Multipoint Fuel Injection.docx

Multipoint Fuel Injection.docx

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MultipointFuelInjection

1.INTRODUCTION

Byreadingthetitle,veryfirstquestionarisesinourmindis“whatisMPFI?

”ActuallyMPFIisnothingbutmultipointfuelinjection.Asthenamesuggests,itisaadvancedfuelinjectiontechnology,nowveryvastusedinpetrolvehiclesinsteadofoldcarburetorsystem.

Eventhoughcarburetorhasservedmillionsofvehiclesoveralongperiodoftime,thismethodofairfuelmixturemanagementisconsideredinefficientandpolluting.Infactcarburetorisoftendefinedas;delicatesystemofsmallcountlessmovingpartsthatalmostalwayssuppliestheundesiredair-fuelratiototheengine.Theoriginofthecarburetordoesnotallowittoadapttothedemandingdrivingconditionthatcarsencountertodaywithoutsacrificingpower,emissioncontrolsandfueleconomy.Nowadays,mostofthemoderncarsusefuelinjectionsystem,mostlycomputercontrolled!

Theelectronicnatureoffuelinjectionmakesitpossibletoadapttochangingdemandsofanenginebydeliveringonlytheamountoffuelrequiredtogeneratethedesiredenginepowerwithoptimalefficiencyandfueleconomy,whileensuringminimumemission.Ithasdistinctadvantagesovertheconventionalcarburetorsystemasincreasedpoweroutput,increasedtorque,improvedvolumetricefficiency,moreuniformfueldistribution,morerapidresponsetothrottlepositionchangesandeasyworkingin'coldstarting'and'enginewarmup'conditions.

2.NEEDFORMPFI

Conventionalcarburetorsystemisassociatedwithcertaindisadvantages.

Disadvantagesofcarburetor

1.Inmulti-cylinderenginethemixturesuppliedtovariouscylindersvariesinqualityandquantity.Firstcylindergetsarichmixturewhilethelastgetslean.Sincetheinductionpassagesareofunequallengthsandofferdifferentresistancestomixtureflow.Themixtureproportionisalsoaffectedduetofuelcondensationininductionmanifold.

ThiscanbeovercomebyusingMPFI.Byusingseperateinjectorforeachcylinder.

2.Carburettorwiththeirchoketube,jets,throttlevalve,inletpipe,bendsdonotgivefreeflowpassageformixture.Thusthereisalossofvolumetricefficiency.

3.Iceformationmaytakesplaceatlowtemperature.ThiscanbeaviodedbyusingMPFI.

4.Fuelmoleculesformedgetadheretothemanifoldwhilepassingthrough,thusreducingqualityofmixture.

AbovedisadvantagescanbeovercomebyusingMPFI.

AdvantagesofMPFI

1.Fuelinjectionallowstheamountoffuelinjectedpercycle,foreachcylindertobevariedinresponsetoinputsderivedfromsensorswhichdefinetheactualrunningconditionoftheengine.

2.Increasedvolumetricefficiencyandhenceincreasedpowerandtorqueduetoabsenceofanyrestrictioninflowpassage.

3.Betterdistributionofair-fuelmixturetoeachcylinder.

4.Lessfuelconsumptionandlesspollution.

5.Morerapidengineresponsetochangesinthrottlepositionandmoreprecisecontrolofequivalenceratioduringcoldstartandenginewarmup.

Dueaboveadvantages,nowadaysmostofthecarsareusingMPFI.BoschL-jetronicisthemostcommonlyusedmulti-pointfuelinjectionsystem.

Therearedifferenttypesofmulti-pointfuelinjectionsystemssuchas,D-JETRONIC,K-JETRONIC,KE-JETRONIC,L-JETRONIC,etc.

3.BOSCHL-JETRONICSYSTEM

Outlineofsystem,

TheL-jetronicisanelectronicallycontrolledfuelinjectionsystemwhichinjectsfuelintermittentlyintointakeports.Itdoesnotrequireanyformofdrive.Itcombinestheadvantagesofdirectairflowsensingandspecialcapabilitiesaffordedbyelectronics.

Thissystemdetectsallchangesresultingfromtheengine（wear,depositsinthecombustionchamberandchangesinvalvesetting）,thusguaranteeingauniformlygoodexhaustgasquality.

Inthissystemvarietyofoperationaldataatanyparticularlocationofthevehicleisregisteredandconvertedintoelectricalsignalsbysensors.ThesesignalsarethenpassedtoECU,whichprocessesthemandcalculatestheexactamountoffueltobeinjected.

3.1DifferentPartsofL-Jetronicsystem

Sensors

1.ThrottlepositionSensor

PrincipleofOperation

Throttlepositionsensorisasshowninfig.6.Itislocatedonthrottlebody.

Itisapotentiometer.PotentiometerisavariableresistorthatconvertsmechanicalmotionintovoltageandthisvoltagesignaliscontinuouslysendtoECU.Thisvoltageisproportionaltothrottlevalveposition.

ThisvaryingvoltagesignaltellsECUtheexactpositionofthrottlevalveandthisenablestheECUtomatchfuelflowwithair-flow.Thenenginereceivesproperairfuelmixture.

2.Airflowsensor

PrincipleofOperation

Airflowsensorisasshowninfig.4.Itismountedonupstreamsideofthrottlevalve.InBOSCHL-JETRONICsystemswinginggatetypeairflowsensorisused.

Thissensorconsistsofvanesorflapsatrightangletoeachother.Airflowingforcestheflapstoswing.Theseflapsareattachedtopotentiometer.Apotentiometersensestheangleofdeflectionandconvertsitintoavoltagesignalinverselyproportionaltoairflowing.ThiscausesthepotentiometertosendvaryingvoltagesignaltoECU.

AfterknowingtheairflowrateECUsendsthesignaltomatchfuelflowwithairflowbyvaryingthetimeofinjectionoffuel.

3.CoolanttemperatureSensor

PrincipleofOperation

Itissituatedinwaterjacketsaroundthecylinder.Thisisathermistor.ItisNTCresistor.Whentemperatureincreasesitselectricalresistancedecreases.ThiscontinuouslysendsvaryingvoltagesignalaboutenginetemperaturetoECU.

IfcoolanttemperatureislowECUincreasestheinjectorpulsewidthtosupplyadditionalfuelforcoldengineoperation.TheECUmayalsoalterignitiontimingtosuitenginetemperature.

Mostoftheenginesusescoolingfan,nowwhenengineiscoldnocoolingisrequired.SensorsendsignalandcoolingfanremainsOFF.WhenenginegetswarmersignalfromsensorcausestheECUtoswitchONthefan.

4.OxygensensororLambdaSensor

PrincipleofOperation

Oxygensensorisasshowninfig.5.Thisisinstalledinexhaustmanifoldorinexhaustpipe.Thisisaboutthesizeofsparkplug.Thefunctionofthissensoristodetectdepartureofairfuelratiofromstiochometricratiobycomparingoxygencontentofexhaustgastothatofatmosphericair.Whenthereisdifferenceinoxygencontenttheoxygensensorproducessmallvoltagesignal.ThissignalissenttoECU.

Oxygensensorvoltageisbetween0.15Vto1.30V.Whenvoltageiscloseto0.45V,theairfuelratioisclosetoidealstiochiometricratio（14.7:

1）.Ifthevoltageishigherthan0.45V,itindicatesthatexhaustgasoxygencontentislowandairfuelratioisrich.Ifthevoltageislessthan0.45V,oxygencontentishighandairfuelratioislean.

AfterreceivingtheoxygensensorsignaltheECUadjuststheinjectorpulsewidthtomodifytheairfuelratio.

5.EnginespeedSensor

PrincipleofOperation

Thisismountedonnotchesincrankshaft.AsthenotchesrotatesitproducesvoltagesignalwhichissendtoECU.

ECUusesthisinformationtocontrolfuelmetering,ignitionsparkadvance,andtheshiftingofelectronictransmissionandtransaxlesbyvaryingtheinjectorpulsewidth.

6.AirtemperatureSensorandAtmosphericpressureSensor

PrincipleofOperation

Itismountedinthepathofairflowonairflowsensor.Airtemperaturesensorisathermistor.Itselectricalresistancedecreasesasthetemperatureincreases.

Atmosphericpressuresensorisabarometricpressuresensor.BoththesensorssendvaryingvoltagesignaltoECU.

Whenatmosphericpressureandairtemperaturechangesthedensityofairchangesbytherelation,

=p/RT.…..

（1）

Airthatishotandatlowatmosphericpressureislessdense.Itcontainslessoxygen,soitisnecessarytovarythefuelrequiredaccordingly.

ECUusesthisinformationandcontrolthepulsewidthtoincreasethefuelflowtomatchwiththeoxygencontent.

Fuelsupplysystem

Thefuelsupplysystemcomprisesofthefollowingcomponents.

Electricfuelpump

Finefilter

Fuelrail

Pressureregulator

Fuelinjectionvalves

Anelectricallydrivenrollercellpumppumpsthefuelfromthefueltankatapressureofapproximately2.5barsthroughafilterintothefuelrail,fuellinesdivergetotheinjectionvalve.Attheendoffuelrailisapressureregulatorwhichmaintainstheinjectionpressureataconstantlevel.Morefuelcirculatesinthefuelsystemthanisneededbytheengineevenundermostextremeconditions.Theexcessfuelreturnstothefueltankbythepressureregulatorbutnotunderpressure.Theconstantflushingthroughofthefuelsystemenablesittobecontinuallysuppliedwithcoolfuel.Thishelpstoavoidtheformationoffuelvaporbubbleandguaranteesgoodhotstartingcharacteristics.

Electricfuelpump

Thefuelpumpisarollercellpumpdrivenbyapermanentmagnetelectricmotor.Therotorplatewhichiseccentricallymountedinthepumphousingisfittedwithmetalrollersinnotchesarounditscircumferencewhicharepressedagainstthepumphousingbythecentrifugalforcesandactsasaseal.

Fuelfilter

ThefuelfilterfiltersoffimpuritiesinthefuelwhichcouldimpairthefunctionoftheInjectionsystem.Thefiltercontainsapaperelementwithanaverageporesizeof10µm,whichbackedupbyafluffstrainer.Thiscombinationensuresahighdegreeoffiltration.Supportplatesecuresthefilterinitsmetalhousing.Thefilterisinstalledinthefuellinedownstreamofthefuelaccumulator.

Fuelrail

Thefuelrailsuppliesallinjectionvalveswithanequalquantityoffuelandensuresthesamefuelpressureatallinjectionvalves.

Pressureregulator

Thepressureregulatorisasshowninfig.8.Thepressureregulatorkeepsthepressurediffe