毕业设计论文 外文文献翻译 汽车专业 汽修 点火系统 中英文对照.docx
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毕业设计论文 外文文献翻译 汽车专业 汽修 点火系统 中英文对照.docx
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毕业设计论文外文文献翻译汽车专业汽修点火系统中英文对照
IgnitionSystem
Thepurposeoftheignitionsystemistocreateasparkthatwillignitethefuel-airmixtureinthecylinderofanengine.Itmustdothisatexactlytherightinstantanddoitattherateofuptoseveralthousandtimesperminuteforeachcylinderintheengine.Ifthetimingofthatsparkisoffbyasmallfractionofasecond,theenginewillrunpoorlyornotrunatall.
Theignitionsystemsendsanextremelyhighvoltagetothesparkplugineachcylinderwhenthepistonisatthetopofitscompressionstroke.Thetipofeachsparkplugcontainsagapthatthevoltagemustjumpacrossinordertoreachground.Thatiswherethesparkoccurs.
Thevoltagethatisavailabletothesparkplugissomewherebetween20,000voltsand50,000voltsorbetter.Thejoboftheignitionsystemistoproducethathighvoltagefroma12voltsourceandgetittoeachcylinderinaspecificorder,atexactlytherighttime.
Theignitionsystemhastwotaskstoperform.First,itmustcreateavoltagehighenough(20,000+)toacrossthegapofasparkplug,thuscreatingasparkstrongenoughtoignitetheair/fuelmixtureforcombustion.Second,itmustcontrolthetimingofthatthesparksoitoccursattheexactrighttimeandsendittothecorrectcylinder.
Theignitionsystemisdividedintotwosections,theprimarycircuitandthesecondarycircuit.Thelowvoltageprimarycircuitoperatesatbatteryvoltage(12to14.5volts)andisresponsibleforgeneratingthesignaltofirethesparkplugattheexactrighttimeandsendingthatsignaltotheignitioncoil.Theignitioncoilisthecomponentthatconvertsthe12voltsignalintothehigh20,000+voltcharge.Oncethevoltageissteppedup,itgoestothesecondarycircuitwhichthendirectsthechargetothecorrectsparkplugattherighttime.
TheBasics
Beforewebeginthisdiscussion,let’stalkabitaboutelectricityingeneral.Iknowthatthisisbasicstuff,buttherewasatimethatyoudidn’tknowaboutthisandtherearepeoplewhoneedtoknowthebasicssothattheycouldmakesenseofwhatfollows.
AllautomobilesworkonDC(DirectCurrent).Thismeansthatcurrentmoveinonedirection,formthepositivebatteryterminaltothenegativebatteryterminal.Inthecaseoftheautomobile,thenegativebatteryterminalisconnectedbyaheavycabledirectlytothebodyandtheengineblockofthevehicle.Thebodyandanymetalcomponentincontactwithitiscalledtheground.Thismeansthatacircuitthatneedstosendcurrentbacktothenegativesideofthebatterycanbeconnectedtoanypartofthevehicle’smetalbodyorthemetalengineblock.
Agoodexampletoseehowthisworksistheheadlightcircuit.Theheadlightcircuitconsistsofawirethatgoesfromthepositivebatteryterminaltotheheadlightswitch.Anotherwiregoesfromtheheadlightswitchtooneoftwoterminalsontheheadlightbulb.Finally,athirdwiregoesfromasecondterminalonthebulbtothemetalbodyofcar.Whenyouswitchtheheadlighton,youareconnectingthewirefromthebatterywiththewiretotheheadlampsallowingbatterycurrenttogodirectlytotheheadlampbulbs.Electricitypassesthroughthefilamentsinsidethebulb,thenouttheotherwiretothemetalbody.Fromthere,thecurrentgoesbacktothenegativeterminalofthebatterycompletingthecircuit.Oncethecurrentisflowingthroughthiscircuit,thefilamentinsidetheheadlampgetshotandglowsbrightly.Lettherebelight.
Now,backtotheignitionsystem,thebasicprincipleoftheelectricalsparkignitionsystemhasnotchangedforover75years.Whathaschangedisthemethodbywhichthesparkiscreatedandhowitisdistribute.
Currently,therearethreedistincttypesofignitionsystem.Themechanicalignitionsystemwasusedpriorto1975.Itwasmechanicalandelectricalandusednoelectronics.Byunderstandingtheseearlysystem,itwillbeeasiertounderstandthenewelectronicandcomputercontrolledignitionsystem,sodon’tskipoverit.Theelectronicignitionsystemstartedfindingitswaytoproductionvehiclesduringtheearly70sandbecamepopularwhenbettercontrolandimprovedreliabilitybecameimportantwiththeadventofemissioncontrols.Finally,thedistributorlessignitionsystembecameavailableinthemid80s.Thissystemwasalwayscomputercontrolledandcontainednomovingparts,soreliabilitywasgreatlyimproved.Mostofthesesystemsrequirednomaintenanceexceptreplacingthesparkplugsatintervalsfrom60,000toover100,000miles.
Let’stakeadetailedlookateachsystemandseehowtheywork.
TheMechanicalIgnitionSystem
Thedistributoristhenervecenterofthemechanicalignitionsystemandhastwotaskstoperform.First,itisresponsiblefortriggeringcoiltogenerateasparkatthepreciseinstantthatitisrequired(whichvariesdependinghowfasttheengineisturningandhowmuchloaditisunder).Second,thedistributorisresponsiblefordirectingthatsparktothepropercylinder(whichiswhyitiscalledadistributor).
Thecircuitthatpowerstheignitionsystemissimpleandstraightforward.WhenyouinsertthekeyintheignitionswitchandturnthekeytotheRunposition,youaresendingcurrentfromthebatterythroughawiredirectlytothepositive(+)sideoftheignitioncoil.Insidethecoilisaseriesofcopperwindingsthatlooparoundthecoiloverahundredtimesbeforeexitingoutthenegative(-)sideofthecoil.Fromthere,awiretakesthiscurrentovertothedistributorandisconnectedtoaspecialon/offswitch,calledthepoints.Whenthepointsareclosed,thiscurrentgoesdirectlytoground.Whencurrentflowsfromtheignitionswitch,throughthewindingsinthecoil,thentoground,itbuildsastrongmagneticfieldinsidethecoil.
Thepointsaremadeupofafixedcontactpointthatisfastenedtoaplateinsidethedistributor,andamovablecontactpointmountedontheendofaspringloadedarm.Themovablepointridesona4,6,or8lobecam(dependingonthenumberofcylinderintheengine)thatismountedonarotatingshaftinsidethedistributor.Thisdistributorcamrotatesintimewiththeengine,makingonecompleterevolutionforeverytworevolutionsoftheengine.Asitrotates,thecampushesthepointsopenandclosed.Everytimethepointsopen,theflowofcurrentisinterruptedthroughthecoil,therebycollapsingthemagneticfieldandreleasingahighvoltagesurgethroughthesecondarycoilwindings.Thisvoltagesurgegoesoutthetopofthecoilandthroughthehigh-tensioncoilwire.
Now,wehavethevoltagenecessarytofirethesparkplug,butwestillhavetogetittothecorrectcylinder.Thecoilwiregoesfromthecoildirectlytothedistributorcap.Underthecapisarotorthatismountedontopoftherotatingshaft.Therotorhasametalstriponthetopthatisinconstantcontactwiththecenterterminalofthedistributorcap.Itreceivesthehighvoltagesurgefromthecoilwireandsendsittotheotherendoftherotorwhichrotatespasteachsparkplugterminalinsidethecap.Astherotorturnsontheshaft,itsendsthevoltagetothecorrectsparkplugwire,whichinturnsendsittothesparkplug.Thevoltageentersthesparkplugattheterminalatthetopandtravelsdownthecoreuntilitreachesthetip.Itthenjumpsacrossthetipofthesparkplug,creatingasparksuitabletoignitethefuel-airmixtureinsidethatcylinder.
ThedescriptionIjustprovidedisthesimplifiedversion,butshouldbehelpfultovisualizetheprocess,butweleftoutafewthingsthatmakeupthistypeofignitionsystem.Forinstance,wedidn’ttalkaboutthecondenserthatisconnectedtothepoint,nordidwetalkaboutthesystemtoadvancethetiming.Let’stakealookateachsectionandexploreitinmoredetail.
TheIgnitionSwitch
Therearetwoseparatecircuitsthatgofromtheignitionswitchtothecoil.Onecircuitrunsthrougharesistorinordertostepdownthevoltageabout15%inordertoprotectthepointsfromprematurewear.Theothercircuitsendsfullbatteryvoltagetothecoil.Theonlytimethiscircuitisusedisduringcranking.Sincethestarterdrawsaconsiderableamountofcurrenttocranktheengine,additionalvoltageisneededtopowerthecoil.Sowhenthekeyisturnedtothespring-loadedstartposition,fullbatteryvoltageisused.Assoonastheengineisrunning,thedriverreleasesthekeytotherunpositionwhichdirectscurrentthroughtheprimaryresistortothecoil.
Onsomevehicles,theprimaryresistorismountedonthefirewallandiseasytoreplaceifitfails.Onothervehicles,mostnotablyvehiclesmanufacturedbyGM,theprimaryresisterisaspecialresisterwireandisbundledinthewiringharnesswithotherwires,makingitmoredifficulttoreplace,butalsomoredurable.
TheDistributor
Whenyouremovethedistributorcapfromthetopofthedistributor,youwillseethepointsandcondenser.Thecondenserisasimplecapacitorthatcanstoreasmallamountofcurrent.Whenthepointsbegintoopenthecurrent,flowingthroughthepointslooksforanalternativepathtoground.Ifthecondenserwerenotthere,itwouldtrytojumpacrossthegapofthepointastheybegintoopen.Ifthiswereallowedtohappen,thepointswouldquicklyburnupandyouwouldhearheavystaticonthecarradio.Topreventthis,thecondenseractslikeapathtoground.Itreallyisnot,butbythetimethecondenserissaturated,thepointsaretoofarapartforthesmallamountofvoltagetojumpacrossthewidepointgap.Sincethearcingacrosstheopeningpointsiseliminated,thepointslastlongerandthereisnostaticontheradiofrompointarcing.
Thepointsrequireperiodicadjustmentsinordertokeept
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