submarine main propulsion diesels - chapter 9
DESCRIPTION
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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
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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.
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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.
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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
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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.*
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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
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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
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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)
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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
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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
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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
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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
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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,
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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.
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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
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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
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rotatingatacriticalspeed.Thiscriticalspeedisdependentonthedimensionsofthecrankshaft,thenumberofcylinders,allrotatingmassesoftheengine,othershaftingandmassesincludingthepropeller,thenumberofpowerstrokesperminute,thearrangementofthecylinders(whethertheyareinlineorinaV),andthecylinderfiringorder.
bedplate,crankcase,orsimilarmembers,resultsinflexuralvibrations.Thecauseofflexuralvibrationliesinthefaultybalanceoftherotatingandreciprocatingmassesoftheengineandthepresenceofthesocalledfreeforcesorrockingcouples.Itmaybemanifestinthehorizontalorverticalplanesandmayinturnbethecauseofvibrationofsurroundingstructures,suchastheship's
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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.
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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
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andstringoverapulley.Anypressureintheworkingcylinderenterstheindicatorcylinderandforcesthesmallindicatorpistonandpencilinaverticaldirectionatthesametimethemainpistonmovesthecardinahorizontaldirectionbymeansofthestringandpulley.
Itisreadilyseenthatanyverticaldistanceonthediagramwillrepresentpressure,andthe
ofexhaustgastemperaturesareofgreatvalueinconjunctionwithindicatorreadingsasaidsingettingthebestresultsfromadieselengine.However,eventhoughtheexhausttemperaturesarenormal,theengineattimesmaynotdevelopitsratedhorsepower.
9E3.Premaxindicator.ThePremaxindicatorisaninstrumentfordeterminingcylinder
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Figure95.Principleofengineindicator.
Figure96.Premaxpressureindicator.
Figure97.Kienepressureindicator.
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compressionandfiringpressures.The 9E4.Kieneindicator.TheKienediesel
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indicatorconsistsessentiallyofapistonsubjecttocylinderpressure,aspringagainstwhichthepistonactsandthetensionofwhichisadjustablebymeansofanindexsleeve,acontrolswitch,andaneonlightcircuitthatshowsifthepistonismoving.Itisattachedtothecylinderindicatorcockinthesamewayasanyotherindicator.Thepressureactingononesideofthepistonintheindicatorisgraduallyincreasedbyincreasingthespringtensionwiththeindexsleeveuntilthisspringpressureisequaltothemaximumcylinderpressurewhichactsontheoppositesideofthepiston.Whenthetwopressuresareequal,thepistonstopsmoving,asshownbystoppingoftheneonlightflashes.Thepressurereadingisthenreadonthescalesleeve.
indicatorisapressureindicatorgageformeasuringthecompressionandfiringpressureofanenginewhileitisrunning.Thecompleteunitconsistsofapressuregageandanaircooledpressurechamberwhichisattachedtothecylinderindicatorcock.
Thecylinderdischargepassesthroughtheindicatorplugupthroughthefiller,screen,andseatpiece.Thisraisesthevalve,allowingthegastopassthroughthedrilledholesintheguidepieceintothepressurechamberandontothegage.Theactionofthegasinthecurvedtubeofthegagetendstostraightenthetube,therebymovingthegageneedleandrecordingthepressureonacalibratedscale.
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