汽车制动Word文档格式.docx

汽车制动Word文档格式.docx

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CarBrakingSystems

Introduction

Aconventionalpassengercarbrakingsystemconsistsofapedaloperatedmastercylinderthatsuppliesfluidunderpressuretofrictionbrakesateachwheel.Toassistthedriver,itiscustomarytoincludeavacuumservothatprovidesahelpingforceinproportiontotheinput.Forsafetyreasons,themastercylinderisoftandemdesignincorporatingtwinpressurechambers.Eachchamberconnectstoatleasttwoofthevehiclebrakestopreventtotallossofbrakesshouldonecircuitfail.

Thefrictionbrakesfallintotwomaincategoriesthataredeterminedbythetypeofrotortowhichthebrakingforceistobeapplied.Thefirsttypeisarotatingdiscthathasfrictionpadsclampedatbothsidesandthesecondtakestheformofarotatingdrum.Inthiscase,frictionliningsareforcedagainsttheinnerdiameterofthedrum.

Mostmoderncarshavediscbrakesfittedatthefrontwheelsbuttherearwheelshaveeitherdiscordrum.Therearbrakesalmostuniversallyincludemanualparking.

Figure1:

Layoutoftypicalbrakingsystem

Thefigure1showsthelayoutofabasiccarbrakingsystem.Themajorfunctionalpartsarethepedal,booster（vacuumservo）,mastercylinder,brakehosesandthediscanddrumbrakesatthewheels.Theparkingbrakeismanuallyoperatedusingacablesystem.

Tandembrakingsystems

Tandembrakingsystemsaretailoredtosuittheweightdistributiononthetwoaxles.ForRearWheelDrive（RWD）carsthesystemissplitfronttorearandforFrontWheelDrive（FWD）carsthesystemdiagonallyconnectsafrontandarearbrake.BothsystemsfacilitatethefitmentofAnti-lockBrakingSystems（ABS）

Thefront/rearsplit

Figure2a:

Thefront/rearsplit

Thissystem,（seeFigure2a）,isthesimplestofallthetandembrakingsystemswitheachaxleconnectedtoaseparatecircuit.

ItisineverydayuseforlargerRWDcarswheretherearaxlecarriessufficientweighttoprovideasatisfactoryvehicledecelerationshouldthefrontcircuitfail.Iftherearsfail,thefrontscanbeapplied.

Thediagonalsplit

.

Figure2b:

Thediagonalsplit

ThesystemshowninFigure2bconnectseachfrontbraketoitsdiagonallyoppositerearbrake.ItisalmostuniversallyusedforFWDearswheretheheavierweightonthefrontwheelsmustbeutilisedineachhydrauliccircuit.Thisistoensureadequateretardationforthepartcircuitfailedcase.Forthiscase,however,thebrakingforceisappliedtoonlyoneofthefrontwheelsandthisgeneratesanadverseforceatthesteeringwheelunlessthesteeringoffsetisreducedtoaminimum

Tandemmastercylinder

Amastercylinderconvertsthedriver’sappliedforceintoahydraulicpressuretoapplythevehicle’sbrakes.Inaddition,itautomaticallyadjustsfortheincreasingvolumeatthebrakestocaterforpadwear.Inthiswaythedriver’spedaltravelremainssubstantiallythesameirrespectiveofhowmuchthebrakepadshaveworn.

Theconventionaltandemmastercylinderconsistsofapairofpistonsspacedapartinasingleboretoformtwochambers.Eachchamberisventedtoafluidreservoiratatmosphericpressureandconnectstoonepartofadualcircuitbrakingsystem.Theprimarypistonoperatesfromthebrakepedalinputandthesecondarypistonrespondstopressuregeneratedintheprimarychamberbymovementoftheprimarypiston.Theinitialmovementofthepistonsisolatesthepressurechambersfromthereservoir.Thematerialforthereservoirissemi-transparenttoascertainthefluidlevelandithousesafloatoperatedswitchtogiveafluidlevelwarning.

Mostmasterdesignsaresimilarbutthemethodofisolatingthereservoirfromthepressurechambersandthewayinwhichfluidisadmitted,whilst“bleeding”thebrakes,varies.

AmericanStandard（AS）

Figure3:

AS/ASTandemmastercylinder

Figure3illustratesatandemmastercylinderhavingpistonsequippedwithASseals.Thesealsnotonlyprovidesealingandrecuperationbutalsoacttoisolatethereservoirfromthepressurechamber.

Actuationofthebrakepedalmovestheprimarypistonsothatitsrecuperationsealpassesoverasmallventilationhole.Onceisolatedfromthereservoir,pressurebuildsupintheprimarychamberandthesecondarypistonmovestocloseitsownventilationhole.Thereafter,thetwopistonsmovefurtherdowntheboredisplacingfluidunderpressuretothebrakes.Thesecondarypistontravelequatestothevolumeofthebrakingcircuittowhichitconnectsandtheprimarypistontravelequatestothevolumeofthewholebrakingsystem.Typicalpistontravelsforadiagonalsplitbrakingsystemare16mmfortheprimaryand8mmforthesecondary.

Toremoveaeratedfluidfromabrake,opentherelevantbrakebleedscrewandactuatethemastercylinder.Thenclosethebleedscrewandreleasetheappliedforcetoallowthemastercylinderplungerstoreturn.ThisallowsfreshfluidfromthereservoirtopassovertheASseals.Repeatuntiltheairisremovedfromthefluid.

CentreValve（CV）

Figure4showsatypicalCVmastercylinderdesign.Inthiscase,thepressuresealisconventionalandthecentralvalveprovidesthemeanstoisolatethepressurechamberfromthereservoir.Inadditionthecentralvalveperformsthefunctionofallowingfluidrecuperationwhilstbleedingthebrakes.

Figure4:

CVTandemmastercylinder

Actuationofthebrakepedalmovestheprimarypistonintoitsboreanditsinitialtravelclosesthecentralvalve.Furthermovementpressurisestheprimarychamberandthesecondarypistonmovestopressurisethesecondarychamber.

Tobleedthebrakes,themastercylinderisactuatedasbeforewiththebleedscrewatthebrakeopen.Onreleaseofthepedalforce,thepistonsreturnandfluidissuckedintothetwochambersthroughthecentralvalves.

AlthoughmorecomplexthantheASmastercylinder,theCVdesignismoreamenableforusewithABS.Insomecases,fluidfromtheABSpumpisforcedbackintothemastercylinder（withbrakesapplied）tofullyreturnthepistons.Inthesecircumstances,therecuperatingsealsmaypassundertheventilationholeswhichresultsindamagetotheseals.ThesameconditionsdonotaffectthesealsoftheCVpistonandthecentralvalvesaredesignedtoopenathighpressureshouldconditionsrequireit.

4.Vacuumservo

Theobjectiveofavacuumservoistoprovideassistancetothedriver’spedalforceinproportiontotheappliedeffort.ThemediumofthevacuumischosensinceitisreadilyavailablefromanICengine’sinletmanifold,particularlywiththethrottleclosed.Thishappenswhenthedrivertransfersfromacceleratortobrakepedal.Anotheradvantageofusingvacuumisthatthelackofairmeansthatthemediumisdryandtheoperatingvalvemechanismdoesnotfreeze.

Figure5:

Vacuumservo

Adisadvantageofusingvacuumisthattheworkingpressureislow（typically0.7bar）andtheassistingpistonmustbesizedaccordingly.VacuumServosizesarehistoricallyquotedininches,i.e.7”,8”and9”orintheirequivalentarea,i.e.Type38,Type50andType64.

Atypicalvacuumservoconsistsofalargepistonhousedwithintwothinmetalshellstoformtwochambers.Thelargechamberconnectstothevacuumsupplyandthesmallchamber,normallyatvacuumpressure,ispressurisedwithatmospherebyoperatingthecentralvalvemechanism.Thevalveassemblyconsistsofaninletandanoutletvalvetogetherwithareactionmechanism.Thereactiondevice,usuallyarubberdisc,abutsboththeinputandoutputrods.Inoperationthetworodsloadthediscandproduceareactiveforceinproportiontotheinputrodarea.AtypicalratiobetweenA2andA1is4:

1.

Whenthedriverpushesonthebrakepedaltoloadtheinputrod,seeFigure5,theoutletvalveclosesandtheinletvalveopenstoadmitairintotheservochamber.Asaresult,apressuredifferentialiscreatedacrossthepistonanditmovestopushtheoutputrodagainstthemastercylinder.TheoutputforceactingovertheareaA2pressurisestherubberdisc.ThispressureactsupontheinputareaA1toprovideareactiontothedriver’sinput.IfA2isfourtimestheareaofA1theboostratiois4:

Whentheinputreactionisinthecorrectrelationshiptotheappliedforce,theinletvalvecloses.Adecreaseininputforceopenstheoutletvalveandthepressuredifferentialacrossthepistonreduces.Shouldthevacuumfail,thebrakesareappliedbydirectlyconveyingthedriver’sforcethroughtheinputandoutputrodstothemastercylinder.

Moderndevelopmentsofthisbasicdesignincludetie-boltsthroughtheservotoconnectthemastercylinderdirectlytothebulkhead.Thisfeaturereducesweightanddeflection.Anotherimprovementincludesadevicethatdeterminesthestopfortheinputrodsothataslightpressuredifferentialdevelopsacrossthepistontobalancetheforceofthereturnspring.Asaresult,theservopistontakesupapositionwithbothvalvesclosedandkeepspedaltraveltoaminimum.

BrakePedalTravelRatio

Figure6:

PedalBrakeTravel

TheamplificationoftheforceappliedbythedriverduetothebrakepedalcanbeobtainedfromFigure6asshownbelow:

PedalRatio=Rp=L2/L1

where:

L2=lengthfrompivottocenterofpedalpad

L1=lengthfrompivottopushrodattachment

PedalTravel=Lp=Lb•Rp

Lb=boosterinputtravel

BoosterInputForce=Fi=Fp•Rp

Fp=pedalforce（appliedbydriver）

BasicEquationforbraking

Figure7:

Forcesactingonvehicleinbraking

Thebasicequationforbrakingissimilartothatforaccelerationexceptthathere,alltheexternalforcesopposemotionasshowninFigure7.