附錄AAutomobileBrakeSystemThebrakingsystemisthemostimportantsystemincars.Ifthebrakesfail,theresultcanbedisastrous.Brakesareactuallyenergyconversiondevices,whichconvertthekineticenergy(momentum)ofthevehicleintothermalenergy(heat).Whensteppingonthebrakes,thedrivercommandsastoppingforcetentimesaspowerfulastheforcethatputsthecarinmotion.Thebrakingsystemcanexertthousandsofpoundsofpressureoneachofthefourbrakes.Twocompleteindependentbrakingsystemsareusedonthecar.Theyaretheservicebrakeandtheparkingbrake.Theservicebrakeactstoslow,stop,orholdthevehicleduringnormaldriving.Theyarefoot-operatedbythedriverdepressingandreleasingthebrakepedal.Theprimarypurposeofthebrakeistoholdthevehiclestationarywhileitisunattended.Theparkingbrakeismechanicallyoperatedbywhenaseparateparkingbrakefootpedalorhandleverisset.Thebrakesystemiscomposedofthefollowingbasiccomponents:the“mastercylinder”whichislocatedunderthehood,andisdirectlyconnectedtothebrakepedal,convertsdriverfootsmechanicalpressureintohydraulicpressure.Steel“brakelines”andflexible“brakehoses”connectthemastercylindertothe“slavecylinders”locatedateachwheel.Brakefluid,speciallydesignedtoworkinextremeconditions,fillsthesystem.“Shoes”and“pads”arepushedbytheslavecylinderstocontactthe“drums”and“rotors”thuscausingdrag,which(hopefully)slowsthecar.Thetypicalbrakesystemconsistsofdiskbrakesinfrontandeitherdiskordrumbrakesintherearconnectedbyasystemoftubesandhosesthatlinkthebrakeateachwheeltothemastercylinder(Figure).Basically,allcarbrakesarefrictionbrakes.Whenthedriverappliesthebrake,thecontroldeviceforcesbrakeshoes,orpads,againsttherotatingbrakedrumordisksatwheel.Frictionbetweentheshoesorpadsandthedrumsordisksthenslowsorstopsthewheelsothatthecarisbraked.Inmostmodernbrakesystems(seeFigure15.1),thereisafluid-filledcylinder,calledmastercylinder,whichcontainstwoseparatesections,thereisapistonineachsectionandbothpistonsareconnectedtoabrakepedalinthedriverscompartment.Whenthebrakeispusheddown,brakefluidissentfromthemastercylindertothewheels.Atthewheels,thefluidpushesshoes,orpads,againstrevolvingdrumsordisks.Thefrictionbetweenthestationaryshoes,orpads,andtherevolvingdrumsordisksslowsandstopsthem.Thisslowsorstopstherevolvingwheels,which,inturn,sloworstopthecar.Thebrakefluidreservoirisontopofthemastercylinder.Mostcarstodayhaveatransparentrreservoirsothatyoucanseethelevelwithoutopeningthecover.Thebrakefluidlevelwilldropslightlyasthebrakepadswear.Thisisanormalconditionandnocauseforconcern.Iftheleveldropsnoticeablyoverashortperiodoftimeorgoesdowntoabouttwothirdsfull,haveyourbrakescheckedassoonaspossible.Keepthereservoircoveredexceptfortheamountoftimeyouneedtofillitandneverleaveacamofbrakefluiduncovered.Brakefluidmustmaintainaveryhighboilingpoint.Exposuretoairwillcausethefluidtoabsorbmoisturewhichwilllowerthatboilingpoint.Thebrakefluidtravelsfromthemastercylindertothewheelsthroughaseriesofsteeltubesandreinforcedrubberhoses.Rubberhosesareonlyusedinplacesthatrequireflexibility,suchasatthefrontwheels,whichmoveupanddownaswellassteer.Therestofthesystemusesnon-corrosiveseamlesssteeltubingwithspecialfittingsatallattachmentpoints.Ifasteellinerequiresarepair,thebestprocedureistoreplacethecompeteline.Ifthisisnotpractical,alinecanberepairedusingspecialsplicefittingsthataremadeforbrakesystemrepair.Youmustneverusecoppertubingtorepairabrakesystem.Theyaredangerousandillegal.Drumbrakes,itconsistsofthebrakedrum,anexpander,pullbacksprings,astationarybackplate,twoshoeswithfrictionlinings,andanchorpins.Thestationarybackplateissecuredtotheflangeoftheaxlehousingortothesteeringknuckle.Thebrakedrumismountedonthewheelhub.Thereisaclearancebetweentheinnersurfaceofthedrumandtheshoelining.Toapplybrakes,thedriverpushespedal,theexpanderexpandstheshoesandpressesthemtothedrum.Frictionbetweenthebrakedrumandthefrictionliningsbrakesthewheelsandthevehiclestops.Toreleasebrakes,thedriverreleasethepedal,thepullbackspringretractstheshoesthuspermittingfreerotationofthewheels.Diskbrakes,ithasametaldiskinsteadofadrum.Aflatshoe,ordisk-brakepad,islocatedoneachsideofthedisk.Theshoessqueezetherotatingdisktostopthecar.Fluidfromthemastercylinderforcesthepistonstomovein,towardthedisk.Thisactionpushesthefrictionpadstightlyagainstthedisk.Thefrictionbetweentheshoesanddiskslowsandstopsit.Thisprovidesthebrakingaction.Pistonsaremadeofeitherplasticormetal.Therearethreegeneraltypesofdiskbrakes.Theyarethefloating-calipertype,thefixed-calipertype,andthesliding-calipertype.Floating-caliperandsliding-caliperdiskbrakesuseasinglepiston.Fixed-caliperdiskbrakeshaveeithertwoorfourpistons.Thebrakesystemassembliesareactuatedbymechanical,hydraulicorpneumaticdevices.Themechanicalleverageisusedintheparkingbrakesfittedinallautomobile.Whenthebrakepedalisdepressed,therodpushesthepistonofbrakemastercylinderwhichpressesthefluid.Thefluidflowsthroughthepipelinestothepowerbrakeunitandthentothewheelcylinder.Thefluidpressureexpandsthecylinderpistonsthuspressingtheshoestothedrumordisk.Ifthepedalisreleased,thepistonreturnstotheinitialposition,thepullbackspringsretracttheshoes,thefluidisforcedbacktothemastercylinderandbrakingceases.Theprimarypurposeoftheparkingbrakeistoholdthevehiclestationarywhileitisunattended.Theparkingbrakeismechanicallyoperatedbythedriverwhenaseparateparkingbrakinghandleverisset.Thehandbrakeisnormallyusedwhenthecarhasalreadystopped.Aleverispulledandtherearbrakesareapproachedandlockedinthe“on”position.Thecarmaynowbeleftwithoutfearofitsrollingaway.Whenthedriverwantstomovethecaragain,hemustpressabuttonbeforethelevercanbereleased.Thehandbrakemustalsobeabletostopthecarintheeventofthefootbrakefailing.Forthisreason,itisseparatefromthefootbrakeusescableorrodsinsteadofthehydraulicsystem.Anti-lockbrakesystemsmakebrakingsaferandmoreconvenient,Anti-lockbrakesystemsmodulatebrakesystemhydraulicpressuretopreventthebrakesfromlockingandthetiresfromskiddingonslipperypavementorduringapanicstop.Anti-lockbrakesystemshavebeenusedonaircraftforyears,andsomedomesticcarwereofferedwithanearlyformofanti-lockbrakinginlate1990s.Recently,severalautomakershaveintroducedmoresophisticatedanti-locksystem.InvestigationsinEurope,whereanti-lockbrakingsystemshavebeenavailableforadecade,haveledonemanufacturetostatethatthenumberoftrafficaccidentscouldbereducedbysevenandahalfpercentifallcarshadanti-lockbrakes.Sosomesourcespredictthatallcarswillofferanti-lockbrakestoimprovethesafetyofthecar.Anti-locksystemsmodulatebrakeapplicationforceseveraltimespersecondtoholdthetiresatacontrolledamountofslip；allsystemsaccomplishthisinbasicallythesameway.Oneormorespeedsensorsgeneratealternatingcurrentsignalwhosefrequencyincreaseswiththewheelrotationalspeed.Anelectroniccontrolunitcontinuouslymonitorsthesesignalsandifthefrequencyofasignaldropstoorapidlyindicatingthatawheelisabouttolock,thecontrolunitinstructsamodulatingdevicetoreducehydraulicpressuretothebrakeattheaffectedwheel.Whensensorsignalsindicatethewheelisagainrotatingnormally,thecontrolunitallowsincreasedhydraulicpressuretothebrake.Thisrelease-applycycleoccursseveraltimepersecondto“pump”thebrakeslikeadrivermightbutatamuchfasterrate.Inadditiontotheirbasicoperation,anti-locksystemshavetwootherthingsincommon.First,theydonotoperateuntilthebrakesareappliedwithenoughforcetolockornearlylockawheel.Atallothertimes,thesystemstandsreadytofunctionbutdoesnotinterferewithnormalbraking.Second,iftheanti-locksystemfailinanyway,thebrakescontinuetooperatewithoutanti-lockcapability.Awarninglightontheinstrumentpanelalertsthedriverwhenaproblemexistsintheanti-locksystem.ThecurrentBoschcomponentAnti-lockBrakingSystem(ABS),isasecondgenerationdesignwildlyusedbyEuropeanautomakerssuchasBWM,Mercedes-BenzandPorsche.ABSsystemconsistsof:fourwheelspeedsensor,electroniccontrolunitandmodulatorassembly.Aspeedsensorisfittedateachwheelsendssignalsaboutwheelrotationtocontrolunit.Eachspeedsensorconsistsofasensorunitandagearwheel.Thefrontsensormountstothesteeringknuckleanditsgearwheelispressedontothestubaxlethatrotateswiththewheel.Therearsensormountstherearsuspensionmemberanditsgearwheelispressedontotheaxle.Thesensoritselfisawindingwithamagneticcore.Thecorecreatesamagneticfieldaroundthewinding,andastheteethofthegearwheelmovethroughthisfield,analternatingcurrentisinducedinthewinding.Thecontrolunitmonitorstherateochangeinthisfrequencytodetermineimpendingbrakelockup.Thecontrolunitsfunctioncanbedividedintothreeparts:signalprocessing,logicandsafetycircuitry.Thesignalprocessingsectionistheconverterthatreceivesthealternatingcurrentsignalsformthespeedsensorsandconvertsthemintodigitalformforthelogicsection.Thelogicsectionthenanalyzesthedigitizedsignalstocalculateanybrakepressurechangesneeded.Ifimpendinglockupissensed,thelogicsectionsendscommandstothemodulatorassembly.ModulatorassemblyThehydraulicmodulatorassemblyregulatespressuretothewheelbrakeswhenitreceivescommandsfromthecontrolutuit.Themodulatorassemblycanmaintainorreducepressureoverthelevelitreceivesfromthemastercylinder,italsocanneverapplythebrakesbyitself.Themodulatorassemblyconsistsofthreehigh-speedelectricsolenoidvalves,twofluidreservoirsandaturndeliverypumpequippedwithinletandoutletcheckvalves.Themodulatorelectricalconnectorandcontrollingrelaysareconcealedunderaplasticcoveroftheassembly.Eachfrontwheelisservedbyelectricsolenoidvalvemodulatedindependentlybythecontrolunit.Therearbrakesareservedbyasinglesolenoidvalveandmodulatedtogetherusingtheselect-lowprinciple.Duringanti-brakingsystemoperation,thecontrolunitcyclesthesolenoidvalvestoeitherholdorreleasepressurethebrakelines.Whenpressureisreleasedfromthebrakelinesduringanti-brakingoperation,itisroutedtoafluidreservoir.Thereisonereservoirforthefrontbrakecircuit.Thereservoirsarelow-pressureaccumulatorsthatstorefluidunderslightspringpressureuntilthereturndeliverypumpcanreturnthefluidthroughthebrakelinestothemastercylinder.Whilediscussingthedifferencesamongengineers,scientists,andmathematiciansinChapter1,wesawthatthewordengineeringisrelatedtobothingeniousanddevise.Creativedesignliesatthecenterofthemechanicalengineeringprofession,andanengineersultimategoalistoproducenewhardwarethatsolvesoneofsocietystechnicalproblems.Beginningeitherfromablanksheetofpaperorfromexistinghardwarethatisbeingmodified,theproductdevelopmentprocessoften