Auto English Unit Seven.docx

Auto English Unit Seven.docx

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AutoEnglishUnitSeven

UnitSeven

EngineCoolimgSysyem

Althoughgasolineengineshaveimprovedalot,theyarestillnotveryefficientatturningchemicalenergyintomechanicalpower.Mostoftheenergyinthegasoline（perhaps70%）isconvertedintoheat,anditisthejobofthecoolingsystemtotakecareofthatheat.Infact,thecoolingsystemonacardrivingdownthefreewaydissipatesenoughheattoheattwoaverage-sizedhouses!

Theprimaryjobofthecoolingsystemistokeeptheenginefromoverheatingbytransferringthisheattotheair,butthecoolingsystemalsohasseveralotherimportantjobs.Whentheengineiscold,componentswearoutfaster,andtheengineislessefficientandemitsmorepollution.Soanotherimportantjobofthecoolingsystemistoallowtheenginetoheatupasquicklyaspossible,andthentokeeptheengineataconstanttemperature.

TheBasics

Insidethecar'sengine,fuelisconstantlyburning.Alotoftheheatfromthiscombustiongoesrightouttheexhaustsystem,butsomeofitsoaksintotheengine,heatingitup.Theenginerunsbestwhenitscoolantisabout200degreesFahrenheit（93degreesCelsius）.Atthistemperature,thecombustionchamberishotenoughtocompletelyvaporizethefuel,providingbettercombustionandreducingemissions.

Theoilusedtolubricatetheenginehasalowerviscosity（itisthinner）,sotheenginepartsmovemorefreelyandtheenginewasteslesspowermovingitsowncomponentsaround.Metalpartswearless.Therearetwotypesofcoolingsystemsfoundoncars:

liquid-cooledandair-cooled.

AirCooling

Someoldercarsareair-cooled.Insteadofcirculatingfluidthroughtheengine,theengineblockiscoveredinaluminumfinsthatconducttheheatawayfromthecylinder.Apowerfulfanforcesairoverthesefins,whichcoolstheenginebytransferringtheheattotheair.

LiquidCooling

Now,mostcarsareliquid-cooled.Thecoolingsystemonliquid-cooledcarscirculatesafluidthroughpipesandpassagewaysintheengine.Asthisliquidpassesthroughthehotengineitabsorbsheat,coolingtheengine.Afterthefluidleavestheengine,itpassesthroughaheatexchanger,orradiator,whichtransferstheheatfromthefluidtotheairblowingthroughtheexchanger.

Plumbing

Thecoolingsysteminyourcarhasalotofplumbing.Thepumpsendsthefluidintotheengineblock,whereitmakesitswaythroughpassagesintheenginearoundthecylinders.Thenitreturnsthroughthecylinderheadoftheengine.Thethermostatislocatedwherethefluidleavestheengine.Theplumbingaroundthethermostatsendsthefluidbacktothepumpdirectlyifthethermostatisclosed.Ifitisopen,thefluidgoesthroughtheradiatorfirstandthenbacktothepump.

Fluid

Carsoperateinawidevarietyoftemperatures,fromwellbelowfreezingtowellover100℉（38℃）.Sowhateverfluidisusedtocooltheenginehastohaveaverylowfreezingpoint,ahighboilingpoint,andithastohavethecapacitytoholdalotofheat.

Waterisoneofthemosteffectivefluidsforholdingheat,butwaterfreezesattoohighatemperaturetobeusedincarengines.Thefluidthatmostcarsuseisamixtureofwaterandethyleneglycol（C2H6O2）,alsoknownasantifreeze.Byaddingethyleneglycoltowater,theboilingandfreezingpointsareimprovedsignificantly.

Thetemperatureofthecoolantcansometimesreach250to275℉（121to135℃）.Evenwithethyleneglycoladded,thesetemperatureswouldboilthecoolant,sosomethingadditionalmustbedonetoraiseitsboilingpoint.

Thecoolingsystemusespressuretofurtherraisetheboilingpointofthecoolant.Justastheboilingtemperatureofwaterishigherinapressurecooker,theboilingtemperatureofcoolantishigherifyoupressurizethesystem.Mostcarshaveapressurelimitof14to15poundspersquareinch（psi）,whichraisestheboilingpointanother45℉（25℃）sothecoolantcanwithstandthehightemperatures.

Antifreezealsocontainsadditivestoresistcorrosion.

WaterPump

Thewaterpumpisasimplecentrifugalpumpdrivenbyabeltconnectedtothecrankshaftof

theengine.Thepumpcirculatesfluidwhenevertheengineisrunning.Thewaterpumpusescentrifugalforcetosendfluidtotheoutsidewhileitspins,causingfluidtobedrawnfromthecentercontinuously.Theinlettothepumpislocatednearthecentersothatfluidreturningfromtheradiatorhitsthepumpvanes.Thepumpvanesflingthefluidtotheoutsideofthepump,whereitcanentertheengine.Thefluidleavingthepumpflowsfirstthroughtheengineblockandcylinderhead,thenintotheradiatorandfinallybacktothepump.Theengineblockandcylinderheadhavemanypassagewayscastormachinedinthemtoallowforfluidflow.Thesepassagewaysdirectthecoolanttothemostcriticalareasoftheengine.

Radiator

Aradiatorisatypeofheatexchanger.Itisdesignedtotransferheatfromthehotcoolantthatflowsthroughittotheairblownthroughitbythefan.

Mostmoderncarsusealuminumradiators.Theseradiatorsaremadebybrazingthinaluminumfinstoflattenedaluminumtubes.Thecoolantflowsfromtheinlettotheoutletthroughmanytubesmountedinaparallelarrangement.Thefinsconducttheheatfromthetubesandtransferittotheairflowingthroughtheradiator.Thetubessometimeshaveatypeoffininsertedintothemcalledaturbulator,whichincreasestheturbulenceofthefluidflowingthroughthetubes.Ifthefluidflowedverysmoothlythroughthetubes,onlythefluidactuallytouchingthetubeswouldbecooleddirectly.Theamountofheattransferredtothetubesfromthefluidrunningthroughthemdependsonthedifferenceintemperaturebetweenthetubeandthefluidtouchingit.Soifthefluidthatisincontactwiththetubecoolsdownquickly,lessheatwillbetransferred.Bycreatingturbulenceinsidethetube,allofthefluidmixestogether,keepingthetemperatureofthefluidtouchingthetubesupsothatmoreheatcanbeextracted,andallofthefluidinsidethetubeisusedeffectively.

RadiatorCap

Theradiatorcapactuallyincreasestheboilingpointofyourcoolantbyabout45℉（25℃）.Howdoesthissimplecapdothis?

Thesamewayapressurecookerincreasestheboilingtemperatureofwater.Thecapisactuallyapressurereleasevalve,andoncarsitisusuallysetto15psi.Theboilingpointofwaterincreaseswhenthewaterisplacedunderpressure.

Whenthefluidinthecoolingsystemheatsup,itexpands,causingthepressuretobuildup.Thecapistheonlyplacewherethispressurecanescape,sothesettingofthespringonthecapdeterminesthemaximumpressureinthecoolingsystem.Whenthepressurereaches15psi,thepressurepushesthevalveopen,allowingcoolanttoescapefromthecoolingsystem.Thiscoolantflowsthroughtheoverflowtubeintothebottomoftheoverflowtank.Thisarrangementkeepsairoutofthesystem.Whentheradiatorcoolsbackdown,avacuumiscreatedinthecoolingsystemthatpullsopenanotherspringloadedvalve,suckingwaterbackinfromthebottomoftheoverflowtanktoreplacethewaterthatwasexpelled.

Thermostat

Thethermostat'smainjobistoallowtheenginetoheatupquickly,andthentokeeptheengineataconstanttemperature.Itdoesthisbyregulatingtheamountofwaterthatgoesthroughtheradiator.Atlowtemperatures,theoutlettotheradiatoriscompletelyblockedOncethetemperatureofthecoolantrisestobetween180and195℉（82-91℃）,thethermostatstartstoopen,allowingfluidtoflowthroughtheradiator.Bythetimethecoolantreaches200to218℉（93-103℃）,thethermostatisopenalltheway.

Thesecretofthethermostatliesinthesmallcylinderlocatedontheengine-sideofthedevice.Thiscylinderisfilledwithawaxthatbeginstomeltataround180℉（differentthermostatsopenatdifferenttemperatures,but180℉isacommonone）.Arodconnectedtothevalvepressesintothiswax.Whenthewaxmelts,itexpandssignificantly,pushingtherodoutofthecylinderandopeningthevalve.

Fan

Likethethermostat,thecoolingfanhastobecontrolledsothatitallowstheenginetomaintainaconstanttemperature.

Front-wheeldrivecarshaveelectricfansbecausetheengineisusuallymountedtransversely,meaningtheoutputoftheenginepointstowardthesideofthecar.Thefansarecontrolledeitherwithathermostaticswitchorbytheenginecomputer,andtheyturnonwhenthetemperatureofthecoolantgoesaboveasetpoint.Theyturnbackoffwhenthetemperaturedropsbelowthatpoint.

Rear-wheeldrivecarswithlongitudinalenginesusuallyhaveengine-drivencoolingfans.Thesefanshaveathermostaticallycontrolledviscousclutch.Thisclutchispositionedatthehubofthefan,intheairflowcomingthroughtheradiator.Thisspecialviscousclutchismuchliketheviscouscouplingsometimesfoundinall-wheeldrivecars.

NewWords

1.dissipate[]v.（使）驱散,浪费

2.primary[]adj.主要的

3.out[]adv.明显地

4.overheat[]vt.使过热

5.wear[]vi.磨损

6.emit[]v.放射（光热，味等）

7.soak[]v.吸收,吸入

8.Fahrenheit[]n.华氏温度的

9.Celsius[]adj.摄氏的

10.vaporize[]vt.vi.（使）蒸发

11.viscosity[]n.粘度，粘性

12.aluminum[]n.铝

13.fin[]n.散热片

14.conduct[]v.传导

15.transfer[]vt.传递,转移

16.circulate[]vt.使循环vi.循环

17.fluid[]n.液体

18.passageway[]n.通路,通道

19.radiator[]n.散热器

20.plumbing[]n.水管装置

21.pump[]n.泵，抽水机

22.thermostat[]n.节温器

23.separate[]v.独立的

24.antifreeze[]n.防冻剂

25.significant[]a.重大的，重要的

26.additional[]adj.附加的

27.cooker[]n.