专业英语ppt课件汽车专业英语ppt课件-与汽车专业英语读译教程配套-宋进桂制作17-U10TA

UNIT10BRAKINGSYSTEMSTEXTABasicBrakingSystemandABS,PROFESSIONALENGLISH,UNIT8BasicBrakingSystemandABSTEXTABasicoperatingprincipleofthehydraulicsystemDiscbrakesDrumbrakesABSComponentsOperatingprincipleTypesofAnti-LockBrakes,本次课学习内容,本次课学习目标,掌握制动系统的有关英文专业词汇掌握制动系统主要部件工作原理训练快速阅读能力，提高阅读速度,NEWWORDS,PHRASESANDEXPRESSIONS,BasicBrakingSystemandABSInordertoreducethespeedofthevehicle,thebrakeshavetoconvertthekineticenergystoredinthevehicletoheatenergy.Abrakingsystemconsistsofanenergy-supplyingdevice,acontroldevice,atransmissiondeviceandthebrake.Today,therearethreetypesofbrakingsystemsinuse:servicebrakingsystem,parkingbrakingsystemandadditionalretarderbrakingsystem.Theservicebrakingsystemandtheparkingbrakingsystemhaveseparatecontrolandtransmissiondevices.Theservicebrakingsystemisnormallyfoot-operated,whiletheparkingbrakingsystemishand-operated.BasicoperatingprincipleofthehydraulicsystemHydraulicsystemsareusedtoactuatethebrakesofallsmallcars.Thesystemtransportsthepowerrequiredtoforcethefrictionalsurfacesofthebrakingsystemtogetherfromthepedaltotheindividualbrakeunitsateachwheel.,Themastercylinderconsistsofafluidreservoirandadoublecylinderandpistonassembly.Doubletypemastercylindersaredesignedtoseparatethefrontandrearbrakingsystemshydraulicallyincaseofaleak.Steellinescarrythebrakefluidtoapointonthevehicleframeneareachofthevehicleswheels.Thefluidisthencarriedtothecalipersandwheelcylindersbyflexibletubesinordertoallowforsuspensionandsteeringmovements.Thehydraulicsystemoperatesasfollows:Whenatrest,theentiresystem,fromthepiston(s)inthemastercylindertothoseinthewheelcylindersorcalipers,isfullofbrakefluid.Uponapplicationofthebrakepedal,fluidtrappedinfrontofthemastercylinderpiston(s)isforcedthroughthelinestothewheelcylinders.Here,itforcesthepistonsoutward,inthecaseofdrumbrakes,andinwardtowardthedisc,inthecaseofdiscbrakes.Themotionofthepistonsisopposedbyreturnspringsmountedoutsidethecylindersindrumbrakes,andbyspringseals,indiscbrakes.,Uponreleaseofthebrakepedal,aspringlocatedinsidethemastercylinderimmediatelyreturnsthemastercylinderpistonstothenormalposition.Thepistonscontaincheckvalvesandthemastercylinderhascompensatingportsdrilledinit.Theseareuncoveredasthepistonsreachtheirnormalposition.Thepistoncheckvalvesallowfluidtoflowtowardthewheelcylindersorcalipersasthepistonswithdraw.Then,asthereturnspringsforcethebrakepadsorshoesintothereleasedposition,theexcessfluidreservoirsthroughthecompensatingports.Itisduringthetimethepedalisinthereleasedpositionthatanyfluidthathasleakedoutofthesystemwillbereplacedthroughthecompensatingports1.Dualcircuitmastercylindersemploytwopistons,locatedonebehindtheother,inthecylinder.Theprimarypistonisactuateddirectlybymechanicallinkagefromthebrakepedalthroughthepowerbooster.Thesecondarypistonisactuatedbyfluidtrappedbetweenthetwopistons.,Ifaleakdevelopsinfrontofthesecondarypiston,itmovesforwarduntilitbottomsagainstthefrontofthemastercylinder,andthefluidtrappedbetweenthepistonswilloperatetherearbrakes.Iftherearbrakesdevelopaleak,theprimarypistonwillmoveforwarduntildirectcontactwiththesecondarypistontakesplace,anditwillforcethesecondarypistontoactuatethefrontbrakes.Ineithercase,thebrakepedalmovesfartherwhenthebrakesareapplied,andlessbrakingpowerisavailable.Discbrakes（自学）Insteadofthetraditionalexpandingbrakesthatpressoutwardagainstacirculardrum,discbrakesystemsutilizeadisc(rotor)withbrakepadspositionedoneithersideofit.Brakingeffectisachievedinamannersimilartothewayyouwouldsqueezeaspinningphonographrecordbetweenyourfingers.Thedisc(rotor)isacastingwithcoolingfinsbetweenthetwobrakingsurfaces.,Thisenablesairtocirculatebetweenthebrakingsurfacesmakingthemlesssensitivetoheatbuildupandmoreresistanttofade.Dirtandwaterdonotaffectbrakingactionsincecontaminantsarethrownoffbythecentrifugalactionoftherotororscrapedoffbythepads.Also,theequalclampingactionofthetwobrakepadstendstoensureuniform,straightlinestops.Discbrakesareinherentlyself-adjusting.Therearethreegeneraltypesofdiscbrake:fixedcaliper,floatingcaliperandslidingcaliper.Thefixedcaliperdesignusestwopistonsmountedoneithersideoftherotor(ineachsideofthecaliper).Thecaliperismountedrigidlyanddoesnotmove.Theslidingandfloatingdesignsarequitesimilar.Infact,thesetwotypesareoftenlumpedtogether.Inbothdesigns,thepadontheinsideoftherotorismovedintocontactwiththerotorbyhydraulicforce.Thecaliper,whichisnotheldinafixedposition,movesslightly,bringingtheoutsidepadintocontactwiththerotor.,Drumbrakes（自学）Drumbrakesemploytwobrakeshoesmountedonastationarybackingplate.Theseshoesarepositionedinsideacirculardrumwhichrotateswiththewheelassembly.Theshoesareheldinplacebysprings.Thisallowsthemtoslidetowardthedrums(whentheyareapplied)whilekeepingtheliningsanddrumsinalignment.Theshoesareactuatedbyawheelcylinderwhichismountedatthetopofthebackingplate.Whenthebrakesareapplied,hydraulicpressureforcesthewheelcylindersactuatinglinksoutward.Sincetheselinksbeardirectlyagainstthetopofthebrakeshoes,thetopsoftheshoesarethenforcedagainsttheinnersideofthedrum.Thisactionforcesthebottomsofthetwoshoestocontactthebrakedrumbyrotatingtheentireassemblyslightly(knownasservoaction).Whenpressurewithinthewheelcylinderisrelaxed,returnspringspulltheshoesbackawayfromthedrum.,ABSStoppingacarinahurryonaslipperyroadcanbeverychallenging.Anti-lockbrakingsystems(ABS)takealotofthechallengeoutofthissometimesnerve-wrackingevent.Infact,onslipperysurfaces,evenprofessionaldriverscantstopasquicklywithoutABSasanaveragedrivercanwithABS.TherearefourmaincomponentstoanABSsystem:speedsensors,pump,valvesandcontroller(Fig.10-1).,Fig.10-1Maincomponentsofanti-lockbrakingsystem,ComponentsSpeedSensorsThespeedsensorsarelocatedateachwheelandprovidethespeedreferencetothecontrolunit.Thespeedsensorsarepermanentlymagnetizedinductivesensorsthatreadpulsesfromatoothwheeloneachhub.Avoltagesignalisgeneratedaseachtoothpassesthroughthemagneticfield.Thesensorsarereplaceable.Thetoothwheelsareintegralwiththewheelhubsandarereplacedwiththecompletehub.ControlUnitThecontrolunitcontainsallthesignalconditioningcircuitryandtheoutputcircuits.Theoutputcircuitscontrolthehydraulicunittoadjustthelinepressuretoeachcaliper.Theunitislocatedunderthedashpanelontheleftside,abovetheglovecompartmentandintheforwardpositionoftheelectronicboxintheenginecompartment.Ifaproblemissensed,thecontrolunitwilllighttheinstrumentpanelwarninglamp.,PumpandValvesThehydraulicunit(Fig.10-2),alsoknownashydraulicactuator,locatedintheenginecompartmentcontainsvalvesandapump.Thevalveshave3positions;pressurebuild-up,pressureholdandpressuredrop.Asthetireslocks,thecontrolunitinformsthevalvetoholdthepressure.Ifthetireremainslocked,thecontrolunitwillallowthevalvetodropthepressureuntilthetirestartstoturn.Thecontrolunitwillallowthevalvetostartbuildingpressuretostartthecycleoveragain.,1.Hydrauliccontrolunit2.Valverelay3.Motorrelay4.Cover5.ABScontrolunitFig.10-2Hydraulicunitoftheanti-lockbrakesystem,Thepumpreturnsthebrakefluidtakenfromthewheelcylinderwhilethepressureislowered.Thepumpisdesignedtomaintainseparationofthe2brakingcircuits.Duringoperationoftheanti-locksystem,apulsingmaybefeltatthebrakepedalandaclickingheardfromthehydraulicunit.ThisisnormalandinformstheoperatorthattheABSisinthefunctioningmode.OperatingprincipleTherearemanydifferentvariationsandcontrolalgorithmsforABSsystems.Wewilldiscusshowoneofthesimplersystemsworks.Thecontrollermonitorsthespeedsensorsatalltimes.Itislookingfordecelerationsinthewheelthatareoutoftheordinary.Rightbeforeawheellocksup,itwillexperiencearapiddeceleration.Ifleftunchecked,thewheelwouldstopmuchmorequicklythananycarcould.Itmighttakeacarfivesecondstostopfrom60mph(96.6kph)underidealconditions,butawheelthatlocksupcouldstopspinninginlessthanasecond.,TheABScontrollerknowsthatsucharapiddecelerationisimpossible,soitreducesthepressuretothatbrakeuntilitseesaacceleration,thenitincreasesthepressureuntilitseesthedecelerationagain.Itcandothisveryquickly,beforethetirecanactuallysignificantlychangespeed.Theresultisthatthetireslowsdownatthesamerateasthecar,withthebrakeskeepingthetiresverynearthepointatwhichtheywillstarttolockup2.Thisgivesthesystemmaximumbrakingpower.WhentheABSsystemisinoperationyouwillfeelapulsinginthebrakepedal;thiscomesfromtherapidopeningandclosingofthevalves.SomeABSsystemscancycleupto15timespersecond.TypesofAnti-LockBrakesAnti-lockbrakingsystemsusedifferentschemesdependingonthetypeofbrakesinuse.Wewillrefertothembythenumberofchannels-thatis,howmanyvalvesthatareindividuallycontrolled-andthenumberofspeedsensors.,Four-channel,four-sensorABS-Thisisthebestscheme.Thereisaspeedsensoronallfourwheelsandaseparatevalveforallfourwheels.Withthissetup,thecontrollermonitorseachwheelindividuallytomakesureitisachievingmaximumbrakingforce.Three-channel,three-sensorABS-Thisscheme,commonlyfoundonpickuptruckswithfour-wheelABS,hasaspeedsensorandavalveforeachofthefrontwheels,withonevalveandonesensorforbothrearwheels.Thespeedsensorfortherearwheelsislocatedintherearaxle.Thissystemprovidesindividualcontrolofthefrontwheels,sotheycanboth