevels.TheDEF/ADBLUE®qualitysensor(5)
monitorsthequalityoftheDEF/ADBLUE®uid.
TopreciselycontrolthequantityofDEF/ADBLUE®uid
injectedfromtheDEF/ADBLUE®tankintotheexhaust
stream,aDEF/ADBLUE®dosingmodule(6)isintegrated
intheoutletareaoftheDOCcanister.Thedosingvalve
isPulseWidthModulation(PWM)controlledbytheECU.
TheECUdeterminestheamountofDEF/ADBLUE®uid
toinjectbasedoninputsfromsensorsintheexhaust
stream.PressurizedDEF/ADBLUE®uidissuppliedto
thedosingmodulefromthesupplymodule(3)atapprox-
imately9bar(130psi).
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