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ValvesandValveTrainThevalvegearofaninternalcombustionengineprovidestimelyadmissionofthefreshchargeintothecylindersandexhaustofspentgasesfromthem.Forthispurposethevalvesatdefinitemomentsopenandclosetheintakeandexhaustportsinthecylinderhead,throughwhichthecylinderscommunicatewiththeintakeandexhaustmanifold.Thevalvegeariscomposedoftiminggears,acamshaft,tappets,valves,springwithfastenersandvalveguides.Thetiminggearsinmostenginesarehousedinaspecialcasefittedatthefrontendoftheengine.Thesearenecessarytotransmitrotationfromthecrankshafttothecamshaft,fuelinjectionpumpshaft,andtooilpumpandothermechanisms.Thegearsaremadeofsteelandusehelicalteethtoreducenoise.Camshaft'sfunctionistoopentheenginevalvespositivelyandtimely,inadefinitesequence,andtocontroltheirclosingagainstthereturnactionofthevalvesprings.Theshaftismadeintegralwithitscamsandbearingjournals.Eachcamcontrolsasinglevalve,eitherintakeorexhaust.Insomeautomobileengines,thecamshaftismadeintegralwithfuelpumpeccentricwheelandoilpumpdrivegear.Thecamshaftbearingsarelubricatedwithoilsuppliedunderpressurefromthemaingalleryinthecylinderblock.Thetappetsservetotransmittheforcefromthecamshafttothepushrods.Thetappetsaresmallcylindricalboresreceivingthepushrods.Theyaremadeofcastironorsteelandlocatedintheguides,whichmaybemadeintegralwiththecylinderblockorremovableasintheengine.Whentheenginesoperate,thetappetscontinuouslyrotateabouttheiraxesforuniformwear.Therotationisensuredbyaconvexsurfaceoftheirbottomsandaslantedsurfaceofthecams.Thepushrodstransmitstheforcefromthetappetstotherockerandaremadeassteelstemswithhardenedtipsorduralumintubeswithsphericalsteeltipspress-fittedat.bothends.Thepushrodtipsbearagainstthetappethollowatoneendagainstthesphericalsurfaceofrockeradjustingscrewatother.Therockerstransmittheforcefromthepushrodtothevalve.Therockersaremadefromsteelandareinstalledonahollowfulcrum.Abronzebushispressfittedintotherockerholetoreducefriction.Thehollowfulcrumissupportedbystandardsonthecylinderhead.Endplayoftherockerispreventedbyacoilspring.Therockerarmisabellcrankmadeofsteel.Atthemiddleoftherockerarm,thereisabosswithaboreintowhichispressedthebushing.Ahardenedcurvedpadisprovidedontheendoftherockerwhereitcontactsthevalvestemtip,whileathreadedholeismachinedinitsotherendtoreceivetheadjustingscrewusedtosetthevalveclearance,theclearancebetweentherockercontactpadandthevalvestemtip,sothatthevalvewillbetightlypressedagainstitsseatwhenhot.Therockerarmfreelyoscillatesabouttherocker-armshaftsupportedbyaseriesofpedestalsorbrackets,whichareboltedtothetopdeckofthecylinderhead.Anenginevalveisadevicedesignedtoopenapassagewhenmovinginonedirectionandtocloseitwhenmovingintheoppositedirection.Eachcylinderofafour-stroke-cycledieselorgasouneengineiscommonlyequippedwithanintakevalveandanexhaustvalve.Thepurposeoftheintakevalveistoallowtheairfuelmixtureorairtoenterthecylinder.Afterthecombustionprocesshasbeencompleted,theburnedgasesarepermittedtoescapefromthecylinderthoughtheexhaustvalve.Toobtainsufficientvalvearea,someautomobileshavetwointakevalvesandtwoexhaustvalves.Avalveconsistsofaheadandstem.Thevalveheadhasanarrowchamferof45°or30°referredtoasvalveface.Thevalvefacefitstightlyagainsttheseat,whichisachievedbygrinding.Forbetterenginebreathing,theinletvalvehasalargerdiameterthantheexhaustone.Asthevalvesarenotequallyheatedintherunningengine,theyaremadefromdifferentmaterials.Theinletvalvesaremadefromchromiumsteel,theexhaustvalvesareofsilchromeheat-resistantsteel.Thecylinderstemofthevalvehasarecessattheupperendforfasteningthevalvespring.Thevalvestemsslideinthecast-ironorcerametallicvalveguides.Thevalvespringprovidestheforcenecessarytoclosethevalveandholdittightlyagainstitsseat.Someenginesusetwospringsoneachvalve,whichreducethesizeofthesprings,improvestheirreliability,andmakestheiroperatingconditionslessarduous.Valveguidesupportsthevalvestemandguidesitsmovementsothatthevalvefaceremainsperfectlyconcentricwiththevalveseatandfitsitwithoutanyskewing.Replaceablevalveguidesarefabricatedfromcastironoracermetsmaterialandpressedinthecylinderhead.Todecreaseoilpenetrationalongthevalvestemtothecombustionchambers,theseatingcollarsarefittedwithrubberringsortheseatsareprovidedwithrubbercaps.Moreuniformheatingandwearofthevalveareensuredwiththevalvesrotatingduringtheoperationoftheengine.Ingeneral,therearetwowaysofrotating:oneisfreerotatetheotherispositiverotate.Asthevalveisopened,springiscompressedanditsincreasingloadcausestheflexiblewashertoflattenoutandforceballsdowntheirrampsagainsttheresistanceofferedbytheirreturnsprings.Astheballsrolldown,theyturnthroughsomeangletheflexiblewasher,seatingcollarandvalvespringtogetherwiththevalve;whenthevalveisclosed,thevalvespringloaddecrease,theflexiblewasherdeflectstoacquireitsinitialconicalshapeandabuttingagainstashoulderinhousing,releasestheballswhicharethenforcedbytheircoilspringstoreturntotheirstartingposition.配气机构内燃机的配气机构保证新鲜混合气可以适时进入汽缸，同时燃烧后的废气及时排出。为实现这一目的，气门在一定的时刻打开和关闭汽缸盖上的进排气道，汽缸通过进排气歧管与进排气管相沟通。配气机构由正时齿轮、凸轮轴、挺柱、气门、带有锁紧装置的弹簧和气门导管组成。大多数发动机的正时齿轮装在专门的壳体内，正时齿轮壳体位于发动机前端。必须使用正时齿轮将