汽车构造英文Chapter 6 Pistonconnecting rods and bearingsWord文档下载推荐.docx
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汽车构造英文Chapter 6 Pistonconnecting rods and bearingsWord文档下载推荐.docx
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Pistonpins
Connectingrods
Forcesinaconnecting-rodassembly
Connecting-rodbearings
Technicalterms
Reviewquestions
Pistonsaredesignedtoresisthighcombustiontemperaturesandtheirpistonringshavetosealagainsthighcombustionpressure.Connectingrodshavetowithstandhighstressesastheytransferthereciprocatingmotionofthepistontothecranksofthecrankshaft.
Thepiston,pistonrings,pistonpin,connectingrodandthebearingformthepistonandconnecting-rodassembly.ThevariouspartsareidentifiedinFigure6.1.
Figure6.1Pistonandconnecting-rodassemblyHOLDENLTD
Figure6.2showsapistonwithitspartsidentified.Thisisashort-skirtpistonwithgroovesforthreepistonrings.Thereareanumberofvariationsinpistondesign,whichincludeshape,mass,provisionforexpansionandtypeofmaterial.
Figure6.2Pistoninhalf-sectionwithitspartsidentified
Pistonsaremadeofcastironoraluminiumalloy.Alluminiumalloypistonsareusedinpetrolandsmallerdieselenginebecausetheyaremuchlighterthancastironpistons.However,aluminiumhasagreaterrateofexpansionthanthecastironcylindersinwhichmostpistonsoperate.Becauseofthis,aluminiumalloypistonsarespeciallydesignedtocontrolexpansion.
Somelargerdieselengines,whichoperateatlowerrpmthanpetrolengines,havecastironpistons.
Pistonclearance
A.pistonisslightlysmallerindiameterthanthecylinderinwhichitoperates.Thisgivesitaworkingclearanceofabout0.02mmto0.05mm.Figure6.3illustratestheclearanceofapistoninitsbore,butthereareparticularwaysinwhichitismeasured.
■Seefigure7.7to7.9inthenextchapterwhichshowhowpistonclearanceismeasured.
Figure6.3Clearanceofapistoninacylinder
Ifapistonhasinsufficientclearance,therewillbenoroomforthepistontoexpandwhenitishotanditwilltendtoseizeonthecylinderwall.Thiswouldscoreboththepistonandthecylinder.
Ifthereistoomuchclearance,pistonslapwilloccur,particularlywhentheengineiscold.Pistonslapiscausedbythesuddentiltingofthepistoninthecylinderasitstartsdownitspowerstroke.Thisisdonewithsuchforcethatitproducesadistinctnoise.
■Pistonslapcanoccurinolderenginesthathaveexcessivepistonclearanceduetoworncylindersorwornorcollapsedpistonskirts.
Pistontemperature
Thepistonissubjectedtothefulleffectsofcombustionandsoithastoberesistanttoheat.TypicaloperatingtemperatureareshowninFigure6.4.Underheavyoperatingconditions,
thesetemperaturescanbecomeevenhigher.
Thereisabigdifferencebetweenthetemperatureoftheheadofthepistonandtheskirtofthepiston.Theheatcausesexpansion,andthisisgreatestatthetopofthepiston.Forthisreason,thelandsatthetopofthepistonaregivenextraclearance.Theycouldbeasmuchas0.75mmsmallerindiameterthantheskirt.
Figure6.4Typicaloperatingtemperatureofvariouspartsofapiston
Controlofpistontemperature
Thereareseveralmethodsusedtopreventpistonsfromexpandingexcessively.Theseincludecamgrinding,usingsteelstruts,controllingtheheatpath,andoilcooling.Allpistonshavesomemeansofcontrollingtheirtemperatureandsomepistonshavemorethanone.
Cam-groundpistons
Apistonthatiscam-groundisslightlyovalinshape.Theshapeisobtainedbyrelievingsomemetalintheareaofthepiston-pinbosses(Figure6.5).Thismakesthediameterofthepistonslightlysmalleratthepistonpinbossesthanatthethrustfaces.Thisallowsthepistontobefittedtothecy1inderwithminimumclearanceatthethrustsides.Theextraclearanceprovidedatthebossesbycamgrindingallowsforpistonexpansion.
■Foranexplanationofpistonthrust,seethelatersection‘Pistonthrusts’.
Figure6.5Cam-groundpiston
A-largestdiameter,C-smallestdiameter,B-extrareliefatlikelyscuffingareasREPCO
Cam-groundpistonsgettheirnamefromtheprocessbywhichpistonswerefinishedtosize.Themachineusesacamtomovethepistonbackwardsandforwardsasitisbeingground.Whencam-groundpistonswarmup,theybecomemoreroundinshape,sotheareaofcontactwiththecy1inderwallincreases(Figure6.6).Grindinggenerallyreferstocastironpistons,aluminiumpistonscanbeturned,ratherthanground.
Figure6.6Asacam-groundpistonwarmsup,skirtexpansionincreasestheareaofcontact
Steel-strutpistons
Specialalloy-steelstrutsarecastintothepistonduringmanufacture.Astrutisfittedtoeachsideofthepiston(Figure6.7).Thesteelfromwhichthestrutsaremadehasaverylowrateofexpansionwhenitisheated.Ontheotherhand,aluminiumalloyhasafairlyhighrateofexpansion.Asthetemperatureofthepistonincreases,thealuminiumalloytriestoexpand,butitisrestrainedbythesteelstruts.Thisholdsthepistonatthethrustfacetoitsspecifiedsize.
Heatpathinpistons
Aswellascontrollingpistonexpansionbystruts,heatiskeptawayfromthethrustfacesofthepistonasmuchaspossible.Thisalsoreducesexpansion.
Figure6.7Analuminium-alloypistonthathassteelstrutstoreduceexpansionHYUNDAI
Figure6.8Pistonwithslotsbehindtheoilring,usedforheatcontrolandoildrainDAIHASU
Onewayofdoingthisiswithhorizontalslotsinthethrustsidesofthepiston,eitherinorbelowtheoil-ringgroove(Figure6.8).Theslotsareusedtobreaktheheatpathbetweentheheadofthepistonandtheskirtatthethrustfaces.Theheatisdirectedtootherpartsof
thepiston,suchasthepiston-pinbosses,wherethereismoreclearanceandalsomoremetaltoabsorbtheheat.Thisreducesexpansionatthethrustfaces.
Pistoncoolingjets
Normallubricationhelpstocoolthepistonsandcylinderwalls,butadwellasthis,someengineshavecoolingjets.Thesearesmallnozzles,orjets,whichsprayajetofoilupintothepiston.Theoilstrikesthepistonhead,absorbsheatandthendropsbackintotheoilpan.Theoilalsolubricatesthepistonpinandcylinderwalls.
Figure6.9showsacoolingjetanditsaction.Thejetissuppliedwithoilfromthemainoilgalleryoftheengine.Ithasaballcheckvalvethatcloseswhentheoilpressureinthegalleryislow,orwhentheengineisstopped.Atnormaloperatingpressure,theballvalveisforcedoffitsseat,sothatthejetspraysoilintothepiston.
Figure6.9PistoncoolingjetthatspraysoilintothepistonFORD
Pistonsaredesignedwithvariousshapes.Theheadofthepistoncanbeflat,domedorhollow.Thepistoncanhaveashortskirt,afullskin,ortheskirtcanbecutaway.
Theshapeofthepistonheadisdesignedtosuitthetypeofcombustionchamberoftheengineandtoimproveturbulenceoftheairasitisbeingcompressed.Theheadofthepistoncanalsobedesignedwithrecessesordepressionstoprovideclearancefortheheadsofthevalves.
Petrolenginesuseaslipper-typepiston(Figure6.10).Cuttingawaytheskirtofthepistonnotonlyreducesthemassofthepiston,butalsoprovidesclearanceforthecrankshaftbalanceweightswhenthepistonisatBDC.Thisenablesshorterconnectingrodstobeused,permittingamorecompactenginedesign.Onsomeengines,thebalanceweightsarecontouredtoavoidinterferencebetweentheweightsandthepistons.
Figure6.10Slipper-typepistonwitharecessedheadandrecessesforthevalves-frontofpistonisidentifiedby‘F’MAZDA
Somepistonshaveaverynarrowtoplandsothatthetopcompressionringisclosetothetopofthepiston.Thisreducesthesizeofthecrevicebetweenthepistonandthecylinderwall,abovethetoppistonring.Thecrevicecancollecttracesofunburnedfuel,whichfindsitswayoutoftheexhaust-so,reducingthecrevicesizereducesthehydrocarbonemissionsfromtheengine.
Pistonthrusts
Duringthepowerstrokes,combustionpressuresforcethepistondownwards.However,thepistondoesnotbearevenlyagainstthewallsofthecylinder,butisthrustagainstthesidesofthecylinder.Thisiscausedbytheangularityoftheconnectingrod(Figure6.11).
Figure6.11ThrustsideoftheenginewhenviewedfromthefrontREPCO
Thecombustionpressuresforcethepistondownwards,andtheconnectingrodoffersresistance,butitdoesthisatanangle.Theresultisasidethrustofthepistonagainstthecylinderwall,asshown.
Tilepistonalsohasasidethrustduringthecompressionstroke,butthisisontheoppositesideofthecylinder.Also,thisisaleaserthrustbecausethedownwardforcefromcompressionismuchlessthanthedownwardforceofcombustion.
Thethrustsaresometimesreferredtoasthemajorandminorthrusts.Becausethethrustduringthepowerstroke(majorthrust)ismostimportant,thissideoftheengineisoftenreferredtoasthethrustsideoftheengine,itisnecessarytoknowaboutthethrustsideofanenginebecausethepistonsinmostengineshavetobeinstalledinaparticularway.Pistonsareoftenprovidedwithamarktoshowhowtheyshouldbefittedinrelationtothefrontoftheengine.
■Thrustsoccuronboththeupstrokesandthedownstrokes,butthemajorthrustisduringthepowerstroke.
Offsetpiston-pinbosses
Pistonsareoftendesignedwiththebossesforthepistonpinslightlyoffset.Withthisdesign,thebossesaremovedalittleaway(offset)fromthecentrelineofthepiston.Thisistowardsthemajorthrustside(Figure6.12).
Figure6.12Pistonwiththepiston-pinholeoffsettowardsthethrustside-theoil-spitholespraysoilontothecylinderwall
Theoffsetofabout1
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