柴油机喷油技术.docx
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柴油机喷油技术.docx
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柴油机喷油技术
8.8COMMON-RAILUNITINJECTORSYSTEM
Generalbackground
Nowbecomingmorewidelyusedbyvirtueofitslower
internalvolumeandgreaterrigidity,thistypeofsystem
canallowhigherinjectionpressurestomeetstricter
emissionlimits,reducenoiselevelsandimprovefuelcon-
sumption.UnlikethejerkpumpsystemdescribedinSec-
tion8.2,whereaseparatelymountedmulti-elementpump
providestheinjectionpressureandregulationoffuel
quantityforthefuelinjectors,eachcylinder-head-moun-
tedunitinjectorcombinesthefunctionsofmetering,tim-
ingandhigh-pressurefuelinjection.Thesefunctionsare
performedbythemovementofaplungeractuallyinthe
unitinjector,viaapush-rodandrockermechanismfrom
theenginecamshaft.Eachenginecylinderistherefore
providedwithaself-containedinjectionpump-and-
nozzleunit,whichreceivesfuelatrelativelylowpressure
fromacommon-railsupplysystem(Figure8.40).
Anotablepioneerintheapplicationofthedieselengine
toheavyvehiclesinAmericawasC.L.Cummins,who
foundedthewell-knowncompanythatbearshisname,
anditwashewhofirstsuccessfullydevelopedtheunit
injectorsysteminthelate1920sforautomotiveuse.This
systemwasoriginallydevelopedtoovercometheunrelia-
bilityofearlyhigh-pressurejerkpumpsandtoavoidthe
stressesassociatedwiththehighpressuregeneratedinthe
pipelinesbetweenthejerkpumpandthecylinder-head-
mountedfuelinjectors.Perhapsitshouldbementioned
thatduringtheearlydevelopmentoftheautomotive
dieselengine,itwasnotuncommonforenginemanufac-
turersthemselvestodesignandbuildtheirownfuelinjec-
tionequipment.
Thepressure-timeunitinjectorsystem
Abasicdistinctionbetweentheunitinjectorandthe
conventionaljerkpumpsystemsoffuelmeteringisthat
theformerisinletmeteredwhilethelatterisoutletme-
teredbyspillcontrolofoutput.Animportantlaterdevel-
opmentoftheunitinjectorsystemproducedbythe
CumminsEngineCompanywastheintroductionin1954
oftheirpressure-timeconceptofinletmeteringforthefuel
enteringtheinjector.
Withaunitinjectoroperatedfromtheenginecamshaft,
itwillbeevidentthatthetimeintervalavailablefor
meteringthefuelthroughafixed-sizeorificeintothe
injectorwillvaryinverselyastheenginespeed.Therefore
ifthefuelsupplypressuretotheinjectorremainscon-
stant,thequantityoffuelmeteredforinjectionwould
decreasewithincreaseinenginespeed.Inordertovary
andcontrolthequantityoffuelthatismeteredforinjec-
tion,sothatitiscorrectlyrelatedtoenginespeedand
load,theCumminspressure-time(PT)systemsuperim-
posesagovernorandthrottlecontrolontheengine-
drivenfueltransferpumpthatsuppliesfueltotheunit
injectors.Hence,thequantityoffuelthatismeteredinto
themissubjecttotwovariables,namelyfuelpressureand
meteringtime.
TheCumminsPTsystemoffuelinjectionhasbeen
continuouslyrefinedsinceitsinception,butessentiallyit
comprisesthefollowingcomponents:
1Anengine-drivenpositivedisplacementfueltransfer
pumpoftheexternalgeartypewithapulsationdamper
tosmooththefuelflow.Thepumpdrawsfuelfromthe
tankviaaprotectivefilter.
2Athrottlecontrol,whichiscombinedwiththefuel
transferpumpforthedrivermanuallytocontrolthe
enginespeedaboveidle,asrequiredbyvaryingoperat-
ingconditionsofspeedandload.Rotationofthe
throttleshaftviatheacceleratorpedalvariesthe
amountofflowandthereforethepressureofthefuel
deliveredtotheunitinjectors.
3Acentrifugallyoperatedgovernor,whichissimilarly
combinedwiththefueltransferpump.Thegovernor
maintainssufficientfuelforidlingwiththethrottle
controlintheidleposition,anditcutsofffueltothe
injectorunitsabovemaximumratedenginespeed.
Duringoperationbetweenidleandmaximumspeeds,
thefuelflowsthroughthegovernortotheinjectorunits
viathethrottlecontrol.
4Ashut-downvalve,whichisalsocombinedwiththe
fueltransferpump,sothatfueldeliverytotheunit
injectorscanbeterminatedtostoptheengine.
5Unitinjectors,whichprovidethemeansofintroducing
fuelintotheenginecombustionchambersandcombine
thefunctionsofmetering,timingandhigh-pressure
injection(Figure8.41).Theyreceivefuelatarelatively
lowcontrolledpressurefromthecommon-railsupply
system.Exceptduringthemeteringstage,thereisa
Advantagesanddisadvantagesofunitinjectorsystems
Theunitinjectorsystemhasgenerallyinvolvedtheex-
pensivecomplicationofadditionalpush-rodswithlower
andupperrockerstoactuatetheinjectorsfromtheengine
camshaft,althoughmorerecentversionsmaybenefit
fromthesimplificationofanoverheadcamshaftand
singlerockeractuationoftheinjectors.
However,thesystemhastheadvantages(notedearlier)
ofeliminatingthenecessityforhigh-pressurefuelpipe-
linesbetweenthepumpandinjectorsandofcombining
allmeteringandinjectionfunctions.Otheradvantages
generallyclaimedarethoseofenablingawiderangeof
enginespeedstobeattainedwithquickthrottleresponse
andlowidlingspeeds,andminimummaintenancere-
quirements.Thesetypicallyincludeadjustingtheunit
injectorsatthesametimeastheenginevalvesevery
60000miles(96000kin)oroneyearandcleaningand
calibratingtheunitinjectorsandfuelpumpevery180000
miles(288000km)ortwoyears.
8.9INTRODUCTIONTOELECTRONICDIESEL
CONTROL
Generalbackground
Itisperhapsonlytobeexpectedthatjustassophisticated
electroniccontrolshavebeenappliedtomanagethefuel
injectionsystemsofpetrolengines,sotheyarenowbeing
adaptedtoperformasimilarrolewithdieselengines.
Theirmainpurposeistomakeitlessdifficultformanu-
facturerstocomplywithstricterexhaustemissionregula-
tionsandachievebetterperformance.
Thedevelopmentofelectroniccontrolsfordieselen-
gines(oftenabbreviatedtoEDC)beganinthemid1970s,
whenDetroitDieselofGeneralMotorsinAmericaap-
pliedthemtoexperimentalcommon-railinjectionsys-
temsusedinthecourseofresearchintodieselengine
combustion.However,itwasnotuntilsometenyears
laterthatelectronicmanagementcontrolsfordistributor,
in-lineandcommon-railfuelinjectionsystemsfirstbegan
toappearafterfurtherresearchbyLucasCAVinthis
country,BoschinGermanyandDetroitDieselin
America.NotableearlyapplicationsofelectronicDiesel
controlhavebeentotheindirect-anddirect-injection
turbochargedenginesusedrespectivelyinBMWpassen-
gercarsandScaniaheavyvehicles.
Basicfeaturesofanelectronicdieselcontrolsystem
Wetakeasanexampletheapplicationofelectroniccon-
troltothein-linefuelinjectionpumpforaheavyvehicle.
Theessentialdifferencesbetweenaconventionalinjection
pumpofthistypeandonethatissubjecttoelectronic
control(Figure8.42)areasfollows:
1Theeliminationofamechanicalgovernor,itsplaceon
thepumpcamshaftbeingtakenbyanimpulsediscthat
rotatesaboutastationaryspeedsensor.
2Theprovisionofalinearsolenoidactuator,whichcon-
stantlyadjuststhepositionofaspring-returnedcontrol
rodforthepumpingelements.
3Theadditionofasensorthatdetectsthechanging
positionofthepumpcontrolrod.
Theelectroniccontrolunitreceivessignalsfromvarious
sensors,thisinformationbeingprocessedandcompared
withpreprogrammedvalues,sothattheappropriate
commandscanbesenttothecontrolrodactuatorandthe
requiredquantitiesoffueloilaredeliveredtotheinjec-
tors.
InthecaseoftheScaniaelectronicdieselcontrolsys-
temforaturbochargedheavy-vehicleengine(Figure
8.43),thesignificanceofthevarioussensorsmaybesum-
marizedasfollows:
AmountofaccelerationTheacceleratorpedalisconnec-
tedtoasensorthatconvertspedaltravelintoanelectrical
signal,therebeingnoneedforamechanicalconnection
betweenthepedalandinjectorpump.
ChargeairpressureThisinformationisrequiredsothat
thequantityoffueloildeliveredtotheinjectorscanbe
accuratelymatchedtotheavailableamountofair.
ChargeairtemperatureItisimportanttorelatethisto
thequantityoffueloildeliveredtotheinjectors,because
coldaircontainsmoreoxygenthanwarmairandallows
morefueltobeburnt.
CoolanttemperatureThisneedstobesensedforauto-
maticallycontrollingthecoldstartfunction,sincethe
quantityoffueloildeliveredtotheinjectorsmustbe
preciselyrelatedtotheprevailingtemperaturewhilst
allowingtheenginetobeprotectedfromoverspeeding.
Forexample,ifthecoolanttemperatureis0~Ctheengine
cannotbespeededuptomorethan1000rev/minforthe
first30seconds.Informationfromthissensoralsocon-
trolsidlingspeed.
EnginespeedAsensorisincorporatedinthefuelinjec-
tionpumpforconstantlymonitoringtheenginespeed,so
thatitcanbemaintainedatpresentvalues,regardlessof
whetherthecruisecontrolisengagedtoretainachosen
roadspeedorthepowertake-offisbeingoperated.
InjectionquantityThisinformationisderivedfroman-
othersensorinthefuelinjectionpumpthatdetectsthe
positionofthecontrolrodforthepumpingelements,
therebyindicatingiftherequiredquantityoffueloilis
beingdeliveredtotheinjectors.
Aswithothermodernelectronicallycontrolledmanage-
mentsystems,severalfunctionsforself-diagnosiscanbe
incorporated,whichindicatewhenafaultoccursandcan
alsospecifywhereithasoccurred.Alightonthedriv
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