汽車專業(yè)英語讀譯教程 第3版 教案 UNIT 7 EMISSION CONTROL SYSTEMS

授課題目UNIT10

BasicBrakingSystemandABS

TEXTA

Basicoperatingprincipleofthehydraulicsystem授課類型理論課首次授課時間學時2學時教學目標1、基本制動系統(tǒng)組成；2、ABS系統(tǒng)組成原理；3、各種輔助制動系統(tǒng)。重點與難點重點：基本制動系統(tǒng)組成的英文難點：ABS系統(tǒng)組成原理。教學手段與方法多媒體教學教學過程：（包括授課思路、過程設計、講解要點及各部分具體內容、時間分配等）1.授課思路在學習了前置課程之后，通過專業(yè)英語的學習，進行與汽車相關技術結構原理的中文英文對接，培養(yǎng)和提高英文專業(yè)資料（信息）的閱讀能力，從而為與專業(yè)英語密切相關的后續(xù)課程奠定了必要的基礎。2.過程設計：（1）課程內容簡介（5分鐘）（2）導入新課（15分鐘）。（3）新課內容（75分鐘）教師根據(jù)學生學習的情況，對課程內容的重點和難點進行講解。（4）課程小結（5分鐘）3.授課內容：Inordertoreducethespeedofthevehicle，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.Itisduringthetimethepedalisinthereleasedpositionthatanyfluidthathasleakedoutofthesystemwillbereplacedthroughthecompensatingports[1].Dualcircuitmastercylindersemploytwopistons，locatedonebehindtheother，inthecylinder.Theprimarypistonisactuateddirectlybymechanicallinkagefromthebrakepedalthroughthepowerbooster.Thesecondarypistonisactuatedbyfluidtrappedbetweenthetwopistons.Ifaleakdevelopsinfrontofthesecondarypiston，itmovesforwarduntilitbottomsagainstthefrontofthemastercylinder，andthefluidtrappedbetweenthepistonswilloperatetherearbrakes.Iftherearbrakesdevelopaleak，theprimarypistonwillmoveforwarduntildirectcontactwiththesecondarypistontakesplace，anditwillforcethesecondarypistontoactuatethefrontbrakes.Ineithercase，thebrakepedalmovesfartherwhenthebrakesareapplied，andlessbrakingpowerisavailable.ABSStoppingacarinahurryonaslipperyroadcanbeverychallenging.Anti-lockbrakingsystems(ABS)takealotofthechallengeoutofthissometimesnerve-wrackingevent.Infact，onslipperysurfaces，evenprofessionaldriverscan'tstopasquicklywithoutABSasanaveragedrivercanwithABS.TherearefourmaincomponentstoanABSsystem:speedsensors，pump，valvesandcontroller(Fig.10-1).ComponentsSpeedSensorsThespeedsensorsarelocatedateachwheelandprovidethespeedreferencetothecontrolunit.Thespeedsensorsarepermanentlymagnetizedinductivesensorsthatreadpulsesfromatoothwheeloneachhub.Avoltagesignalisgeneratedaseachtoothpassesthroughthemagneticfield.Thesensorsarereplaceable.Thetoothwheelsareintegralwiththewheelhubsandarereplacedwiththecompletehub.ControlUnitThecontrolunitcontainsallthesignalconditioningcircuitryandtheoutputcircuits.Theoutputcircuitscontrolthehydraulicunittoadjustthelinepressuretoeachcaliper.Theunitislocatedunderthedashpanelontheleftside，abovetheglovecompartmentandintheforwardpositionoftheelectronicboxintheenginecompartment.Ifaproblemissensed，thecontrolunitwilllighttheinstrumentpanelwarninglamp.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，withthebrakeskeepingthetiresverynearthepointatwhichtheywillstarttolockup[2].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，sotheycanbothachievemaximumbrakingforce.Therearwheels，however，aremonitoredtogether;theybothhavetostarttolockupbeforetheABSwill