中英圖文對(duì)照汽車(chē)技術(shù)解析+機(jī)械專業(yè)英語(yǔ)詞匯

CHAPTER1AUTOMOTIVEBASICS

1.1PrincipalComponents主要構(gòu)成零部件

Today'saveragecarcontainsmorethan15,000separate,individual

partsthatmustworktogether.Thesepartscangroupedintofour

majorcategories:engine,body,chassisandelectricalequipment.現(xiàn)

在，-一般汽車(chē)由大約一萬(wàn)五千多個(gè)獨(dú)立的零部件組成。這些部件分為四大類，即

發(fā)動(dòng)機(jī)（引擎系統(tǒng)），車(chē)身，底盤(pán)和電氣設(shè)備。

layoutofacommercalvhicle

1.2Engine發(fā)動(dòng)機(jī)/引擎系統(tǒng)

Theengineactsasthepowerunit.Theinternalcombustionengineis

mostcommon:thisobtainsitspowerbyburningaliquidfuelinside

theenginecylinder.Therearetwotypesofengine:gasoline（also

calledaspark-ignitionengine）anddiesel（alsocalleda

compression-ignitionengine）.Bothenginesarecalledheatengines;

theburningfuelgeneratesheatwhichcausesthegasinsidethe

cylindertoincreaseitspressureandsupplypowertorotateashaft

connectedtothetransmission.

引擎的作用是動(dòng)力產(chǎn)生的部件。內(nèi)燃機(jī)引擎最為常見(jiàn)。引擎通過(guò)燃燒其汽缸內(nèi)的

液體汽油來(lái)獲得能量（將汽油的能量轉(zhuǎn)化為動(dòng)能來(lái)驅(qū)動(dòng)汽車(chē)）。引擎分為汽油（火

花引燃式）發(fā)動(dòng)機(jī)和柴油（壓燃式）發(fā)動(dòng)機(jī)。這兩類發(fā)動(dòng)機(jī)都被稱為熱力機(jī)。

燃燒的汽油產(chǎn)生熱量，促使汽缸內(nèi)的氣體氣壓增強(qiáng)，從而提供能量以旋轉(zhuǎn)（啟動(dòng)）

傳輸軸。

1.3Body車(chē)身

Anautomobilebodyisasheetmetalshellwithwindows,doors,a

hood,andatrunkdeckbuiltintoit.Itprovidesaprotectivecovering

fortheengine,passengers,andcargo.Thebodyisdesignedtokeep

passengerssafeandcomfortable.Thebodystylingprovidesan

attractive,colorful,modernappearanceforthevehicle.

汽車(chē)車(chē)身是由金屬薄片殼、車(chē)窗、車(chē)門(mén)、發(fā)動(dòng)機(jī)罩和行李艙蓋等共同構(gòu)成。車(chē)身

為發(fā)動(dòng)機(jī)，乘客和貨物提供了一個(gè)防護(hù)罩。車(chē)身的設(shè)計(jì)應(yīng)該遵循保證乘客安全和

舒適的原則。車(chē)身的風(fēng)格使汽車(chē)具有了引人注目、彩色的，現(xiàn)代的外立面。

1.4Chassis底盤(pán)

Thechassisisanassemblyofthosesystemsthatarethemajor

operatingpartofavehicle.Thechassisincludesthetransmission,

suspension,steering,andbrakesystems.

底盤(pán)由汽車(chē)的主要操作系統(tǒng)組裝而成。包括傳動(dòng)系、行駛系、轉(zhuǎn)向系和制動(dòng)系四

部分。

Transmissionsystems—conveysthedrivetothewheels.Themain

componentsareclutch,gearbox,driveshaft,finaldrive,and

differential.

傳動(dòng)系-將驅(qū)動(dòng)力傳遞到車(chē)輪。主要組成部件是離合器、變速箱、傳動(dòng)軸、主減

速器和差速器。

Suspension—absorbstheroadshocks.

懸掛系-吸收路面震動(dòng)。

Steering—controlsthedirectionofthemovement.

轉(zhuǎn)向系-控制汽車(chē)的行駛方向。

Brake—slowsdownthevehicle.

制動(dòng)系-使汽車(chē)減速緩行。

1.5ElectricalEquipment電氣設(shè)備

Theelectricalsystemsupplieselectricityfortheignition,horn,lights,

heater,andstarter.Theelectricitylevelismaintainedbyacharging

circuit.Thiscircuitconsistsofthebattery,alternator（orgenerator）.

Thebatterystoreselectricity.Thealternatorchangestheengine's

mechanicalenergyintoelectricalenergyandrechargesthebattery.

電氣設(shè)備為汽車(chē)點(diǎn)火、喇叭、車(chē)燈、發(fā)熱器和啟動(dòng)器提供電力。通過(guò)循環(huán)充電

來(lái)維持電量。

NewWords

Principalcomponent主要部件

category種類，類型

body車(chē)身

chassis底盤(pán)

layout布置

powerunit動(dòng)力裝置

internalcombustionengine內(nèi)燃機(jī)

cylinder汽缸

gasoline汽油

spark火花

ignition點(diǎn)燃，點(diǎn)火

diesel柴油機(jī)

compression壓縮

shaft軸

transmission傳動(dòng)系

sheetmetal金屬板

shell外殼

hood（發(fā)動(dòng)機(jī)）罩

trunkdeck行李艙蓋

cargo貨物

styling樣式

assembly總成，裝配

suspension懸掛，懸置

shock沖擊

steering轉(zhuǎn)向，操縱

brake剎車(chē)，制動(dòng)器

clutch離合器

gearbox變速器

driveshaft傳動(dòng)軸

finaldrive主減速器，后橋

differential差速器

slowdown（使）慢下來(lái)，減速

horn喇叭

starter起動(dòng)機(jī)

charge充電

alternator交流發(fā)電機(jī)

ReviewQuestions

1.Listthemainpartsofanautomobile?

2.Whatarethecommontypesofavehicleaccordingtobodystyling?

3.Whichsystemsdoesachassisincludeandwhatarethemainfunctionsofthe

chassis?

4.Whyaresuspensionsystemsusedonvehicles?

CHAPTER2INTERNALCOMBUSTIONENGINE內(nèi)燃發(fā)動(dòng)機(jī)

2.1principleofoperation發(fā)動(dòng)機(jī)的工作原理/操作原理

2.1.1Engineandpower發(fā)動(dòng)機(jī)與能量

Engineisusedtoproducepower.

發(fā)動(dòng)機(jī)產(chǎn)生動(dòng)能。

Thechemicalenergyinfuelisconvertedtoheatbytheburningofthefuelata

controlledrate.Thisprocessiscalledcombustion.Ifenginecombustionoccurswith

thepowerchamber,theengineiscalledinternalcombustionengine.Ifcombustion

takesplaceoutsidethecylinder,theengineiscalledanexternalcombustionengine.

汽油燃料通過(guò)受控速度的燃燒講自身的化學(xué)能轉(zhuǎn)化為熱能。這個(gè)過(guò)程稱作燃燒。

如果發(fā)動(dòng)機(jī)的內(nèi)燃在燃料室中發(fā)上，發(fā)動(dòng)機(jī)被稱作內(nèi)燃發(fā)動(dòng)機(jī)。如果內(nèi)燃發(fā)生在

汽缸外，發(fā)動(dòng)機(jī)則被稱作外燃發(fā)動(dòng)機(jī)。

Engineusedinautomobilesareinternalcombustionheatengines.汽車(chē)的發(fā)動(dòng)機(jī)

是內(nèi)燃發(fā)動(dòng)機(jī)。

Heatenergyreleasedinthecombustionchamberraisesthetemperatureofthe

combustiongaseswiththechamber.Theincreaseingastemperaturecausesthe

pressureofthegasestoincrease.Thepressuredevelopedwithinthecombustion

chamberisappliedtotheheadofapistontoproduceausablemechanicalforce,

whichisthenconvertedintousefulmechanicalpower.燃燒汽缸中釋放的熱能將汽

缸內(nèi)的內(nèi)燃其他溫度升高。氣體溫度的升高導(dǎo)致其他壓強(qiáng)增大。汽缸內(nèi)的壓強(qiáng)不

斷產(chǎn)生以用于活塞頭產(chǎn)生可用的機(jī)械動(dòng)力，隨后轉(zhuǎn)變成為有用的機(jī)械動(dòng)能。

2.1.2EngineTerms發(fā)動(dòng)機(jī)術(shù)語(yǔ)

Linkingthepistonbyaconnectingrodtoacrankshaftcausesthegastorotatethe

shaftthroughhalfaturn.

連動(dòng)桿將汽缸活塞與機(jī)軸聯(lián)接起來(lái)，這種連接促使氣體

Thepowerstroke“usesup“thegas,someansmustbeprovidedtoexpeltheburntgas

andrechargethecylinderwithafreshpetrol-airmixture:thiscontrolofgas

movementisthedutyofthevalves;aninletvalveallowsthenewmixturetoenterat

therighttimeandanexhaustvalveletsouttheburntgasafterthegashasdoneitsjob.

Enginetermsare:

TDC(TopDeadCenter):thepositionofthecrankandpistonwhenthepistonis

fartherawayfromthecrankshaft.上止點(diǎn)

BDC(BottomDeadCenter):thepositionofthecrankandpistonwhenthepistonis

nearesttothecrankshaft.下止點(diǎn)

Stroke:thedistancebetweenBDCandTDC;strokeiscontrolledbythecrankshaft.

沖程

Bore:theinternaldiameterofthecylinder.內(nèi)孑L(汽缸的內(nèi)直徑)

Sweptvolume:thevolumebetweenTDCandBDC.活塞排量

Enginecapacity:thisisthesweptvolumeofallthecylindere.g.afour-stroke

havingacapacityoftwoliters(2000cm)hasacylindersweptvolumeof50cm.發(fā)動(dòng)

機(jī)容積

Clearancevolume:thevolumeofthespaceabovethepistonwhenitisatTDC.

汽缸余隙容積

Compressionratio=(sweptvol+clearancevol)\(clearancevol)壓縮率

Two-stroke:apowerstrokeeveryrevolutionofthecrank.二沖程一曲柄旋轉(zhuǎn)一圈

作功一次。

Four-stroke:apowerstrokeeveryotherrevolutionofthecrank.四沖程-曲柄旋轉(zhuǎn)

兩圈作功一次。

2.1.3TheFour-strokeSpark-ignitionEngineCycle

Thespark-ignitionengineisaninternal-combustionenginewithexternally

suppliedinignition,whichconvertstheenergycontainedinthefueltokineticenergy.

Thecycleofoperationsisspreadoverfourpistonstrokes.Tocompletethefull

cycleittakestworevolutionsofthecrankshaft.

Theoperatingstrokesare:

Thisstrokeintroducesamixtureofatomizedgasolineandairintothe

cylinder.Thestrokestartswhenthepistonmovesdownwardfroma

positionnearthetopofthecylinder.Asthepistonmovesdownward,

avacuum,orlow-pressurearea,iscreated.

Duringtheintakestroke,oneoftheportsisopenedbymovingthe

inletvalve.Theexhaustvalveremainstightlyclosed.

Compressionstroke

Asthepistonmovesupwardtocompressthefuelmixturetrappedin

thecylinder,thevalvesareclosedtightly.Thiscompressionaction

heatstheair/fuelmixtureslightlyandconfinesitwithinasmallarea

calledthecombustionchamber.

Powerstroke

Justbeforethepistonreachesthetopofitscompressionstroke,an

electricalsparkisintroducedfromasparkplugscrewedintothe

cylinderhead.

Thesparkignitesthecompressed,heatedmixtureoffuelandairin

thecombustionchambertocauserapidburning.Theburningfuel

producesintenseheatthatcausesrapidexpansionofthegases

compressedwithinthecylinder.Thispressureforcesthepiston

downward.Thedownwardstroketurnsthecrankshaftwithgreatforce.

Exhauststroke

Justbeforethebottomofthepowerstroke,theexhaustvalveopens.

Thisallowsthepiston,asitmovesupward,topushthehot,burned

gasesoutthroughtheopenexhaustvalve.

Then,justbeforethepistonreachesitshighestpoint,theexhaust

valveclosesandtheinletvalveopens.Asthepistonreachesthe

highestpointinthecylinder,knownasTDC,itstartsbackdownagain.

Thus,onecycleendsandanotherbeginsimmediately.

2.1.4EngineOverallMechanics

Theenginehashundredsofotherparts.Themajorpartsofengineareengine

block,engineheads,pistons,connectingrods,crankshaftandvalves.Theotherparts

arejoinedtomakesystems.Thesesystemsarethefuelsystem,intakesystem,ignition

system,coolingsystem,lubricationsystemandexhaustsystem.Eachofthesesystems

hasadefinitefunction.Thesesystemswilldiscussedindetaillater.

NEWWORD

Piston活塞

Connectingr)d連桿

Crankshaft曲軸

Powerioke活塞行程

Expel排出

Valve氣閥

inlet(intake)vdve進(jìn)氣閥

exhaustvalve排氣閥

term術(shù)語(yǔ)

TDC上止點(diǎn)

BDC下止點(diǎn)

Bore缸徑

sweptvolume有效容積

enginecapacity發(fā)動(dòng)機(jī)排量

clearancevobme余隙容積，燃燒室容積

compressionatio壓縮比

revolution旋轉(zhuǎn)，轉(zhuǎn)數(shù)

everyoher每隔?個(gè)

cycle循環(huán)

spreadover分布，遍及

intakeiroke進(jìn)氣行程

compression^roke壓縮行程

knock敲缸，敲打

exhaust仃oke排氣行程

enginebbck發(fā)動(dòng)機(jī)缸體

lubrication潤(rùn)滑

2.2EngineBlockandCylinderHead

2.2.1EngineBlock

Theengineblockisthebasicframeoftheengine.Allotherengine

partseitherfitinsideitorfastentoit.Itholdsthecylinders,water

jackets,andoilgalleries.Theengineblockalsoholdsthecrankshaft,

whichfastenstothebottomoftheblock.Thecamshaftalsofitsinside

theblock,exceptonoverhead-camengines(OHC).Inmostcars,this

blockismadeofgrayiron,oranalloy(mixture)ofgrayironandother

metals,suchasnickelorchromium.Engineblocksarecastings.

Someengineblocks,especiallythoseinsmallercars,aremadeofcast

aluminum.Thismetalismuchlighterthaniron.However,ironwears

betterthanaluminum.Therefore,thecylindersinmostaluminum

enginesarelinedwithironorsteelsleeves.Thesesleevesarecalled

cylindersleeves.Someengineblocksaremadeentirelyofaluminum.

汽虹體

2.2.2CylinderHead

Thecylinderheadfastenstothetopoftheblock,justasarooffits

overahouse.Theundersideformsthecombustionchamberwiththe

topofthepiston.Themostcommoncylinderheadtypesarethehemi,

wedge,andsemi-hemi.Allthreeofthesetermsrefertotheshapeof

theengine'scombustionchamber.Thecylinderheadcarriesthe

valves,valvespringsandtherockersontherockershaft,thispartof

thevalvegearbeingworkedbythepush-rods.Sometimesthe

camshaftisfitteddirectlyintothecylinderheadandoperatesonthe

valveswithoutrockers.Thisiscalledanoverheadcamshaft

arrangement.Likethecylinderblock,theheadismadefromeither

castironoraluminumalloy.

K虹工

2.2.3Gasket

Thecylinderheadisattachedtotheblockwithhigh-tensilesteelstuds.

Thejointbetweentheblockandtheheadmustbegas-tightsothat

noneoftheburningmixturecanescape.Thisisachievedbyusing

cylinderheadgasket.Thisisasandwichgasket,i.e.asheetof

asbestosbetweentwosheetsofcopper,boththesematerialsbeing

abletowithstandthehightemperatureandpressureswithinthe

engine.

2.2.4OilPanorSump

Theoilpanisusuallyformedofpressedsteel.Theoilpanandthe

lowerpartofthecylinderblocktogetherarecalledthecrankcase;

theyenclose,orencase,thecrankshaft.Theoilpumpinthe

lubricatingsystemdrawsoilfromtheoilpanandsendsittoall

workingpartsintheengine.Theoildrainsoffandrunsdownintothe

pan.Thus,thereisconstantcirculationofoilbetweenthepanandthe

workingpartsoftheengine.

NewWords

engineblock缸體

cylinderhead氣缸蓋

fasten使固定

waterjacket水套

oilgallery油道

camshaft凸輪軸

overhead-cam(OHC)頂置凸輪

grayiron灰心鐵

alloy合金

nickel銀

chromium銘

casting鑄件

headcover汽缸蓋罩

intakemanifold進(jìn)氣總管

distributor分電器

oilpan油底殼

aluminum鋁

belinedwith鑲有

cylindersleeve氣缸套

hemi半球形

wedge楔型，楔入

semi-hemi準(zhǔn)半球形

rocker搖臂

push-rod推桿

gasket襯墊

high-tensile高強(qiáng)度的

stud螺栓

gas-tight密封的

asbestos石棉

crankcase曲軸箱，曲柄箱

encase封閉，把…包起來(lái)

drainoff排出,流出

ReviewQuestion

1.WhatdoTDC,BDC,stroke,compressionratioandenginecapacitystandfor?

2.Howdoyoucalculatesweptvolumeandcompressionratio?

3.Whatcontrolsthelengthofthestroke?

4.Listthemainpartsoftheengineoverallmechanics?

5.Whatarethemainfunctionoftheengineblock?

2.3PistonConnectingRodandCrankshaft

2.3.1PistonAssembly

Thepistonisanimportantpartofafour-strokecycleengine.Mostpistonsaremade

fromcastaluminum.Thepiston,throughtheconnectingrod,transferstothe

crankshafttheforcecreatebytheburningfuelmixture.Thisforceturnsthe

crankshaft.Thin,circular,steelbandsfitintogroovesaroundthepistontosealthe

bottomofthecombustionchamber.Thesebandsarecalledpistonrings.Thegrooves

intowhichtheyfitarecalledringgrooves.Apistonpinfitsintoaroundholeinthe

piston.Thepistonpinjoinsthepistontotheconnectingrod.Thethickpartofthe

pistonthatholdsthepistonisthepinboss.

Thepistonitself,itsringsandthepistonpinaretogethercalledthepiston

assembly.

2.3.2.Piston

Towithstandtheheatofthecombustionchamber,thepistonmustbestrong.It

alsomustbelight,sinceittravelsathighspeedsasitmovesupanddowninsidethe

cylinder.Thepistonishollow.Itisthickatthetopwhereittakethebruntoftheheat

andtheexpansionforce.Itisthinatthebottom,wherethereislessheat.Thetoppart

ofthepistonisthehead,orcrown.ThethinpartistheskirtThesectionsbetweenthe

ringgroovesarecalledringlands.

Thepistoncrownmaybeflat,concave,domeorrecessed.Indieselengine,the

combustionchambermaybeformedtotallyorinpartinthepistoncrown,depending

onthemethodofinjection.Sotheyusepistonswithdifferentshapes.

2.3.3PistonRings

AsFig.2-9shows,pistonringsfitintoringgroovesneartheofthepiston.In

simplestterms,pistonringsarethin,circularpiecesofmetalthatfitintogroovesin

thetopsofthepistons.

Inmodemengines,eachpistonhasthreerings.(Pistoninolderengines

sometimeshadfourrings,orevenfive.)Thering'soutsidesurfacepressesagainstthe

cylinderwalls.Ringsprovidetheneededsealbetweenthepistonandthecylinder

walls.Thatis,onlytheringscontactthecylinderwalls.Thetoptworingsaretokeep

thegasesinthecylinderandarecalledcompressionrings.Theloweronepreventsthe

oilsplashedontothecylinderborefromenteringthecombustionchamber,andis

calledanoilring.Chrome-facecast-ironcompressionringsarecommonlyusedin

automobileengines.Thechromefaceprovideaverysmooth,wear-resistantsurface.

Duringthepowerstoke,combustionpressureonthecombustionringsisvery

high.Itcausesthemtountwist.Someofthehigh-pressuregasgetsinbackofthe

rings.Thisforcetheringfaceintofullcontactwiththecylinderwall.Thecombustion

pressurealsoholdsthebottomoftheringtightlyagainstthebottomoftheringgroove.

Therefore,highcombustionpressurecausesatightersealbetweentheringfaceand

thecylinderwall.

活塞環(huán)

2.3.4PistonPin

Thepistonpinholdstogetherthepistonandtheconnectingrod.Thispinfits

intothepistonpinholesandintoaholeinthetopendoftheconnectingrod.Thetop

endofismuchsmallerthantheendthatfitsonthecrankshaft.Thissmallendfits

insidethebottomofthepiston.Thepistonpinfitsthroughonesideofthepiston,

throughthesmallendoftherod,andthenthroughtheothersideofthepiston.It

holdstherodfirmlyinplaceinthecenterofthepiston.Pinsaremadeofhigh-strengh

steelandhaveahollowcenter.Manypinsarechrome-platedtohelpthemwear

better.

2.3.3Connectingrod

Theconnectingrodismadeofforgedhigh-strengthsteel.Ittransmitsand

motionfromthepistontothecrankpinonthecrankshaft.Theconnectingrodlittle

endisconnectedtothepistonpin.Abushmadefromasoftmetal,suchasbronze,is

usedforthisjoint.Thelowerendoftheconnectingrodfitsthecrankshaftjournal.

Thisiscalledthebigend.Forthisbig-endbearing,steel-backedleadortinshell

bearingareused.Thesearethesameasthoseusedforthemainbearings.Thesplitof

thebigendissometimesatanangle,sothatitissmallenoughtobewithdrawn

throughthecylinderbore.Theconnectingrodismadefromforgedalloysteel.

活朦迂桿級(jí)

2.3.5Crankshaft

Thecrankshaft,inconjunctionwiththeconnectingrod,covertsthe

reciprocatingmotionofthepistontotherotarymotionneededtodrivethevehicle.It

isusuallymadefromcarbonsteelwhichisalloyedwithasmallproportionof

nickel.Themainbearingjournalsfitintothecylinderblockandthebigendjournals

alignwiththeconnectingrods.Attherearendofthecrankshaftisattachedthe

flywheel,andatthefrontendarethedrivingwhellsforthetiminggears,fan,

coolingwaterandalternator.

Thethrowofthecrankshaft,thedistancebetweenthemainjournalandthe

bigendcenters,controlsthelengthofthestroke.Thestrokeisdoublethethrow,

andthestroke-lengthisthedistancethatthepistontravelsfromTDCtoBDCandvice

versa.

2.3.6Flywheel

Theflywheelisthemadefromcarbonsteel.Itfitsontotherearofthecrankshaft.

Aswellaskeepingtheenginerotatingbetweenpowerstrokesitalsocarriesthe

clutch,whichtransmitsthedrivetothetransmission,andhasthestarterringgear

arounditscircumference.Thereisonlyoneworkingstrokeinfoursoaflywheelis

neededtodrivethecrankshaftduringthetimethattheengineisperformingthe

non-powerstrokes.

NewWords

Comprise由。。。。。。名威

Inter慣性，慣量

Radius半徑，范圍

Circular圓形的

Steelband鋼圈

Fitinto放入，放進(jìn)

Groove凹槽

Pistonpin活塞銷

Pinboss活塞銷凸臺(tái)

Withstand抵抗

Hollow空的

Brunt沖力

Crown活塞頂

Skirt裙部

Ringland環(huán)帶

Concave凹的，凹入的

Dome圓頂

Recessed隱蔽的

Cylinderwall氣缸壁

Cylinderbore缸筒

Splash飛濺

chrome-face表面鍍銀的

Untwist朝相反方向的

Inplace在適當(dāng)位置

Chrome-plated鍍銘的

Forge偽造，仿造

Crankpin曲軸銷

Bush軸瓦，套筒

Bronze青銅

Crankshaftjournal曲軸軸頸

Steel-backed專岡背的

Lead鉛

Tin錫

Splint切口，中斷，分配，分離

Inconjunctionwith連同

Reciprocatingmotion往復(fù)運(yùn)動(dòng)

Rotary旋轉(zhuǎn)的

Carbonsteel碳鋼

Journal軸頸

Alignwith匹配

Overlap重疊

Timinggear正時(shí)齒輪

Throw擺幅

Viceverse反之亦然

Impulse脈沖

Spaceout隔開(kāi)，分隔

Throughout遍及

Diagram圖表

Firingorder點(diǎn)火順序

Companion成對(duì)

Circumference圓周

2.4ValveSystem

Thevalvesystemismadeupofthosepartsneededtoopenandclosethevalvesat

justtherighttime.

2.4.1ValveOperation

Tocoordinatethefour-strokecycle,agrouppartscalledthevalvetrainopens

andclosesthevalves(movesthemdownandup,respectively).Thesevalve

movementsmusttakeplaceatexactlytherightmoments.Theopeningofeachvalve

iscontrolledbyacamshaft.

1.Camshaft(OHC)ValveTrainOverhead

Thecamisanegg-shapedpieceofmetalonashaftthatrotatesincoordination

withthecrankshaft.Themetalshaft,calledthecamshaft,typicallyhasindividual

camsforeachvalveintheengine.Asthecamshaftrotates,thelobe,orhighspotof

thecam,pushesagainstpartsconnectedtothestemofthevalve.Thisactionforces

thevalvetomovedownward.Thisactioncouldopenaninletvalve,oropenan

exhaustvalveforanexhauststroke.

Asthecamshaftcontinuestorotate,thehighspotmovesawayfromthevalve

mechanism.Asthisoccurs,valvespringpullthevalvetightlyclosedagainstits

opening,calledthevalveseat.

Valveinmodemcarenginesarelocatedinthecylinderheadatthetopthe

engine.Thisisknownasanoverheadvalve(OHC)configuration.Inaddition,when

thecamshaftislocatedoverthecylinderhead,thean*angementisknownasoverhead

camshaft(OHC)design.Somehigh-performanceenginehavetwoseparate

camshafts,oneforeachsetofinletandexhaustvalves.Theseenginesareknownas

overhead-camshaft(DHOC)engine.

2.Push-rodValveTrain

Thecamshaftalsocanbelocatedinthelowerpartoftheengine,withinthe

engineblock.Totransferthemotionofthecamupwardtothevalve,additionalparts

areneeds.

Inthisarrangement,thecamlobspushagainstroundmetalcylinderscalled

followerupward(awayfromthecamshaft).Thecamfollowerridesagainstapush

rod,whichpushesagainstarockerarm.Therockerarmpivotsonashaftthroughits

center.Asonesideoftherockerarmmovesup,theothersidemovesdownJustlike

aseesaw.Thedownward-movingsideoftherockerarmpushesonthevalvestemto

openthevalve.

Becauseapush-rodvalvetrainhasadditionalparts,itismoredifficultto

runathighspeeds.Push-rodenginestypicallyrunatslowerspeedsand,

consequently,producelesshorsepowerthanoverhead-camshaftdesignsofequalsize.

(Remember,poweristherateatwhichworkisdone.)

2.4.2ValveClearance

Whentheenginerunsincompressionstrokeandpowerstroke,thevalves

mustclosetightlyontheirseatstoproduceagas-tightsealandthuspreventthegases

escapingfromthecombustionchamber.Ifthevalvesdonotclosefullytheengine

willnotdevelopfillpower.Alsothevalveheadswillbeliabletobebruntbythe

passinghotgases,andthereisthelikelihoodofcrowntouchinganopenvalve,

whichcanseriouslydamagetheengine.

Sothatthevalvescanclosefullysomeclearanceisneededintheoperating

mechanism.Thismeansthattheoperatingmechanismmustbeabletomove

sufficientlyfarenoughawayfromthevalvetallowthevalvestobefullyclosed

againstitsseatbythevalvespring.However,iftheclearanceissettoogreatthiswill

causealightmetallictapingnoise.

2.4.3ValveTiming

Thetimeatwhichvalvesopenandclose(valvetiming)andthedurationof

thevalveopeninginstatedindegreesofcrankshaftrotation.Forexample,theintake

valvenormallybeginstoopenjustbeforethepistonhasreachedthetopdeadcenter.

ThevalveremainsopenasthepistontravelsdowntoBDCandevenpastBDC.This

isintakevalveduration.Anexampleofthiscouldbestatedasfollows:IOat

17BTDC,ICat51ABDC(or,intakeopens17beforetopdeadcenter,intakecloses

51afterbottomdeadcenter).Intakevalvedurationinthiscaseis248ofcrankshaft

rotation.

Thisleaves129durationforthecompressionstrokesincecompressionends

whenthepistonreachesTDC.Atthispointthepowerstrokebegins.Thepower

strokeendswhentheexhaustvalvebeginstoopenapproximatelyat51beforebottom

deadcenter.Thedurationofthepowerstrokeinthiscaseisalso129.

Sincetheexhaustvalveisopeningat51BBDC,thisbeginstheexhauststroke.

TheexhauststrokecontinuesasthepistonpassesBDCandmovesupwardtopast

TDC.Withtheexhaustvalveclosingat17TTDC,thedurationoftheexhauststroke

is248.

Itisapparentfromthisdescriptionthattheexhaustvalvestaysopenforashort

periodoftimeduringwhichtheintakevalveisalsoopen.Inotherwords,theendof

theexhauststrokeandthebeginningoftheintakestrokeoverlapforashortperiodof

time.Thisiscalledvalveoverlap.Valvetimingandvalveoverlapvaryondifferent

engines.

OpeningtheintakevalvebeforeTDCandclosingitafterBDCincreasethefill

ofair-fuelmixtureinthecylinder.Openingtheintakevalveearlyhelpsovercomethe

staticinertiaoftheair-fuelmixtureatthebeginningoftheintakestroke,while

leavingtheintakevalveopenafterBDCtakesadvantageofthekentiaofthemoving

air-fuelmixture.Thisincreasevolumetricefficiency.

Asthepistonmovesdownonthepowerstrokepastthe90ATDCposition,

pressureinthecylinderhasdropped,andtheleveragetothecrankshafthasdecreased

duetoconnectingrodangleandcrankshaftposition.Thisendstheeffectivelengthof

thepowerstroke,andtheexhaustvalvecannowbeopenedtobeginexpellingthe

burnedgases.Theexhaustvalveremainsopenuntilthepistonhasmoveduppastthe

TDCposition.Thishelpstoremoveasmuchoftheburnedgasesasispossibleand

increasevolumetricefficiency.

2.4.4CamDesignandControlDynamics

Thefunctionofthecamistoopenandclosethevalvesasfaraspossible,as

fastaspossibleandassmoothlyaspossible.Theclosingforceforthevalvesis

appliedbythevalvespringwhichalsomaintaincontactbetweenthecamandthe

valves.Dynamicforceimposelimitsoncamandvalvelift.

Theentirevalve-trainassemblycanbeviewasaspring\masssysteminwhich

theconversionfromstoredtofreeenergycausesforcevibration.Valve-train

assemblieswithoverheadcamshaftscanberepresentedwithsufficientaccuracybya

1-masssystem(consistingofthemovingmass,thevalve-trainassemblystiffness

andcorrespondingdamping).

Forsystemwithvalvebottom-mountedcamshaftandpushrods,a2-mass

systemisbeingincreasinglyused.

Themaximumpermissiblecontactstress,usuallyregardedastheparameter

whichlimitscam-loberadiusandtherateofopeningontheflank,currentlylies

between600-700Mpadependinguponthematerialparingsused.

2.4.5CamshaftDriveMechanism

Eachcammustrevolveonceduringthefour-strokecycletoopenavalve.Acycle,

remember,correspondswithtworevolutionsofthecrankshaft.Therefore,the

camshaftmustrevolveatexactlyhalfthespeedofthecrankshaft.Thisis

accomplishedwitha2:1gearratio.Agearconnectedtothecamshafthastwicethe

numberofteethasagearconnectedtothecrankshaft.Thegearsarelinkedinoneof

threeways:

l.BeltDrive

Acog-typebeltcanbeused.Suchbeltsaremadeofsyntheticrubberand

reinforcedwithinternalsteelorfiberglassstrands.Thebeltshaveteeth,orslotted

spacestoengageanddriveteethongearwheels.Abelttypicallyisusedon

engineswithoverhead-camvalvetrains.

2.ChainDrive

Onsomeengines,ametalchainisusedtoconnectthecrankshaftandcamshaft

gears.Mostpush-rodenginesandsomeOHCengineshavechains.

3.GearDrive

Thecamshaftandcrankshaftgearscanbeconnecteddirectly,