内燃机英文资料3

1、Chapter3FUELFEEDSYSTEMSOFINTERNALCOMBUSTIONENGINESFuelsandOperationofFuelSystemsThefuelsystemofaninternal-combustionengineisintendedtoproduceacombustilemixturecomposedofthefuelstoredinthefueltankandatmosphericairanddeliverbothtothecylinders.Carburettorenginesuselightgradesoffuel-gasolineandkerosene,

2、whiledieselenginesutilizeheavyoil.Lightfuelsarecharacterizedbysuchmainpropertiesasvolatility,combustibility,heatingvalueanddetonationability,thelatterbeingdescribedbytheoctanenumber.ThegradesofgasolineconformtoappropriateStateStandardsandareavailableinA-66,A-72andA-74grades(thefigureindicatestheocta

3、nenumber).Thequalityofadieselfuelischaracterizedbythefollowingmainproperties:specificgravity,viscosity,freezingpoint,absenceofmechanicalimpurities,cokecontent.Inviewofthehighviscosity,differentgradesofdieselfuelareemployedinsummerandwinter-IIJIinsummerandII3inwinter.Fuelsystemsandtheirelementsdiffer

4、greatlyincarburetoranddieselengines.Someofthedevicesusedinfuelsystemsserveacommonpurposeandaredesignedalongsimilarlines;theseincludefueltanks,filtersandsedimentbowls,fuelpipelines,aircleaners,intakeandexhaustpipes(headers)andsilencers.Thedesignofdeviceslikeprimingpumpsandtheelementsoffuelfeedsystems

5、forproducingtheworkingmatterdifferinprincipleinthevarioustypesofengines.Toensureanuninterruptedsupplyofcombustiblemixturethefuelsystemshouldprovidefor:storage,purificationanddeliveryoffuel;purificationanddeliveryofair;preparationofcombustiblemixture;dischargeanddampingoftheexhaustgasenergy.Theprinci

6、palprocessconsistsinthepreparationoftheworkingmixturewhichcomprisesthefollowingthreestages;pulverizationoffuelnecessaryforitsrapidandcompletevaporization(byincreasingthesurfaceofevaporation);vaporizationoffuel;mixingofthefuelvapourswithair.ThefuelsystemofcarburetorenginesisdesignedasshowninFig.32.Fu

7、elpump6deliversfuelfromtank8tocarburetor2.Theprincipalelementinthefuelsystemisthecarburetor,wherethecombustiblemixtureisprepared.Mechanicalimpuritiesareremovedfromthefuelinfiltersedimentbowls5and7.Airissuppliedtothecarburetorthroughaircleaner1.Thealmost-readycombustiblemixturepreparedinthecarburetor

8、isadmittedthroughintakepipe3(wheretheremainsoffuelvaporizecompletely)totheenginecylinders.Aftercombustionofthemixturetheexhaustgasesaredischargedintotheatmospherethroughexhaustpipe4andsilencer.Anotherfuelsystemisemployedindieselengines(Fig.33)一41XFig,32.DiagramoffuelfeedsystemofcarburettorengineFig.

9、33.DiagramoffuelfeedsystemofadieselAirisdrawnintothecylindersthroughaircleaner9andintakepipe8.Primingpump4deliversfuelfromtank1viafuelline2tohigh-pressurepump6;thefuelistheninjectedintothecombustionchamberbynozzle7.Atthesametimethefuelisforce-cleanedincoarse-mesh3andfine-mesh5filters.Inthisdesignthe

10、principalelementsareahigh-pressurepump,andanozzlwhichfinelyatomizesthefuel;Theexhaustgasesareremovedfromthecylindersinthesamewayasincarburetorengines.Theoreticallyspeaking,about14.8-15kgofairisrequiredforfullcombustionof1kgoffuel.Inactualpracticetheair/fuelratiomaybemoreoritmaybeless.Theair/fuelrati

11、ocharacterizesthequalityofthecombustiblemixtureandhasanimportanteffectontheengineperformance.Thequalityofthecombustiblemixtureisbestassessedbytheexcessaircoefficienta,whichistheratiooftheamountofairper1kgoffuelintheactualmixturetotheamountofairtheoreticallynecessaryforfullcombustionofthesamequantity

12、offuel.Dependingonthecompositionand,hence,onthevalueoftheexcessaircoefficient,themixtureisreferredtoasnormalwhena=1,leanwhena1andrichwhena1.Mostenginesrunforconsiderableperiodsatpowerslessthanmaximum,andforthisreasonitismostexpedienttoprovidethemwithaleanmixtureforthecurrentduties.Whentheengineisshi

13、ftedovertofullpoweritshouldbesuppliedwithanormalandevenwithasomewhatenrichedmixture.Richmixturesshouldbealsoemployedatlowspeeds,whentheconditionsofatomizingandmixingthefuelwithairdeteriorateduetothesmallvelocityoftheairdrawnintotheengine.FuelSyetemsofCarburettorEnginesThecombustiblemixtureisprepared

14、inthecarburetor,andtheprocessitselfiscalledcarburetion.Present-daycarburetorsatomizefuelbypulverization,andareoftencalledpulverizationcarbuettorsforthisreason.AdiagramofasimplecarburetorisshowninFig.34.Floatchamber1maintainsthefuelattheconstantlevelnecessaryfornormaloperationofthecarburetor.Thefueli

15、sdeliveredintothechamberalongpipeline2byapump,orlessfrequentlybygravity,directlyfromthetank.Thechambercommunicateswiththeatmospherethroughahole.Whenthefuellevelsinks,float4goesdown,opensneedlevalve3andadmitsfuelintothechamber.Whenthefuellevelreachesitsnormalvalue,thefloatgoesupandstopstheinflowoffue

16、l.Thenormallevelisreachedwhenthefuelinthechamberis4-2mmelowtheedgeofpulverizer8Thislevelensureseasysuctionoffuelfromthepulverizerandpreventsleakagewhenthecarburettorisinoperative.Jettube9,withacalibratedholeofdefinitediameter,metersouttheamountoffueltobesupplied.Thepulverizer,whichtakestheformofathi

17、ntube,communi-cateswiththefloatchamberthroughthejet.Mixingchamber6isastraightorbenttube,oneofwhoseendsisconnectedtotheengineintakepipeandtheothertotheaircleaner.Thefuelismixedwithairpreciselyinthischamber.Fig.34.carburetorwithDiffuserDiagramofelementaryupcomingflow7,mountedinthemixingchamberantheend

18、ofthepulverizer,incerasesthevelocityoftheairstreaminthemixingchamberandtherebyprovidesamoreintensiveatomizationoffuel.Throttle5changesthecrosssectionpresentedtothecombustiblemixturetowhichthethrottleisopenedtothecombustiblemixture.Thethrottleiscontrolledbythedriverfromthecab.Thedegreetowhichthethrot

19、tleisopeneddeterminestheamountofmixturepassedandaccordinglychangesthepoweroftheengine.Asthecrankshaftrotates,therarefactionbuiltupduringadmis-sionstrokescausesatmosphericairtoflowthroughthecarburettor.Thevelocityofairincreasesconsiderablyinthediffusorandastrongrarefactionappearsabovethepulverizer.Du

20、etothedifferenceinpressure,fuelissprayedoutofthepulverizerandmixesandevapo-ratesomthehigh-velocityairflow.Thecombustiblemixturepre+paredinthismannerisdrawnintothecylindersduringadmissionstrokes,theamountbeingadjust-edbythepositionofthethrottle.Asimplecarburettorcanpro-videarequiredcompositionoffaela

21、ndairmixtureonlyatsomeoneduty,sinceifthenumberofenginerevolutionsandtheloadareincreased,themixturesup-pliedbythecarburettorwillbecomericherastherarefactionatthediffusorwillalsoincrease.Carburettorsarecxtremelycomplicatedindesign.There-quiredfuel/airratioatvariousdutiredisensuredbyspecialautomaticmet

22、eringdevices.Fig.35.CombinationaircleanerAsthefuel/aivratioisregulat-edantomatically,carburettorcontrolboilsdowntoadjustingtheamountofthesuppliedmix-turebymeansofthethrottle.Ordinarily,thethrottlecambeoperatedbothbyhandandbyafottpedal.Thelinkagebe-tweenthetwosystemsissode-signedthatthefootpedalopera

23、tesindependentlyofthehandcontrol.Fuelisstoredinafueltankdesignedtoaccommodaleenoughfuelforadayswork.Thefuellevelinthetankisreadoffanelectricindicatorwhosepick-uptogetherwithfloatismountedontheupperwallofthetank.Thefuelpumpdeliversfuelfromthetankintothecarburettorfloatchamber.Mostpopulararediaphragm-

24、typepumps,whichvaryonlyslightlyindesign.Fuelfiltersandsedimentbowlscleanfuelfrommechanicalimpu-ritiesandwater,measuresnecessarytopreventthecarburettorductsandjetsfromcloggingup.Aircleanersconsiderablyincreasetheservicelifeofengines;Theycatchairdustbeforeitcntersthecylinders.Testdatashowthat,intheabs

25、enceofaircleaners,cylinderwearIncreasesnindfoldandthatofpistonrings-tenfold.Wearbecomesmuchworsewhentheenginehastooperateatanearthworkoratconstructionsites,wherethereisanabundanceofabrasiveparticlesintheair.Combinationinertiaandoilbathaircleanershavebecomeespe-ciallywidespread(Fig.35).Alargevarietyo

26、fcombiantionaircleanerscanbefoundonpres-ent-dayengines.Whentakenpropercareof,theycatchabout95to97percentofdust.Whentheengineoperates,theairsuckedinbythecleanerisdrawnunderneathitscap1,goesdownandthen,abruptlychangingitsdirection,strikesagainsttheoilinreservoir4,Theheaviestdustparticlesareejectedbyin

27、ertiaandsticktotheoilafterwhich,purifiedfinallyfromdustandoil,itflowstothecarburettorviapipe5.SpecificFeaturesofDieselEngineFuelSystemsIndieselenginesthecombustiblemixtureisprepareddirectlyintheenginecylinders,afeaturegivingrisetocertaindifficultiesinthewayofmixtureformation.Themixtureformationproce

28、ssdependsgreatlyontheshapeofthecombustionchamber,whichmustdirecttheairsoastomixitwithfuelrapidlyanduniformly.However,whatevertheshapeofthecombustionchamber,theprocessrequiresafinepulverizationoffuel,whichcanbeattainedonlywhentheinjectionpressureissuf-ficientlyhigh(120-140kg/cm2).Thefinerthefuelispul

29、verized,thebetteritwillmixwithairandvaporize,themoreeffectivewillbethecombustionprocessandthemoreefficienttheoperationofthediesel.Thefuelfeedsystemofdieselshasahigh-pressurepumpandnoz-zles.Thepumpandnozzlesaremanufacturedwithahighdegreeofaccuracyandarethereforeverysuseeptibletowear.Toensuretheirprol

30、ongedandreliableoperation,allmechanicalimpuritiesmustbethoroughlyremovedfromthefuel.Thefueliscleanedwiththeaidofcoarse-meshandfine-meshfiltersarrangedinseriesinthefuelflowsystem.Sincefine-meshfiltersofferaconsiderableresistancetotheflow,aspecialprimingpumpisneededforthefueltopassthroughthemtothehigh

31、-pressurepump.Thepressuredevelopedbytheprimingpumpreaches1.5kg/cm2intractorand3.9-4.2kg/cm2intruckdiesels.Ahigh-pressurefuelpumpuniformlysuppliescarefullymeteredoutportionsoffuel,conformingtothegivenloadontheengine,tothecombustionchambersatadefinitepressureandatadefinitemoment.Toillustratetheprincip

32、leofoperationanddesignofonesuchpump,letusconsiderthefuelpumpusedindieselenginetypeKIM-100(Fig,36).Thepumphasfoursections,thesameasthenumberofcylinders.Fuelflowsfromthefilteralonglow-pressurechammel11.Thesectioilbodycontainssleeve5withintakeport12atthetop,throughwhiclithefuelisadmittedanddischarged(i

33、ncuttingoff).Nonreturnvalve7ispressedbyaspringagainstseat6.Thesleeveaccommodatesplunger4(itsheadisshownseparately).Theplungerismoveddownbyspring1,whichrestsagainsttheshoulderoffollower2,andup(deliverystroke)bycamshaft10andthefollower.Theamountoffuelsuppliedtothecylindercanbechangedbyturningtheplunge

34、rinthesleeve.Thefuclwillbedeliveredintothefuellinewhentheendedgeoftherisingplungeroverlapstheintakeportofthesleeve.Deliveryiscompletedwhenthecutting-off(shaped)edgeoftheplungerheadbeginstoopentheintakeportofthesleeveastheplungermovesupward.Atthisinstantthefuelwillbegintoflowfromthespaceabovetheplung

35、erthroughtherecessintheplungerheadintothelow-pressurechannel,thepres-sureabovetheplungerwilldropsharplyandthenonreturnvalvewillclose.Thedeliverystrokeoftheplungerand,hence,theamountoffuelsupplieddependonthepositionthecutting-offedgeassumesrelativetotheintakeportofthesleeve.Thispositioncanbechangedby

36、turningtheplungeraboutitsaxis.Themaximumfeedwillbeobtainedwhenthefullheightoftheshapedplungerheadgoespasttheintakeport.Whentheplungerheaddoesnotoverlaptheportatall(i.e.,whenslotaintheplungerheadandtheintakeportcoincied),thefeedwillbeshutoff.Thelowerendsoftheplungerscarrybronzetoothedsectors3meshedwi

37、thrack&connectedwithpullrod9bymeansofaguide,Thedrivercanturnalltheplungerssimultaneouslyintheirsleevesbyshiftingthepullrodtotherightortotheleftbyhandorwiththeaidofacentrifugalgovernor.Automaticcontrolofenginerevolutionsimprovesthedrivingconditions,increasestheengineefficiencyandreducesfuelconsump-ti

38、on.ASectionAA金龍ss豐矗AFig.36.High-pressurefuelpumpoftypeKM-100dieselengine11Thispurposeisservedbyall-dutygovernorswhichadjusttheenginespeedatvariousduties.Thegovernorisactuatedbytheengineandactsupontherackofthehigh-pressurefuelpump.Fig.37Closed-typenozzle1-atomizer,2-valve,3-infeedchannel,4-body,5-valveliftlimiter,6-spring.7-rodGovernorsemployedinpresent-daytractorenginesareprovieded,asarule,withcorrectiongearswhichenrichthemixtureforshort-timeoverloadsontheengine,afeaturewhi