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外文资料5HowAutomaticTransmissionsWorkIfyouhaveeverdrivenacarwithanautomatictransmission,thenyouknowthattherearetwobigdifferencesbetweenanautomatictransmissionandamanualtransmission:Thereisnoclutchpedalinanautomatictransmissioncar.Thereisnogearshiftinanautomatictransmissioncar.Onceyouputthetransmissionintodrive,everythingelseisautomatic.Boththeautomatictransmission(plusitstorqueconverter)andamanualtransmission(withitsclutch)accomplishexactlythesamething,buttheydoitintotallydifferentways.Itturnsoutthatthewayanautomatictransmissiondoesitisabsolutelyamazing!Inthisarticle,we'llworkourwaythroughanautomatictransmission.We'llstartwiththekeytothewholesystem:planetarygearsets.Thenwe'llseehowthetransmissionisputtogether,learnhowthecontrolsworkanddiscusssomeoftheintricaciesinvolvedincontrollingatransmission.SomeBasicsJustlikethatofamanualtransmission,theautomatictransmission'sprimaryjobistoallowtheenginetooperateinitsnarrowrangeofspeedswhileprovidingawiderangeofoutputspeeds.Withoutatransmission,carswouldbelimitedtoonegearratio,andthatratiowouldhavetobeselectedtoallowthecartotravelatthedesiredtopspeed.Ifyouwantedatopspeedof80mph,thenthegearratiowouldbesimilartothirdgearinmostmanualtransmissioncars.You'veprobablynevertrieddrivingamanualtransmissioncarusingonlythirdgear.Ifyoudid,you'dquicklyfindoutthatyouhadalmostnoaccelerationwhenstartingout,andathighspeeds,theenginewouldbescreamingalongnearthered-line.Acarlikethiswouldwearoutveryquicklyandwouldbenearlyundriveable.Sothetransmissionusesgearstomakemoreeffectiveuseoftheengine'storque,andtokeeptheengineoperatingatanappropriatespeed.Thekeydifferencebetweenamanualandanautomatictransmissionisthatthemanualtransmissionlocksandunlocksdifferentsetsofgearstotheoutputshafttoachievethevariousgearratios,whileinanautomatictransmission,thesamesetofgearsproducesallofthedifferentgearratios.Theplanetarygearsetisthedevicethatmakesthispossibleinanautomatictransmission.Gears外文资料6Thisautomatictransmissionusesasetofgears,calledacompoundplanetarygearset,thatlookslikeasingleplanetarygearsetbutactuallybehavesliketwoplanetarygearsetscombined.Ithasoneringgearthatisalwaystheoutputofthetransmission,butithastwosungearsandtwosetsofplanets.Let'slookatsomeoftheparts:FirstGearInfirstgear,thesmallersungearisdrivenclockwisebytheturbineinthetorqueconverter.Theplanetcarriertriestospincounterclockwise,butisheldstillbytheone-wayclutch(whichonlyallowsrotationintheclockwisedirection)andtheringgearturnstheoutput.Thesmallgearhas30teethandtheringgearhas72,sothegearratiois:Ratio=-R/S=-72/30=-2.4:1Sotherotationisnegative2.4:1,whichmeansthattheoutputdirectionwouldbeoppositetheinputdirection.Buttheoutputdirectionisreallythesameastheinputdirection--thisiswherethetrickwiththetwosetsofplanetscomesin.Thefirstsetofplanetsengagesthesecondset,andthesecondsetturnstheringgear;thiscombinationreversesthedirection.Youcanseethatthiswouldalsocausethebiggersungeartospin;butbecausethatclutchisreleased,thebiggersungearisfreetospinintheoppositedirectionoftheturbineSecondGearThistransmissiondoessomethingreallyneatinordertogettherationeededforsecondgear.Itactsliketwoplanetarygearsetsconnectedtoeachotherwithacommonplanetcarrier.Thefirststageoftheplanetcarrieractuallyusesthelargersungearastheringgear.Sothefirststageconsistsofthesun(thesmallersungear),theplanetcarrier,andthering(thelargersungear).Theinputisthesmallsungear;theringgear(largesungear)isheldstationarybytheband,andtheoutputistheplanetcarrier.sotheformulais:1+R/S=1+36/30=2.2:1Theplanetcarrierturns2.2timesforeachrotationofthesmallsungear.Atthesecondstage,theplanetcarrieractsastheinputforthesecondplanetarygearset,thelargersungear(whichisheldstationary)actsasthesun,andtheringgearactsastheoutput,sothegearratiois:1/(1+S/R)=1/(1+36/72)=0.67:1Togettheoverallreductionforsecondgear,wemultiplythefirststagebythesecond,2.2x0.67,togeta1.47:1reduction.Thismaysoundwacky,butitworks.ThirdGear外文资料7Mostautomatictransmissionshavea1:1ratiointhirdgear.You'llrememberfromtheprevioussectionthatallwehavetodotogeta1:1outputislocktogetheranytwoofthethreepartsoftheplanetarygear.Withthearrangementinthisgearsetitiseveneasier--allwehavetodoisengagetheclutchesthatlockeachofthesungearstotheturbine.Ifbothsungearsturninthesamedirection,theplanetgearslockupbecausetheycanonlyspininoppositedirections.Thislockstheringgeartotheplanetsandcauseseverythingtospinasaunit,producinga1:1ratio.OverdriveBydefinition,anoverdrivehasafasteroutputspeedthaninputspeed.It'saspeedincrease.Inthistransmission,engagingtheoverdriveaccomplishestwothingsatonce.IfyoureadHowTorqueConvertersWork,youlearnedaboutlockuptorqueconverters.Inordertoimproveefficiency,somecarshaveamechanismthatlocksupthetorqueconvertersothattheoutputoftheenginegoesstraighttothetransmission.Inthistransmission,whenoverdriveisengaged,ashaftthatisattachedtothehousingofthetorqueconverter(whichisboltedtotheflywheeloftheengine)isconnectedbyclutchtotheplanetcarrier.Thesmallsungearfreewheels,andthelargersungearisheldbytheoverdriveband.Nothingisconnectedtotheturbine;theonlyinputcomesfromth