submarine main propulsion diesels - chapter 9

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ENGINEPERFORMANCEANDOPERATION

A.COMBUSTION,ANDEFFICIENCY9A1.Combustion.Engineefficiencyisacomparisonoftheamountofpowerdevelopedbyanenginetotheenergyinputasmeasuredbytheheatingvalueofthefuelconsumed.Inordertounderstandthevariousfactorsresponsiblefordifferencesinengineefficiency,itisnecessarytohavesomeknowledgeofthecombustionprocesswhichtakesplaceintheengine.

Inthedieselengine,ignitionofthefuelisaccomplishedbytheheatofcompressionalone.Tosupportcombustion,airisrequired.Approximately14poundsofairarerequiredforthecombustionof1poundoffueloil.However,toinsurecompletecombustionofthefuel,anexcessamountofairisalwayssuppliedtothecylinders.Theratiooftheamountofairsuppliedtothequantityoffuelinjectedduringeachpowerstrokeiscalledtheairfuelratioandisanimportantfactorintheoperationofanyinternalcombustionengine.Whentheengineisoperatingatlightloadsthereisa,largeexcessofairpresent,andevenwhentheengineisoverloaded,thereisanexcessofairovertheminimumrequiredforcompletecombustion.

Theinjectedfuelmustbedividedintosmallparticles,usuallybymechanicalatomization,asitissprayedorinjectedintothecombustionchamber.Itisimperativethateachofthesmallparticlesbecompletelysurroundedbysufficientairtoeffectcompletecombustionofthefuel.Toaccomplish

1.Thefuelmustenterthecylinderatthe,propertime.Thatis,thefuelinjectionvalvemustopenandcloseincorrectrelationtothepositionofthepiston.

2.Thefuelmustenterthecylinderinafinemistorfog.

3.Thefuelmustmixthoroughlywiththeairthatsupportsitscombustion.

4.Sufficientairmustbepresenttoassurecompletecombustion.

5.Thetemperatureofcompressionmustbesufficienttoignitethefuel.

Figure91isareproductionofapressuretimediagramofamechanicalinjectionengine.Thelowercurvypartofwhichisadottedline,isthecurveofcompressionandexpansionwhennofuelisinjected.AtAtheinjectionvalveopens,fuelentersthecombustionchamberandignitionoccursatB.ThepressurefromAtoBshouldfallslightlybelowthecompressioncurvewithoutfuelduetoabsorptionofheatbythefuelfromtheair.TheperiodfromAtoBistheignitiondelay.FromBthepressurerisesrapidlyuntilitreachesamaximumatC.Thismaximum,insomeinstances,mayoccurattopdeadcenter.AtDtheinjectionvalvecloses,thefueliscutoff,butburningofthefuelcontinuestosomeundeterminedpointalongtheexpansionstroke.

TheheightofthediagramfromBtoCiscalledthefiringpressureriseandthe



this,theairinthecylindermustbeinmotionwithgoodfuelatomization,combinedwithpenetrationanddistribution.Inmechanicalinjectionenginesthisisaccomplishedbyforcingscavengingairintothecylinderwithawhirlingmotiontocreatethenecessaryturbulence.Thisisusuallydone,inthe2cycleengine,byshapingtheintakeairports,orbycastingthemsothattheircentersareslightlytangentialtotheaxisofthecylinderbore.

Beforeproceedingwiththestudyofthecombustionprocess,theconditionsconsideredessentialtogoodcombustionshouldbereviewed:

slopeofthecurvebetweenthesetwopointsistherateatwhichthefuelisburned.

Poorcombustionofthefuelisusuallyindicatedbyasmokyexhaust,butsomesmokemaybetheresultofburninglubricatingoilthathaspassedtheringsintothecombustionchamber.Incompletecombustionisindicatedbyblacksmoke,orifthefuelisnotigniting,itmayappearasbluesmoke.Immediatelyafterstartinganengine,whenrunningatlightloadsoratoverloads,orwhenchangingfromoneloadtoanother,smokeislikelytoappear.

Asmokyexhaustfromtheenginedoesnotindicatewhetheroneorallthecylindersare

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Figure91.Pressuretimediagramofcombustionprocess.causingitAblacksmokingcylinderusuallyshowsahigherexhausttemperaturewhichcanbeobservedfrompyrometersinstalledintheindividualexhaustlinesfromthecylinders.Openingtheindicatorcockoneachcylindertoobservethecoloroftheexhaustisanothercheck.Stillanothermethodiscuttingoffthefuelsupplytoonecylinderatatimetoseewhateffectithasontheengineexhaust.Thislattershouldneverbedonewhentheengineisoperatingatfullloadasoverloadingoftheothercylinderswillresultiftheengineis

radiationandconvectiontothesurroundingair.

2.Heatrejectedandlosttotheatmosphereintheexhaust.

3.Inefficientcombustionorlackofperfectcombustion.

Alossduetoimperfectorincompletecombustionisanimportantitem,becausesuchlosseshaveaseriouseffectonthepowerthatcanbedevelopedinthecylinderasshownbythepressurevolumediagramorindicatorcard.



governorcontrolled.

9A2.Enginelosses.Itisobviousthatnotalloftheheatcontentofafuelcanbetransferredintousefulworkduringthecombustionprocess.Themanydifferentlossesthattakeplaceinthetransformationofheatenergyintoworkmaybedividedintotwoclasses,thermodynamicandmechanical.Thenetusefulworkdeliveredbyanengineistheresultobtainedbydeductingthetotallossesfromtheheatenergyinput.

Thermodynamiclossesarecausedby:

1.Losstothecoolingsystemandlossesby

Completecombustionisnotpossibleintheshorttimepermittedinmodernenginedesign.However,theselossesmaybekepttoaminimumiftheengineiskeptadjustedtotheproperoperatingcondition.Incompletecombustioncanfrequentlybedetectedbywatchingexhausttemperatures,notingtheexhaustcolor,andbeingalertforunusualnoisesintheengine.

Heatenergylossesfromboththecoolingwatersystemsandlubricatingoilsystemarealwayspresent.Someheatisconductedthroughtheenginepartsandradiatedtotheatmosphereorpickedupbythesurroundingairbyconvection.Theeffectoftheselossesvariesaccordingtothepartofthecycleinwhichtheyoccur.The

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heatappearinginthejacketcoolingwaterisnotatruemeasureofcoolinglossbecausethisheatincludes:

1.Heatlossestojacketsduringcompression,combustion,andexpansionphasesoftheworkingcycle.

2.Heatlossesduringtheexhauststroke.

3.Heatlossesabsorbedbythewallsoftheexhaustpassages.

4.Heatgeneratedbypistonfrictiononcylinderwalls.

Heatlossestotheatmospherethroughtheexhaustareinevitablebecausetheenginecylindermustbeclearedofthestillhotexhaustgasesbeforeanotherfreshairchargecanbeintroducedandanotherpowerstrokebegun.Theheatlosttotheexhaustisdeterminedbythetemperaturewithinthecylinderwhenexhaustbegins.Itdependsupontheamountoffuelinjectedandtheweightofaircompressedwithinthecylinder.Impropertimingoftheexhaustvalves,whetherearlyorlate,willresultinincreasedheatlosses.Ifearly,thevalvereleasesthepressureinthecylinder

Figure92.Heatbalanceforadieselengine.

pumpinglossescausedbyoperationofwaterpumps,lubricatingoilpumps,andscavengingairblowers,powerrequiredtooperatevalves,andsoforth.Frictionlossescannotbeeliminated,buttheycanbekeptataminimumbymaintainingtheengineinitsbestmechanicalcondition.Bearings,pistons,andpistonringsshouldbeproperlyinstalledandfitted,shaftsmustbeinalignment,andlubricatingandcoolingsystemsshouldbeattheirhighestoperatingefficiency.



beforealltheavailableworkisobtainediflate,thenecessaryamountofairforcompletecombustionofthenextchargecannotberealized,althoughasmallamountofadditionalworkmaybeobtained.Thetimingoftheexhaustvalveisacompromise,thebestpossiblepositionofopeningandclosingbeingdeterminedbytheenginedesigner.Itisessentialthatthevalvebetightandproperlytimedinordertomaintainthelosstotheexhaustataminimum.Thisisalsotrueforairinletvalvesettingon4cycletypeengines.

Ifanindicatorcardistakenofadieselenginecylinder,itispossibletocalculatethehorsepowerdevelopedwithinthecylinder.Thiscalculationdoesnottakeintoaccountthepowerlossresultingfrommechanicalorfrictionlosses,aswillbediscussedlater,butitreflectstheactualworkproducedwithinthecylinder.

Mechanicallossesareofseveralkinds,notallofthempresentineveryengine.Thesumtotalofthesemechanicallossesdeductedfromtheindicatedhorsepowerdevelopedinthecylinderswillgivethebrakehorsepowerfinallydeliveredasusefulworkbytheengine.Thesemechanicalorfrictionlossesincludebearingfriction,pistonandpistonringfriction,and

9A3.Compressionratioandefficiencies.a.Compressionratio.Thetermcompressionratioisusedquiteextensivelyinconnectionwithengineperformanceandvarioustypesofefficiencies.Itmaybedefinedastheratioofthetotalvolumeofacylindertotheclearancevolumeofthecylinder.Itmaybebestexplainedbyreferencetothepressurevolumeindicatorcardofadieselcylinder.InFigure93,thevolumeisreducedfromsquareroot(C)+squareroot(D)tosquareroot(C)duringcompression.Thecompressionratioisthenequalto(squareroot(C)+squareroot(D))/squareroot(C)

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Figure93.Compressionratio.

Compressionratioinfluencesthe

thefuelwouldfireordetonatebeforethepistoncouldreachthecorrectfiringposition.

Thetemperatureentropy(TS)diagramofanyparticularcycleindicatestheamountofheatinputandtheamountofheatrejected.Forexample,inFigure94,theTSdiagramofamodifieddieselcycle,theheatinputisrepresentedbytheareaFBDGandtheheatrejectedtotheexhaustbytheareaFAEG.Theheatrepresentedindoingusefulworkisrepresentedbythedifferencebetweenthesetwo,orareaABDE.Theefficiencyofthecyclecanthenbeexpressedas



thermalefficiencyofanengine.Theoreticallythethermalefficiencyincreasesasthecompressionratioisincreased.Theminimumvalueofadieselenginecompressionratioisdeterminedbythecompressionrequiredforstarting,which,tolargeextentisdependentonthetypeoffuelused.Themaximumvalueofthecompressionratioisnotlimitedbythefuelusedbutislimitedbythestrengthofthepartsoftheengineandtheallowableenginewgt/bhpoutput.

b.Cycleefficiency.Theefficiencyofanycycleisequaltotheoutputdividedbytheinput.Thedieselcycleshowsoneofthehighestefficienciesofanyengineyetbuiltbecauseofthehighercompressionratiocarriedandbecauseofthefactthatcombustionstartsatahighertemperature.Inotherwords,theheatinputisatahigheraveragetemperature.Theoretically,thegasolineengineusingtheOttoorconstantvolumecyclewouldbemoreefficientthanthedieselifitcouldusecompressionratiosashighasthelatter.

ThegasolineengineoperatingontheOttocyclecannotuseacompressionratiocomparabletothedieselengineduetothefactthatthefuelandairaredrawnintogetherandcompressed.Ifhighcompressionratioswereused,

(H1H2)/H1whereH1istheheatinputalonglinesBCandCD(thelinesrepresentingtheconstantvolumeandconstantpressurecombustion),andH2istheheatrejectedalonglineEA(thelinerepresentingtheconstantvolumeexhaust).Sinceheatandtemperatureareproportionaltoeachother,thecycleefficiencyisactuallycomputedfrommeasurementsmadeofthetemperature.Thespecificheatofthemixtureinthecylinderiseitherknownorassumed,andwhencombinedwiththetemperature,theheatcontentcanbecalculatedatanyinstant.Thus,itisseenthattemperatureisameasureofheat,andthattheheatisproportionaltothetemperatureofthegas.

c.Volumetricefficiency.Thevolumetricefficiencyofanengineistheratioofthevolumethatwouldbeoccupiedbytheairchargeatatmospherictemperatureandpressuretothecylinderdisplacement(theproductofthe

Figure94.Temperatureentropydiagramofmodifieddieselcycle.

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areaoftheboretimesthestrokeofthepiston).Thevolumetricefficiencydeterminestheamountofairavailableforcombustionofthefuel,andhenceinfluencesthemaximumpoweroutputoftheengine.

Volumetricefficiencyisactuallythecompletenessoffillingofthecylinderwithfreshairatatmosphericpressure.Thevolumetricefficiencyofanenginemaybeincreasedbyenlargingtheareasofintakeandexhaustvalvesorports,andbyhavingallvalvesproperlytimed

calculatedaspreviouslyexplained,theindicatedthermalefficiencycanbecomputed.

Indicatedthermalefficiency=(IndicatedhpX42.42Btuperminuteperhp)/(RateofheatinputoffuelinBtuperminute)X100percent

Inlikemannertheoverallthermalefficiencycanbefoundfromthebrakehorsepowerortheactualpoweravailableattheengineshaft.*



sothatasmuchairaspossiblewillenterthecylinders.Sinceanyburnedgaseswillreducethechargeoffreshair,thesuperchargingeffectgainedbyearlyclosingoftheexhaustvalvesorportswillreducethevolumetricefficiency.Insomeengines,thevolumetricefficiencyisalsoincreasedbyusingspecialapparatustoutilizeairat2to3psiovertheatmosphericpressure.Thisprocedureiscommonlycalledsupercharging.

d.Thermalefficiency.Thermalefficiencymayberegardedasameasureoftheefficiencyandcompletenessofcombustionoftheinjectedfuel.Thermalefficienciesaregenerallyconsideredasbeingoftwokinds,indicatedthermalefficiencyandoverallthermalefficiency.

Ifallthepotentialheatinthefuelweredeliveredaswork,thethermalefficiencywouldbe100percent.Thisisnotpossibleinpractice,ofcourse.Todeterminethevaluesoftheaboveefficienciestheamountoffuelinjectedisknown,andfromitsheatingvalue,orBtuperpound,thetotalheatcontentoftheinjectedfuelcanbefound.Fromthemechanicalequivalentofheat(778footpoundsareequalto1Btu),thenumberoffootpoundsofworkcontainedinthefuelcanbecomputed.Iftheamountoffuelinjectedismeasuredoveraperiodoftime,therateatwhichtheheatisputintotheenginecanbeconvertedintopotentialpower.Then,iftheindicatedhorsepowerdevelopedbytheengineis

Overallthermalefficiency=Brakehorsepower/HeatinputoffuelX100percent

e.Mechanicalefficiency.Themechanicallossesinanenginedecreasetheefficiencyoftheengineandrepresenttheskillwithwhichtheenginepartsweredesignedaswellastheskillwithwhichtheoperatormaintainstheengine.Aspreviouslystated,thebrakehorsepowerisequaltotheindicatedhorsepowerminusthemechanicallosses.Theratioofbrakehorsepowertoindicatedhorsepower,then,isthemechanicalefficiencyoftheenginewhichincreasesasthemechanicallossesdecrease.

Mechanicalefficiency=Brakehorsepower/IndicatedhorsepowerX100percent

*Thispowerreferredtoasshafthorsepower,istheamountavailableforusefulwork.Itisthepoweravailableatthepropeller.Thereisafurtherlossofpowerbetweenthemainpropulsionengine(measuredasbrakehorsepower)andshafthorsepowerduetothefrictioninthereductiongears,hydraulicorelectrictypecouplings,lineshaftbearings,stuffingboxes,sterntubebearings,andstrutbearings.Theselossesinsomecasesareconsiderableandthetotallossmaybeashighas7or8percent.Therefore,theyshouldnotbeneglectedinmakingcomputations.

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B.ENGINEPERFORMANCE

9B1.Engineperformance.a.General.Manyfactorsaffecttheengineperformanceofanengine.Someofthesefactorsareinherentintheenginedesignotherscanbecontrolledbytheoperator.Thefollowinglistofvariableconditionsaffectingtheperformanceof

whichtheenginewilloperatewithasmokyexhaust.

f.Injectionrate.Therateofinjectionisimportantbecauseitdeterminestherateofcombustionandinfluencesengineefficiency.Injectionshouldstartslowly



adieselengineisnotcomplete,butcontainsalltheimportantfactorsthatshouldbefamiliartooperatingpersonnel.

b.Fuelcharacteristics.Thecetanenumberofthefuelhasanimportanteffectonengineperformance.Fuelswithlowcetaneratinghavehighignitionlag.Aconsiderableamountoffuelcollectsinthecombustionspacebeforeignitionoccurs,withtheresultthathighmaximumpressuresarereached,andthereisatendencytowardknocking.Thistendstoincreasewearoftheengineandreduceitsefficiency.Fuelswithhighcetaneratingshavelowautoignitiontemperaturesandhenceareeasierstartingthanfuelswithlowcetaneratings.Therefore,dieselengineperformanceisimprovedbytheuseofhighcetanenumberfueloils.

c.Airtemperature.Thetemperatureoftheairinthecylinderdirectlyaffectsthefinalcompressiontemperature.Ahighintaketemperatureresultsindecreasedignitionlagandfacilitateseasystarting,butisgenerallyundesirablebecauseitdecreasesthevolumetricefficiencyoftheengine.

d.Quantityoffuelinjectedperstroke.Thequantityoffuelinjecteddeterminestheamountofenergyavailabletotheengine,andalso(foragivenvolumetricefficiency)theairfuelratio.

e.Injectiontiming.Theinjectiontiminghasapronouncedeffectonengineperformance.Formanyengines,theoptimumisbetween5degreesto10degreesbeforetopdeadcenter,butitvarieswithenginedesign.Earlyinjectiontendstowardthedevelopmentofhighcylinderpressures,becausethefuelisinjectedduringapartofthecyclewhenthepistonismovingslowlyandcombustionisthereforeatnearlyconstantvolume.Extremeinjectionadvancewillcauseknocking.Lateinjectiontends"todecreasethemeanindicatedpressure(mip)oftheengineandtolowerthepoweroutput.Extremelylateinjectiontendstoward

sothatalimitedamountoffuelwillaccumulateinthecylinderduringtheinitialignitionlagbeforecombustionbegins.Itshouldproceedatsucharatethatthemaximumriseincylinderpressureismoderate,butitmustintroducethefuelasrapidlyaspermissibleinordertoobtaincompletecombustionandmaximumexpansionofthecombustionproducts.

g.Atomizationoffuel.Theaveragesizeofthefuelparticlesaffectstheignitionlagandinfluencesthecompletenessofcombustion.Smallsizedparticlesaredesirablebecausetheyburnmorerapidly.Opposedtothisrequirementisthefactthatsmallparticleshavealowpenetration,andthereisthereforeatendencytowardincompletemixingofthefuelandthecombustionair,whichleadstoincompletecombustion.

h.Combustionchamberdesign.Theamountofturbulencepresentinthecombustionchamberofanengineaffectsthemixingofthefuelandthecombustionair.Highturbulenceisanaidtocompletecombustion.

9B2.Power.Engineperformanceofaninternalcombustionenginemaybemeasuredintermsoftorque,orpowerdevelopedbytheengine.Thepowerthatanyinternalcombustionengineiscapableofdevelopingislimitedbymeaneffectivepressure,lengthofstroke,cylinderbore,andthespeedoftheengineinrevolutionsperminute(rpm).

a.Meanindicatedpressure.Theaverageormeanpressureexertedonthepistonduringeachexpansionorpowerstrokeisknownasthemeanindicatedpressure.Meanindicatedpressureisofgreatimportanceinenginedesign.Itcanbeobtainedfromindicatorcardsmathematicallyordirectlyfromtheplanimeter.Excessivemeanpressuresresultinoverloadingtheengineandconsequenthightemperatures.Temperaturesgreaterthanthosecontemplatedintheenginedesignmaycausecrackedcylinderheads,liners,andwarpedvalves.Therearetwokindsofmeaneffectivepressures.One,mip,or



incompletecombustion,asaresultof mean

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indicatedpressureisthatdevelopedinthecylinderandcanbemeasured.Theotherisbmeporbrakemeaneffectivepressureandiscomputedfromthebhpdeliveredbytheengine.

NOTE.Maximumpressuredevelopedhasnobearingonmep.

b.Lengthofstroke.Thedistancethepistontravelsfromonedeadcentertoitsoppositedeadcenterisknownasthelengthofstroke.Thisdistanceisoneofthefactorsthatdeterminesthepistonspeedwhichislimitedbythefrictionalheatgeneratedandtheinertiaofthemovingparts.Inmodernengines,pistonspeedreachesapproximately1600feetperminute.Ifthelengthofstrokeistooshort,excessivesidethrustwillbeexertedonatrunktypepiston.Thelengthofstroke,however,cannotbetoogreatbecauseofthelackofoverheadspaceavailableonsubmarinetypeengines.

c.Cylinderbore.Thecylinderboreisitsdiameter,andfromthisthecrosssectionalareaofthepistonisdetermined.Itisuponthisareathatthegaspressureactstocreatethedrivingforce.Thispressureisthemeanindicatedpressurereferredtoabove,expressedandcalculatedforanareaof1squareinch.Theratiooflengthofstroketocylinderboreissomewhatfixedinenginedesign.Thereareafewinstancesinwhichthestrokehasbeenlessthanthebore,butinalmosteverycasethestrokeislongerthanthebore.Thisratioinamoderntrunkpistontypeengineisabout1.25,whileinacrossheadtypeengineinusetodayitisabout1.50.

d.Revolutionsperminute.Thisisthespeedatwhichthecrankshaftrotates,andsincethepistonisconnectedtotheshaft,itdetermines,withthelengthofstroke,thepistonspeed.Sincethepistonmovesupanddowneach

singleacting,2strokecycleengine,thereisapowerstrokeforeachrevolution.

Havingdefinedthefactorsinfluencingthepowercapableofbeingdeveloped,thegeneralformulaforcalculatinghorsepowerisasfollows:

IHP=(PXLXAXN)/33,000

P=Meanindicatedpressure,inpsiL=Lengthofstroke,infeetA=EffectiveareaofthepistoninsquareinchesN=Numberofpowerstrokesperminute

Thehorsepowerdevelopedwithinthecylinderasaresultofcombustionofthefuelcanbecalculatedbymeasuringthemeanindicatedpressureandenginespeed.Thenwiththeboreandstrokeknown,thehorsepowercanbecomputedforthetypeofenginebeingused.Thispoweriscalledindicatedhorsepowerbecauseitisobtainedfromthepressuremeasuredfromanengineindicatorcard.Itdoesnottakeintoaccountthepowerlossduetofriction,aswillbediscussedlater.Example:

Givena12cylinder,2cycle,singleactingenginehavingaboreof8inchesandastrokeof10inches.Itsratedspeedis720rpm.Whenrunningatfullloadandspeed,themeanindicatedpressureismeasuredandisfoundtobe105psi.Whatistheindicatedhorsepowerdevelopedbytheengine?

Solution:

Fromtheformula

IHP=(PXLXAXN)/33,000

P=105L=10/12A=3.1416(8/2)2N=720

IHP=(105X(10/12)X3.1416(8/2)2)



revolution,thepistonspeedisequaltotwicethestroketimestherevolutionsperminute(rpm),andisusuallyexpressedinfeetperminute.Ifthestrokeis10inches,andthespeedofrotationis750rpm,thepistonspeedis

750X2X(12/10)=1,250feetperminute.

Thepowerdevelopedbytheenginedependsupontheengine'sspeedandthetypeofengine.Ifitisasingleacting,4strokecycleenginetherewillbeonepowerstrokeforeverytworevolutionsofthecrankshaft.Ifitisa

X720

IHP=96.96

Sincethisisjustthehorsepowerdevelopedinonecylinder,iftheloadisperfectlybalancedamongallcylinders,thetotalindicatedhorsepoweroftheengineis

IHP=12X96.96=1163.5

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e.Brakehorsepower.Asstatedabove,brakehorsepoweristhepowerdeliveredbytheengineindoingusefulwork.Numerically,itisequaltotheindicatedhorsepowerminusthemechanicallosses.

BHP=IHPminusthemechanicallosses.

Fromtheexampleabove,theIHPwasfoundtobe1163.5.Ifthebrakehorsepowerofthisenginewas900asdeterminedinatestlaboratory,thenthemechanicallosseswouldbe

1163.5900=263.5horsepower

or

(263.5/1163.5)X100=22.6percentoftheindicatedhorsepowerdevelopedinthecylinders

or90/1163.5=77.4percentmechanicalefficiency.

Enginepowerisfrequentlylimitedbythemaximummeanpressureallowed.Tofindthebmepoftheaboveengine,firstobtainthepowerdevelopedinonecylinder.Thus,

900/12=75.0bhp

Fromthegeneralformulafor

bedeterminedfromtheindicatedhorsepowerundervaryingconditionsofoperation.Itshouldbenotedthatasarule,indicatorcardstakenonengineshavingaspeedover450rpmarenotreliableandthereforenoindicatormotionsareprovided.

9B3.Engineperformancelimitations.Thepowerthatcanbedevelopedbyagivensizecylinderwhosepistonstrokeisfixedislimitedonlybythepistonspeedandthemeaneffectivepressure.Thepistonspeedislimitedbytheinertiaforcessetupbythemovingpartsandtheproblemoflubricationduetofrictionalheat.

Themeanindicatedpressureislimitedby:

1.Heatlossesandefficiencyofcombustion.

2.Volumetricefficiencyortheamountofairchargedintothecylinderandthedegreeofscavenging.

3.Completemixingofthefuelandairwhichrequiresfineatomization,sufficientpenetration,andaproperlydesignedcombustionchamber.

Thelimitingmeaneffectivepressures,bothbrakeandindicated,areprescribed



horsepower,

HP=(PXLXAXN)/33,000

75=PX(10/12)X3.1416X(8/2)2720/33,000

P=(75X33,000)/(10/12X3.1416X(8/2)2X720)

P=82.1psi

Hence,fortheaboveengineundertheconditionsstatedthebmepis82.1whilethemipis105psi.

Thebrakehorsepoweristhepoweravailableattheengineshaftforusefulwork.Brakehorsepowercannotusuallybemeasuredafteranengineisinstalledinservice,unlesstheenginedrivesanelectricgenerator.Thebrakehorsepowerisdeterminedbyactualtestsintheshopsofthemanufacturerbeforedeliveryoftheengine.Frictionallossesarequiteindependentoftheloadontheengine.Hence,unlessthebrakehorsepowerhasbeenmeasuredatvariousloadsandspeeds,themechanicallossescannot

bythemanufacturerortheBureauofShipsandshouldneverbeexceeded.Inadirectdriveship,themeaneffectivepressuresdevelopedaredeterminedbytherpmoftheshaft.Inelectricdriveships,thehorsepowerandmepcanbedeterminedreadilyfromtheelectricalreadings,takingintoaccountgeneratorefficiency.

Thedieseloperatorshouldrememberthatthetermoverloadingmeansexceedingthelimitingmeaneffectivepressure.

9B4.Operation.Allsubmarinetypedieselenginesareratedatagivenhorsepowerandagivenspeedbythemanufacturer.Thesefactorsshouldordinarilyneverbeexceededintheoperationoftheengine.Usingtheratedspeedandbhp,itispossibletodeterminearatedbmepwhicheachindividualcylindershouldneverexceed,otherwisethatcylinderwillbecomeoverloaded.Theratedbmepholdsonlyforratedspeed.Ifthespeedoftheenginedropsdownbelowratedspeed,thenthecylinderbmepwhichshouldnotbeexceededgenerallydropsdowntoalowervalueduetopropellercharacteristics.Thebmepshouldneverexceedthenormalmepatlowerenginespeed.Usuallyit

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shouldbesomewhatloweriftheenginespeedisdecreased.

Navytypeenginesaregenerallyratedhigherforemergencyusethanwouldnormallybethecasewithcommercialengines.TheeconomicalspeedformostNavytypedieselenginesisfoundtobeabout90percentofratedspeed.Forthisspeedtheoptimumloadconditionshavebeenfoundtobefrom70percentto80percentoftheratedloadoroutput.Thus,wespeakofrunningtheenginesatan8090combinationwhichwillgivetheenginepartsalongerlifeandwillkeeptheengineitselfmuchcleanerandinbetteroperatingcondition.The8090means

Dieselenginesdonotoperatewellatexceedinglylowbmepsuchasthatoccurringatidlingspeed.Thistypeofenginerunningtendstogumuppistons,rings,valves,andexhaustports.Ifanengineisrunatidlingspeedforlongperiodsoftime,itwillrequirecleaningandoverhaulmuchsoonerthanifithadbeenrunat50percentto100percentofload.

Someenginemanufacturersdesigntheirenginefuelsystemssothatitisimpassibletoexceedtheratedbmeptoanygreatextent.Thisisdonebylimitingthemaximumthrottleorfuelcontrolsettingbymeansofapositivestop.Thisregulatesthemaximumamountoffuel



thatwearerunningtheenginewith80percentofratedloadat90percentofratedspeed.

thatcanenterthecylinderandthereforethemaximumloadofthecylinder.

C.LOADBALANCE

9C1.Indications.Loadbalancemeanstheadjustmentoftheenginesothattheloadwillbeevenlydistributedamongallthecylindersoftheengine.Eachcylindermustproduceitsshareofthetotalworkdonebytheengineinordertohaveabalancedload.Iftheengineisdevelopingitsratedfullload,ornearlyso,andonecylinderormoreisproducinglessthanitsshareoftheload,theremainderofthecylindersobviouslymustbedoingmorethantheirshareofthetotalworkandhenceareoverloaded.

Anoverloadedconditionofanengine,orofoneormoreofitscylinders,maybeindicatedby:

1.Blacksmokeintheexhaust.

2.Highexhausttemperature.

3.Highlubricatingoilandcoolingwatertemperature.

4.Hotbearingsandhightemperaturesofotherengineparts(ingeneral,ahotrunningengine).

5.Excessivevibrationoftheengine.

6.Unusualsoundoftheengine.

Whenblacksmokeisobservedintheexhaustfromthemufflers,itisnotpossibletodetermineimmediatelywhethertheentireengineorjustoneofthecylindersisoverloaded.However,byopeningtheindicatorcocksontheindividualcylinders,thecoloroftheirexhaustscanbedetermined.

Hightemperaturesoftheexhaustgases

fromindividualcylindersindicateanoverloadedconditionofthesecylinders.Ahighcommonexhausttemperatureintheexhaustheaderindicatesaprobableoverloadingofthewholeengine.Theseconditionsareindicatedbypyrometersinstalledinallmodernengines.Aconstantcheckonthepyrometerreadingswillindicateaccuratelywhenanycylinderisfiringproperlyandcarryingitscorrectshareoftheload.Anysuddenchangeinthereadingoftheexhausttemperatureofanycylindershouldbeinvestigatedimmediately.Thedifferenceinexhausttemperaturesbetweenanytwocylindersshouldnotexceed25degreesFforawellbalancedengine.Howeveracertaintoleranceisallowedusually50degreesto75degreesispermissible.

Thermometersareprovidedinthelubricatingoilandcoolingwatersystems.Moderndieselengineshavethermometersinstalledinthecoolingsystemsofindividualcylinders.Anabnormalriseinanyofthesetemperaturesmayindicateanoverloadedconditionandshouldbeinvestigatedasquicklyaspossible.

Ingeneral,excessiveheatinanypartoftheenginemayindicateoverloading.Anoverheatedbearingmaybetheresultofoverloadingacylinder.Anabnormallyhotcrankcasecouldresultfromoverloadingtheengineasawhole.Excessivetemperaturesofsomeenginepartscanbecheckedbytouch.

Ifallcylindersarenotdoinganequal

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amountofwork,theforceexertedby outshouldbewithin10to20psiofeach



individualpistonswillbeunequal.Inthisevent,theunequalforcesmaycauseanuneventurningmomenttobeexertedonthecrankshaftandvibrationswillbesetup.Theskilledoperatorcantellbythefeelandthesoundofanenginewhenapoordistributionofloadexists.This,ofcourse,comesfromlongexperience,butitisimportantthatthebeginneravailhimselfofeveryopportunitytoobserveenginesrunningunderallconditionsofloadingandperformance.

9C2.Causesofunbalance.Intheprecedingsectionsomeofthegeneralcausesofunequalloaddistributionwerediscussed.Topreventunbalanceinanengine,theforemostconsiderationisthattheenginemustbeinexcellentmechanicalcondition.Aleakyvalveorfuelinjector,leakycompressionrings,oranyothersuchmechanicaldifficultieswillmakeitimpossiblefortheoperatortobalancetheloadunlesshesecurestheengineanddismantlesatleastapartofit.Therefore,theenginemustbeplacedinpropermechanicalconditionbeforetheloadcanbebalanced.

Sincetheheatofcompressionisreliedupontoignitethefuelinjectedinthedieselengine,theamountofthiscompressionmustbemaintainedwithinfixedlimits.Inordertohavethesametypeofcombustionineachcylinder,thedegreeofcompressioninallcylindersshouldbeapproximatelythesame.Forexample,lowcompressionpressureinonecylindermaypreventallthefuelfromburning,ormayevenpreventignitionofthefuelinthatcylinder.Thiswouldresultinareducedamountofworkornoworkbeingdonebythiscylinder.Thecommoncausesoflowcompressionare:

1.Stickingcompressionrings.

2.Excessiveringorcylinderwear.

3.Leakycylinderheadgasketorcrackedcylinderhead.

4.Leakyvalveincylinderhead.

otherinallcylindersofaproperlyadjustedengine.

Inordertohavetheloadequallydistributed,eachcylindermustreceivethesameamountoffuel.Itisherethattheeffectofanimproperlyadjustedfuelpumpisevident.Acylinderreceivingmorefuelthannecessaryforagivenloadwilldevelopmorepowerthanrequired.

Anyadjustmentofthefuelpumpmustbeundertakenonlybyapersonthoroughlyfamiliarwiththetypeofpumpbeingused.Heshouldfirstdeterminebeyondalldoubtthattheengineisinpropermechanicalcondition.Agreatmanyfactorsmaycausethecylindertofireunevenly.Someofthesecausesareacloggedorimproperlytimedfuelinjectionvalve,improperlytimedairintakeorexhaustvalve,airorwaterinthefuelsystem,improperrockerarmvalveclearance,dirtorotherforeignmatterinthefueloilwhichmaybepluggingupthestrainersandfilters,andanyotherfactorthatcontributestopoorcombustion.Ifacylinderisfiringincorrectly,alwayschecktheaboveconditionsbeforemakinganyadjustmentstothefuelpump.

Changingtheamountoffuelbeingdeliveredbyadjustingthepumpshouldbedoneonlywhenitiscertainthatthecauseofthetroubleisinthepump.Thispointcannotbeemphasizedtoostrongly.Forinstance,ifthefailureofacylindertofirecorrectlywasduetoacloggedfuelinjectionvalvetipandtheoperatorincreasedthefuelsupplytothecylinderwiththeintentionofincreasingthepowerdevelopedbythatparticularcylinder,theincreaseinfuelmightwashthevalvecleanandcausethecylindertobecomebadlyoverloadedfromtheexcessfuelsupplied.Thecorrectprocedurewouldhavebeentoreplacethecloggedinjectionvalvewithaspareandtocleantheonethatwasremoved.Thedecreaseinpowerdeliveredbyacylindermayalsobeduetosomeforeignmatterunderavalveorpistonring,andoncecleared,thecylinderwouldbecomeoverloadedifthefuelsupplyhad,inthemeantime,been



5.Crackedcylinderliner.

6.Excessiveclearancevolume.

Incorrectingthese,itisgenerallynecessarytoreplacethedefectivepart.However,insomecasessuchasastickingringorvalve,itisnecessaryonlytocleanthepartandreplaceit.Thesecoldcompressionpressureswithfuelcut

increased.

Theoperatorwhoalwaysmaintainshisplantingoodmechanicalconditionwillberequiredtomakefew,ifany,adjustmentstothe

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fuelsystemwhileitisrunning.Thefuelsupplytoanindividualcylindershouldnotbeadjusteduntilafteranexhaustivesearchhasrevealedthateveryotherconditionisnormalinallrespects.

Afteranoverhaulinwhichpistonringsofcylinderlinershavebeenrenewed,considerableadjustmentoftheenginemaybenecessary.Lubricatingoilwillleakbytheringsintothecombustionspaceuntilaftertheringshaveproperlyseated.Thecompressionwillalsoincreaseasthesealbetweentheringsandthelinerbecomesmoreeffective.Thelubricatingoilwillburninthecylinder,givinganincorrectindicationoffueloilcombustion,andifthepumphasbeenproperlysetwhentheenginewasstarted,theenginewillbeoverloaded,oratleastunbalanced.Asthecompressionrisestonormalpressure,thepowerdevelopedwillincreaseasalsowilltheconditionsofpressureandtemperatureunderwhichthecombustiontakesplace.Hence,whenanoverhaulhasbeencompleted,theenginemustbecarefullywatcheduntiltheringsareseated,andthecompressionsettothelevelspecifiedintheinstructionsforthattypeofengine.Thisadjustmentwillbefacilitatedbytheuseoffrequentcompressiontests.Iftheengineisnotfittedsothatthecompressioncanbereadilyvaried,theengineshouldberununderlightloaduntilitiscertainthattheringshaveseated.

9C3.Effectofunbalance.Ingeneral,

developedwithinacylinderisdirectlyproportionaltothepowerproducedbythatcylinder,anyincreaseinonewillcauseacorrespondingincreaseintheother.Hence,ifthepowerisnotevenlydistributedthroughoutthecylinders,themeanindicatedpressuresintheindividualcylinderswillvary.Temperaturevariesdirectlyasthepressure,sothatadecreaseinpressurewillresultinacorrespondingdecreaseintemperature,Thequalityofcombustionobtaineddependsupontheheat,andheatuponthetemperature,sothatwithadecreaseinpressure,combustionwillnotbesogoodasbefore.Thispoorcombustionwilllowerthethermalefficiency,andtheoutputoftheenginewillbereduced.

Ifanengineisdeveloping600bhp,anditsmechanicalefficiencyis80percent,theindicatedhorsepowerbeingdevelopedis750.Iftheenginehas10workingcylinders,eachcylindershouldbeproducing75indicatedhorsepower.Whenthisisnotthecasetheengineisunbalanced.Theeffectherewouldbetoincreasethemeanindicatedpressureofthosecylindersdoinglessthantheirshareofthework,andtodecreasethatofthosecylindersproducingmorethan75indicatedhorsepower.

Theturningmomentactingonthecrankshaftisproportionaltotheforceactingonthepiston.Thisforce,inturn,istheresultofthemeanindicatedpressuredevelopedinthecylinder.Iftheseforcesfromdifferentcylindersarenotequal,



theeffectofunbalanceisanoverheatedengine.Clearancesareestablishedbytheenginedesignertoallowforsufficientexpansionofmovingpartswhenoperatingatthedesignedtemperatures.Consequently,anengineoperatingattemperaturesinexcessofthoseforwhichitwasdesignedmaysuffermanycasualties.Excessiveexpansionofthemovingpartswillcauseseizuresandaburningupoftheengine.Ifthetemperaturesriseabovetheflashpointofthelubricatingoilvaporsinthecrankcase,anexplosionmayresult.Thehightemperaturesmaydestroythelubricatingoilfilmbetweenadjacentsurfacesofthemovingpartsandresultinfurtherincreasedtemperaturesduetotheincreasedfriction.Infact,theeffectisthesameasforoverheatingfromanycause.

Sincethemeanindicatedpressure

thereisanuneventurningmomentactingalongthelengthofthecrankshaft,andvibrationsresult.Thesevibrations,ifsufficientlysevere,mayshaketheenginelooseinitsfoundation,cracktheenginehousing,framework,andbedplate,destroythebearings,andevenbreakthecrankshaft.Itisobviousthatabadlyvibratingenginecanresultinseriousdamageandshouldbestoppedimmediately.

Toavoidalltheharmfuleffectsofoverloadingandunbalancingofload,theloadonadieselengineshouldbeequallydistributedamongtheworkingcylindersandnocylinder,ortheengineitself,shouldeverbeoverloaded.Inconclusion,thecorrectproceduretofollowinbalancinganengineis:

1.Maintaintheengineinpropermechanicalcondition.

184

2.Adjustthefuelsysteminaccordancewiththemanufacturer'sinstructions.

3.Operatetheenginewithinthetemperaturelimitsspecifiedintheinstructions.

4.Keepthecylindertemperaturesandpressuresasevenlydistributedaspossible.

5.Trainyourselftodetectabadconditionbythesensesoftouchandhearing.

D.ENGINEDYNAMICSANDVIBRATIONS

9D1.Balancing.Itisnotpossibletobalanceoutalltheforcesproducingvibrationinanengine.However,theprimaryorprincipalforcesmaybealmostentirelybalancedbytheadditionofweightstothecrankshaftorconnectingrodsattheproperplaces.Balancingbytheadditionofweightssoastocreateforcesequalandoppositetothoseofinertiaisknownascounterbalancing.Usually,aftercounterbalancing,therearestillsomesmallforcesremainingthathavenotbeencompletelybalancedout.Theseremainingforcesareproducedbythereciprocatingparts,sinceitispossibletocompletelycounterbalanceallprimaryrotatingforces.

excessivevibrationinservice.Thisisduetothelowspeedsusedwiththebalancingmachines.Dieselenginesintheservicemustoperateoverawidespeedrangeusually,andforthisreasontheyarenotaccepteduntilaftertheyhavebeentriedatallspeedsatwhichtheymustoperatewheninstalledinservice.

Inanyevent,allrotatingpartsoftheengineshouldbeasaccuratelybalancedaspossible.

9D2.Flywheels.Aflywheelstoresupenergy,theamountofwhichdependsupontherotatingspeed,theweight,andthediameterofthewheel.Inmostmarineenginesheavyflywheelsarenot



Allrotatingpartsaresubjectedtotwokindsofunbalance.Theyarecalledstaticunbalanceanddynamicunbalance.Theunbalancedconditioninbothcasescanbereadilydeterminedandcorrectedbycounterbalancing.

Astaticbalancingtestisconductedbyplacingthetwoendsoftherotatingpartonperfectlysmooth,horizontal,andparallelrails.Ifstaticallyunbalanced,thepartwillrollontherailsuntilitscenterofgravityreachesitslowestpositionandthenitwillcometorest.If,however,itscenterofgravityliesalongitsaxisitwillremainatrestwhenplacedinanyposition,anditistheninstaticbalance.

Itfrequentlyoccursthatthecenterofgravityofabodyliesinitsaxisofrotationbutthatitsirregularshapeorcompositiongeneratesadisturbingforcewhenthebodyisrotated.Inthiscasethebodywouldbeinstaticbalanceandindynamicunbalance.Ingeneral,beforebalancing,mostrotatingpartsareinbothstaticanddynamicunbalance.

Inallcases,completebalancingcanbeobtainedbyattachingweightstotherotatingbody,ifthepositionanddegreeofunbalancingareknown.Fordeterminingthisunbalanceallnavalshipyardsareequippedwithbalancingmachines.Experiencewithlargeandhighspeedmachineryhasshownthatbalancingmachinesshowgoodresultsbutdonotinsureagainst

necessary,astheotherrotatingmassesontheshaftservethesamepurpose.Thesemassesaretheclutchandgenerator,andwithalargenumberofcylindersfiring,thepowerstrokeissmoother,andthereislessneedforaflywheel.

Theflywheelservesthreepurposes,namely:

1.Topreventtheenginefromstallingwhenrunningatidlingspeed.

2.Toreducethevariationsinspeedatallloads.

3.Tohelpcarrytheengineovercenterswhenstarting.

Whenthespeedoftheshafttendstoincrease,theflywheelabsorbsenergy.Whenittendstodecrease,theflywheelgivesupitsenergytotheshaftinanefforttokeepitrotatingatauniformspeed.

9D3.Torsionalvibrations.Thetwistinganduntwistingoftheshaftsystemresultintorsionalvibrations.Allshaftshavesomeflexibilityandwithweightsattachedtothem,suchaspistons,gearsandcamshaftsindieselengines,theyhavewhatisknownasanaturalfixedfrequency.Whenthefrequencyofthepowerstrokeimpulsescoincideswiththenaturalfrequencyoftheentireshaftsystem,atorsionalvibrationisproduced,andtheshaftisthensaidtobe

185

rotatingatacriticalspeed.Thiscriticalspeedisdependentonthedimensionsofthecrankshaft,thenumberofcylinders,allrotatingmassesoftheengine,othershaftingandmassesincludingthepropeller,thenumberofpowerstrokesperminute,thearrangementofthecylinders(whethertheyareinlineorinaV),andthecylinderfiringorder.

bedplate,crankcase,orsimilarmembers,resultsinflexuralvibrations.Thecauseofflexuralvibrationliesinthefaultybalanceoftherotatingandreciprocatingmassesoftheengineandthepresenceofthesocalledfreeforcesorrockingcouples.Itmaybemanifestinthehorizontalorverticalplanesandmayinturnbethecauseofvibrationofsurroundingstructures,suchastheship's



Withoutgoingintofurtherdetail,itissufficienttosaythattorsionalcriticalspeedsdependuponthenumberofpowerimpulsesperrevolutionandthenaturalrateofvibrationofthecombinedshaftsystem.Specialinstrumentsareavailablefordeterminingthedegreeoftorsionalvibrationandthenaturalfrequencyofanyparticularshaftsystem.

Tochangetherangeorpointofmaximumvibrationofthecriticalspeedsforagiveninstallationitisnecessarytomakeachangeinthemassesontheelasticshaftsystem.Itisevidentthereforethatinenginesoperatingataconstantspeed,itismuchsimplertochangethenaturalfrequencyinordertoavoiddangerouscriticalspeedsthanitisinamarineenginerequiringawiderangeofoperatingspeeds.

Criticalspeedsandmodeofvibrationsaredeterminedwiththeaidofaninstrumentrecordingtorsionalvibrations.Theenginebuilderscalculatethecriticalspeedsandfurnishaguaranteethat,withtheenginecoupledtotheloadforwhichitisdesigned,nodangerouscriticalspeedswilloccurwithintheoperatingspeeds.

Torsionalvibrationsneednotnecessarilyshaketheframingoftheengineandmaynotevenbenoticeabletotheoperator.Thisfacthasbeenborneoutinseveralcasualtiesinwhichthecrankshaftbrokewithoutwarning.Excessivewearofgearsorofattachedauxiliariesandrepeatedbreakageofshaftingorotherpartsattachedtoitcanverywellbecausedbytorsionalvibrations.Mostinstallationsinnavalvesselshavebeencheckedandtestedtodeterminetheexactlocationoftorsionalvibrations,theiramplitudes,andfrequencies.

9D4.Flexuralvibrations.Thebendingofthepartsoftheengineframingsuchasthe

hullinmarineinstallations.Thistypeofvibrationdoesnotdependonthewaytheengineiscoupledtoitsload,andifanenginedoesnotvibrateontest,novibrationswilldevelopafteritisplacedinservice.

9D5.Torsionalvibrationdampening.Therearecertainforcesactinginresistancetotorsionalvibrations.Theseforcesareduetothefrictionofthebearingsthatcarrytheshaftingandtheworkabsorbedinthemetaloftheshaftinresistingthetwistingcalledhysteresis.Propellersinthewaterarethemostinfluentialfactor.Alloftheseforcesmaybesaidtobetheresultofnaturalcauses,andtheyacttodampenout,orreducetheamplitudeofthetorsionalvibrations.Inadditiontothesenaturalforces,thereareothermethodsemployedtoreduceoreliminatetheseverityofthevibrations.Thismaybeaccomplishedbychangingthefiringorderofthecylindersintheengine,orbychangingtherotatingweights,ortheflexibilityoftheshaftings.

Inadditiontotheabovedampeningfactorsandmethodstherearevarioustypesofcommercialtorsionalvibrationdampeners,suchasthatusedontheFM38D81/810cylinderengine.Eachsuchdampenermustbedesignedforaspecificshaftsystemoperatingwithaparticulartypeengine.Vibrationdampenersareusuallylocatedatornearapointofmaximumtorsionalvibrationamplitudealongtheshaft,generallyattheforwardendofanengine.

Thereareseveraldifferenttypesofdampeners.All,however,accomplishthesamepurpose.Theytendtoreducetheswingingmotionoftheshaft.Thisisaccomplishedbyhavingafreelyrotatingdiskordisksactingagainstafixeddiskwhichcreatesfrictionandtherebyactsasabrake.Thispreventstheshaft,fromtwistinganduntwistingwhilerotatingonitsaxis.



186

E.ENGINEPRESSUREINDICATOR

9E1.General.Efficientuninterruptedperformanceoftheenginedependsuponthemaintenanceofequalcorrectcompressionwithoutfuel,andfiringpressureswithfuelamongthevariouscylinders.Poorenginecompressioncauseslossofpower,pooracceleration,smokyexhaust,andstartingdifficulties.Anabnormallyhighfiringpressureinoneormorecylindersmaycauseenginewear,unevenrunning,andoverheating.Thesecompressionpressuresmaybemeasuredbyinstrumentsknownaspressureindicators.Compressionreadingswithoutfuelaretakenaftertheengineiswarmedupandthefuelcutoffonthatparticularcylinder.Firingpressurereadingsaretakenwiththeenginewarmedupandoperatingunderastatedloadatastatedspeed.

9E2.Typesofengineindicators.Therearetwogeneralclassesofenginepressurerecordingindicators.Inthefirst,theinstrumentmeasuresgraphicallythecylinderpressureandatthesametimeindicatesthepositionofthepistonatanypointofitsstrokeorcycle.Inotherwords,theindicatordrawsadiagramofthepressureinthecylinderwithrespecttothemovementofthepiston.Sincethemovementofthepistonisameasureofthevolumedisplaced,thediagramisdrawntotheordinatesofpressureandvolume.Inthesecondgeneralclass,theindicatorrecordsthemaximumpressuresonly.

Figure95showsthefundamentalprincipleoftheoperationofanengineindicatorinwhichthemovementofthepistonisrecorded.Theindicatorequipmentincludesasmallcylinderthatcanbeattachedtothemainworkingcylinderoftheengine,apistonandrodthatworkinthissmallcylinder,withapencilontheendoftherod.Thepencilpointbearsonthepapertackedtothedrumwhichismovedbyhook

horizontaldistancewillrepresentpistonmovement.Asanexample,inatwostrokecycleengine,onecompleterevolutionorcyclewouldproduceadiagramliketheoneshownintheillustration.Thisdiagramiscalledanindicatorcard.Iftheindicatorspringiscalibratedsothatthenumberofpoundsofpressurerequiredtoraisethepencil1inchisknown,thentoreadthepressureatanypointonthecardallthatisnecessaryistomeasurethedistanceininchesfromtheatmosphericlineXYonthediagramtothepointatwhichtheamountofpressureisdesired,andmultiplythisbythecalibrationnumberofthespring.Thetotallengthofthediagramrepresentsthestrokeofthepiston.Thishorizontalscalethencanbelaidoffininches,feet,pistonstroke,orvolumeofpistondisplacement.

Thistypeofindicatorislittleusedbyoperatingpersonnelonfleettypesubmarinestoday,mainlybecausethereisnoprovisionmadeonmodernenginesfortheattachmentoftheequipmentnecessarytotaketheindicatorcard,andalsobecausetherearenomeansofcompressionadjustmentotherthancompleteoverhauloftheengine.

Theothertypeofindicator(indicatingmaximumpressuresonly)isusedtosomeextentfortakingmaximumcylinderpressures,tocheckagainstmanufacturer'stestdataandpreviousshipboardpressuretests.ThetwomostcommonlyusedindicatorsofthistypearethePremaxindicatorandtheKieneindicator.

Themethodnormallyusedtochecktheequaldistributionofpoweramongthevariouscylindersistocomparetheexhaustgastemperaturesofthecylindersbymeansofthermocouplesplacedintheexhaustelbowsofeachcylinder.Pyrometerreadingshaveprovedtobeagoodcheckonthegeneralrunningconditionsofanengine,andtherecords



andstringoverapulley.Anypressureintheworkingcylinderenterstheindicatorcylinderandforcesthesmallindicatorpistonandpencilinaverticaldirectionatthesametimethemainpistonmovesthecardinahorizontaldirectionbymeansofthestringandpulley.

Itisreadilyseenthatanyverticaldistanceonthediagramwillrepresentpressure,andthe

ofexhaustgastemperaturesareofgreatvalueinconjunctionwithindicatorreadingsasaidsingettingthebestresultsfromadieselengine.However,eventhoughtheexhausttemperaturesarenormal,theengineattimesmaynotdevelopitsratedhorsepower.

9E3.Premaxindicator.ThePremaxindicatorisaninstrumentfordeterminingcylinder

187

Figure95.Principleofengineindicator.

Figure96.Premaxpressureindicator.

Figure97.Kienepressureindicator.

188

compressionandfiringpressures.The 9E4.Kieneindicator.TheKienediesel



indicatorconsistsessentiallyofapistonsubjecttocylinderpressure,aspringagainstwhichthepistonactsandthetensionofwhichisadjustablebymeansofanindexsleeve,acontrolswitch,andaneonlightcircuitthatshowsifthepistonismoving.Itisattachedtothecylinderindicatorcockinthesamewayasanyotherindicator.Thepressureactingononesideofthepistonintheindicatorisgraduallyincreasedbyincreasingthespringtensionwiththeindexsleeveuntilthisspringpressureisequaltothemaximumcylinderpressurewhichactsontheoppositesideofthepiston.Whenthetwopressuresareequal,thepistonstopsmoving,asshownbystoppingoftheneonlightflashes.Thepressurereadingisthenreadonthescalesleeve.

indicatorisapressureindicatorgageformeasuringthecompressionandfiringpressureofanenginewhileitisrunning.Thecompleteunitconsistsofapressuregageandanaircooledpressurechamberwhichisattachedtothecylinderindicatorcock.

Thecylinderdischargepassesthroughtheindicatorplugupthroughthefiller,screen,andseatpiece.Thisraisesthevalve,allowingthegastopassthroughthedrilledholesintheguidepieceintothepressurechamberandontothegage.Theactionofthegasinthecurvedtubeofthegagetendstostraightenthetube,therebymovingthegageneedleandrecordingthepressureonacalibratedscale.

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submarine main propulsion diesels - chapter 9

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