lesson-1-HOW-DOES-A-MARINE-DIESEL-ENGIEN-WORK

-1-LESSON1HOWDOSEAMARINEDIESELENGINEWORK第1页共3页ENGLISHOFMARINEENGINEERINGLESSON1HOWDOESAMARINEDIESELENGINEWORK?Thedieselengineisatypeofinternalcombustionenginewhichignitesthefuelbyinjectingitintohot,highpressureairinacombustionchamber.Themarinedieselengineisatypeofdieselengineusedonship.Theprincipleofitsoperationisasfollow:Achargeoffreshairisdrawnorpumpedintotheenginecylinderandthencompressedbythemovingpistontoveryhighpressure.Whentheairiscompressed,itstemperaturerisessothatitignitesthefinesprayoffuelinjectedintothecylinder.Theburningofthefueladdsmorebeattotheaircharge,causingittoexpandandforcetheenginepistontodoworkonthecrankshaftwhichinturndrivetheship’spropeller.Theoperationbetweentwoinjectionsiscalledacycle,whichconsistsofafixedsequenceofevents.Thiscyclemaybeachievedeitherinfourstrokesortwo.Inafour-strokedieselengine,thecyclerequiresfourseparatestrokesofthepiston,i.e.suction,compression,expansionandexhaust.Ifwecombinethesuctionandexhaustoperationswiththecompressionandexpansionstroke,thefour-strokeenginewillbeturnedintoatwo-strokeone,asisshowninFigure1(a)-(d).图略。Thetwo-strokecyclebeginswiththepistoncomingupfromthebottomofitsstroke,i.e.bottomdeadcenter(BDT),withtheairinletportsorscavengeportsinthesidesofthecylinderbeingopened(Fig.1(a)).Theexhaustportsareuncoveredalso.Pressurizedfreshairchargesintothecylinder,blowingoutanyresidualexhaustgasesfromthelaststrokethroughtheexhaustports.Asthepistonmovesaboutonefifthofthewayup,itclosestheinletportsandtheexhaustports.Theairisthencompressedasthepistonmovesup(Fig.1(b)).Whenthepistonreachesthetopofitsstroke,i.e.thetopdeadcenter(TDC),boththepressureandthetemperatureoftheairrisetoveryhighvalues.Thefuelinjectorinjectsafinesprayoffueloilintothehotairandcombustiontakesplace,producingmuchhigherpressureinthegases.Thepistonisforceddownwardasthehigh-pressuregasesexpand(Fig.1©)untilituncoverstheexhaustports.Theburntgasesbegintoexhaust(Fig.1(d))andthepistoncontinuesdownuntilitopenstheinletports.Thenanothercyclebegins.Inthetwo-strokeengine,eachrevolutionofthecrankshaftmakesonepowerorworkingstroke,whileinthefour-strokecycleengine,ittakestworevolutionstomakeonepowerstroke,thatiswhyatwo-strokecycleenginewilltheoreticallydeveloptwicethepowerofafour-strokeengineofthesamesize.Inefficientscavengingandotherlosses,however,reducethepoweradvantagetoabout1.8.Eachtypeofenginehasitsapplicationonboardship.Thelowspeed(i.e.90to120r/min)mainpropulsiondieseloperatesonthetwo-strokecycle.Atthislowspeedtheenginerequiresno-2-LESSON1HOWDOSEAMARINEDIESELENGINEWORK第2页共3页reducinggearboxbetweenitandpropeller.Thefour-strokeengine(usuallyrotatingatmediumspeed,between250to750r/min)isusedforalternatorsandsometimesformainpropulsionwithagearboxtoprovideapropellerspeedofbetween90to120r/min.READINGMATERIALWORKINGCYCLESAdieselenginemaybedesignedtoworkonthetwo-strokeoronthefour-strokecycle.Bothofthemareexplainedbelow.TheFour-StrokeCycleFigure2showsdiagrammaticallythesequenceofeventsthroughoutthetypicalfour-strokecycleoftworevolutions.ItisusualtodrawsuchdiagramsstartingatTDC(firing),buttheexplanationwillatTDC(scavenge).Topdeadcenterissometimesreferredtoasinnerdeadcenter(IDC).Proceedingclockwiseroundthediagram,bothinlet(orsuction)andexhaustvalvesareinitiallyopen.(ALLmodernfour-strokeengineshavepoppetvalves.).Iftheengineisnaturallyaspirated,orisasmallhigh-speedtypewithacentrifugalturbocharger,theperiodofvalveoverlap,i.e.whenbothvalvesareopen,willbeshort,andtheexhaustvalvewillclosesome100aftertopdeadcenter(ATDC).Propulsionengineandthevastmajorityofauxiliarygeneratorenginesrunningatspeedsbelow1,000r/minwillalmostcertainlybeturbochargedandwillbedesignedtoallowagenerousthroughflowofscavengeairatthispointinordertocontroltheturbinebladetemperature.Inthiscasetheexhaustvalvewillremainopenuntilexhaustvalveclosure(EVC)at50-600ATDC.Asthepistondescendstoouterorbottomdeadcenter(BDC)onthesuctionstroke,itwillinhaleafreshchargeofair.Tomaximizethis,balancingthereducedopeningasthevalveseatsagainsttheslightramorinertiaeffectastheincomingcharge,theinlet(suction)valvewillnormallybeheldopenuntilabout25-350ACBT(145-1550BTDC).Thiseventiscalledinletvalveclosure(IVC).Thechargeisthencompressedbytherisingpistonuntilithasattainedatemperatureofsome550℃.Atabout10-20oBTDC(firing),dependingonthetypeandspeedoftheengine,theinjectoradmitsfinelyatomizedfuelwhichigniteswithin2-70(dependingonthetypeagain)andthefuelburnsoveraperiodof30-500whilethepistonbeginstodescendontheexpansionstroke,thepistonmovementusuallyhelpingtoinduceairmovementtoassistcombustion.Atabout120-1500ATDCtheexhaustvalveopens(EVO),thetimingbeingchosentopromoteaveryrapidblow-downofthecylindergasestoexhaust.Thisisdone:(a)topreserveasmuchenergyasispracticabletodrivetheturbocharger,and(b)toreducethecylinderpressuretoaminimumbyBDCtoreducepumpingworkonthe‘exhaust’stroke.Therisingpistonexpelstheremainingexhaustgasandatabout70-800BTDCtheinletvalveopens(IVO)sothattheinertiaoftheout-flowinggas,plusthepositivepressuredifference,whichusuallyexistsacrossthecylinderbynow,producesathroughflowofairtotheexh