engineering science the life behind an internal combustion
TRANSCRIPT
ENGINEERING SCIENCE
LECTURE NOTES
The Life Behind anInternal Combustion EngineBY HAZEL WEE LING, ST HILDA’S COLLEGE, 2015–2019AND SHAUN TANG, UNIVERSITY COLLEGE, 2012–2016
The internal combustion engine ranks as one of the most important inventions ever made, providing controllable power from a truly portable unit since 1859. In this class, we will look at the theoretical thermodynamic cycles behind these machines – using knowledge of the First Law for a closed system – and at how real engines work.
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FirstLawforOpenSystemsIntroductionManythermodynamicmachinesmaybemodelledasopensystems.TheFirstLawofthermodynamics
forclosedsystemsisnotreadilyapplicabletosuchmachines,sowemustrecastitintoanewform,the
steadyflowenergyequation.Wewillalsomeetanewproperty,enthalpy,whichisparticularlyuseful
forthesecalculations.
SteadyFlowinOpenSystemsSteadyFlowEnergyEquationForaclosedsystemtheFirstLawstates
whereUistheinternalenergyofthesystemduetothermalmotionofthe
molecules.WecanextendthisconcepttoincludethetotalenergyEofthesystem,comprisinginternal,gravitationalandgrosskineticenergy,sothat
where
z=heightaboveanarbitrarylevelm
c=bulkvelocityofthefluidms-1.
ThusgzJkg-1(orkJkg-1)isthepotentialenergyperunitmass,and1/2c2Jkg
-1(orkJkg
-1)isthegross
kineticenergyperunitmass.Usuallyinclosedsystemstheinitialandfinalvaluesofzareequal,andthe
valuesofcarezero,sothatthetotalenergyEisthesameastheinternalenergyU.
Consideranopensystemwithasteadymassflowm˙kgs-1intoandoutofthe
systemboundary,asillustratedbelow.Theheatflowrateintothesystemis
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Nowconsiderwhathappensduringonesecond.AnelementoffluidAispushed
intothesystem,andanelementoffluidBisexpelledasshowninthefigure.The
systemboundarycanbeanalysedasaclosedsystembutwithamovingboundary.TheFirstLawforaclosedsystemgives
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NegligibletermsintheSFEE
Workinareversiblesteadyflowprocess
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istheareaunderap–vcurve,betweenthecurveandthevaxisoffigure(a).Nowp1v1andp2v2arethetworectanglesinfigure(b),sothattheshaftworkws
isgivenbyminustheareabetweenthecurveandthepaxisoffigure(c),oranalyticallyby
Steadyflowdevices
Compressor
Acompressororpumpisadeviceforincreasingthepressureofafluid,illustratedschematicallyinthe
figureabove.Thetermcompressorisgenerallyusedforgases,andthetermpumpisusedforliquids.
Inacentrifugalcompressorarotatingimpellerflingsthefluidradiallyoutward,thusincreasingits
pressure.Inanaxialcompressor,thefluidispushedbybladesonarotatingwheelthroughstationary
bladesintoasmallerspace.Areciprocatingcompressoroperatesinadifferentfashion.
Consideracompressorwhichtakesfluidatpressurep1anddeliversitatpressurep2wherep2>p1.Ifweassumethat
_thecompressorisadiabatic
_kineticandpotentialenergychangesarenegligible
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Turbine
Aturbineisadeviceforextractingworkfromahighpressurefluid,illustratedschematicallyinthe
figureabove.Thebasicprincipleisthatthefluidexpandsthroughstationarynozzlesorbladesand
impactsagainstbladesmountedonarotatingwheel.Thisprocesscanberepeatedmanytimesin
stages.Ateachstagethefluidlosessomeofitsoriginalpressure.Commonexamplesarethesteam
turbineandthegasturbine.
Consideraturbinewhichtakesfluidatpressurep1andexhaustsitatpressurep2wherep1>p2.Ifweassumethat
_theturbineisadiabatic
_kineticandpotentialenergychangesarenegligible
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Example1:
Problem:Asteadyflowpumpcompressesliquidwaterat20oCfrom0.0233barto100bar,asshownschematicallyin
figure7.13.Thepumpisreversible.Calculatetheworkrequiredperkgofwater.Ifthepumpisalsoadiabatic,calculatethe
changeinspecificenthalpyofthewater.
Solution:Liquidwaterisalmostincompressible,soonthep–vdiagramoffigure7.14thelinefrom1to2isalmostvertical.
Atlowtemperaturesthespecificvolumeofwateriseffectivelyconstantandequaltov=1lkg-1=10
-3m
3kg
-1.Iftheprocessisreversible,thentheshaftworkis
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Example2:
Problem:Aboilertakesinwateratapproximately20
ocand100bar,heatsittoboilingtemperature,
vaporisesit,andsuperheatsthesteamto500oC,asshowninthefigurebelow.Theboileroperatesat
constantpressure,soonthep–vdiagram,theline2to3ishorizontal.Calculatetheheatinputperkg
offluid.
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Example3:
Problem:Steamat5000Cand100barispassedthroughanadiabaticturbineandexhaustsata
pressureof0.0233barandadrynessfractionof0.8,asshowninthefigure.Calculatetheshaftwork
producedbytheturbineperkgoffluid.
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Onthep–vdiagram,thelineformsagentlecurvefrom3to4.Fromthepreviousexample,h3=3374.6kJkg
-1.Wecanwriteh4as
Example4:
Problem:Wetsteamat0.0233baranddryness0.8iscondensedtogivesaturatedwaterat200C,as
illustratedinfigureabove.Calculatetheheatreleasedbythecondensationperkgoffluid.
Solution:Thewetsteamiscondensedinacondenser.Typicallythewetsteamimpactsagainstasurface
cooledbycoldwater.Noshaftworkisdone,sows=0,
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Onthep–vdiagram,thelinefrom3to4ishorizontal.Fromthepreviousexamples,h4=2047.3kJkg-1andh5=83.9kJkg-1.Notethatthesaturationpressureat200Cis0.0233bar.Hence
CyclesRankineCycleItwillbeobviousthatthelastfourexamplesdescribeprocesseswhichcanbejoinedtoformacycle.Thiscycle,calledtheRankinecycle,isillustratedinthefigurebelow,andisthebasisofmostelectrical
generatingstations,suchastheDidcotpowerstation.Itconvertsheatintowork.
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Rankinecycleonap-vdiagramItisusefultorepresenttheRankinecycleonap–vdiagramasinthefigureabove.Labeltheend
pointsofeachprocessclearlyandmarkarrowstoshowthedirectionofthecycle.Notethearrowsgo
clockwisearoundthecycle.Theprocessesare1→2feedpump,2→3boiler,3→4turbineand4→1
condenser.Notethatintheboilerandthecondenser,thepressureremainsconstant.Thefeedpump
lineisalmostverticalasthespecificvolumevdecreasesonlyslightlyfrom1→2.Point1liesonthe
saturatedliquidline,point2isinthesubcooledregion,point3inthesuperheatregion,andpoint4
insidethesaturatedliquidandvapourregion.
Internalcombustionengines
IntroductionTheinternalcombustionengine,initstwocommonforms,thespark-ignitionengineandthe
compression-ignitionorDieselengine,mustrankasoneofthemostimportantinventionsevermade,
providingcontrollablepowerfromatrulyportableunit.Inthischapterwewilllookatthetheoretical
cyclesbehindthesemachines,usingourknowledgeoftheFirstLawforaclosedsystem,andalsoat
howrealengineswork.
IdealisedCycles(AirStandardCycles)WhenweconsideredtheRankinecycle,theworkingfluidateverypointinthecyclewassteam,or
water,apuresubstance.Thesamewatersimplywentaroundandaroundthecycle,undergoingeach
processinturn,inaclosedsystem.Theinternalcombustionengineisratherdifferent.
Thespark-ignitionenginedrawsairfromtheatmosphereintoachamber,mixesitwithfuel,
compressesthefuel-airmixture,ignitesitwithaspark,allowstheresultinghotgasestoexpandanddo
work,thenexpelsthegasesintotheatmosphere.
Thecompression-ignitionengineisverysimilarexceptthattheairaloneis
compressed,afterwhichthefuelisinjected,ignitingspontaneously.Thustherearetwofeatureswhich
areimmediatelyapparent.
- Theinternalcombustionengineisnotstrictlyaclosedsystem,asairisdrawninandlater
expelled,alongwithcombustionproducts.True,thatsameaircouldeventuallybedrawnintotheengineagain,makingitacycleasillustratedinthefigurebelow,butthisisstretchingthe
definitionsomewhat.
- Theworkingfluidisnotapuresubstance,andinfactchangesconsiderablyinitscomposition
andthermalproperties,fromprocesstoprocess.Furthermore,themassofworkingfluid
containedwithintheenginevarieswithtime.Thecombustionprocessinarealengineisnotthe
simplestoichiometricequationyouhaveseeninthelastchapter,butisverycomplicated,with
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theformationofmanyotherproducts,suchasoxidesofnitrogenduetothehightemperatures,
andhydrocarbonsduetoprematurequenchingofthecombustionontherelativelycold
chamberwalls.
Tosimplifytheanalysisoftheinternalcombustionengine,weusetheairstandardcycle,inwhichtheworkingfluidisassumedtobeair,whichcirculateswithintheengineasinatrueclosedsystem.This
givesagoodfirstapproximation,becausetheprincipalconstituentofairisN2,whichremains
unchangedduringthecycle(exceptfortheformationofsmallamountsofNOandNO2).
InternalcombustionengineOttoCycleTheOttocycleisanairstandardcycleapproximatingthespark-ignitionengine
cycle.Therearefourprocesses,asillustratedinthefigureabove.
- 1→2representsreversibleadiabaticcompressionoftheair,requiringworkWin,frompressure
p1andvolumeV1top2andV2- 2→3representstheadditionofheatQintotheairatconstantvolumeV2=V3,raisingthe
pressuretop3- 3→4representsreversibleadiabaticexpansionoftheair,producingworkWout,topressure
p4andvolumeV4=V1- 4→1representstherejectionofheatQoutfromtheairatconstantvolumeV4=V1,reducing
thepressuretop1.
FromtheFirstLaw,wecanrelatetheworkandheattermsby
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Butthecompressionandexpansionprocesses,1→2and3→4,areboth
reversibleandadiabatic,soiftheairbehavesasanidealgas,canberepresentedby
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DieselCycleTheDieselcycleisanairstandardcycleapproximatingthecompression-ignition
enginecycle.Therearefourprocesses,asillustratedinthefigurebelow.
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GasTurbineEnginesIntroductionThegasturbineenginehasrevolutionisedairtravelwithitsamazingpowerto
weightratio.Inthischapterwewilllookatthetheoreticalcyclebehindthegas
turbine,andalsohowarealgasturbineengineworks.
IdealisedCycle(AirStandardCycles)Asforthediscussionofinternalcombustionenginecycles,weuseairstandardcycleswhenwefirstconsidergasturbineengines.Thatis,weassumethatthe
workingfluidisair,whichcirculateswithintheengineasinatrueclosedsystem.
ThisgivesagoodapproximationbecausetheprincipalconstituentofairisN2.
JouleCycleTheJoulecycleisanairstandardcycleapproximatingthegasturbineengine.
Therearefourprocessesasillustratedinthefigurebelow.
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Thecompressionandexpansionprocesses,1→2and3→4,arebothreversibleandadiabatic,soif
theairbehavesasanidealgas,theycanberepresentedusingthepolytropiclawby
Joulecycleefficiencyvspressureratio
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ThusthethermalefficiencyoftheairstandardJoulecycleisdeterminedsolelybythepressureratio.
Increasingthepressureratiorpincreasesthethermalefficiency.
Thisisillustratedforγ=1.4infigurethefigureabove.Notethattheefficiencyfallstozeroatrp=1.Theworkratiorwisdefinedastheratioofthenetworkouttotheturbinework
Schematicdiagramofagasturbineengine
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Additionalnoteson• Behaviourofsteam-watermixtures• PolytropicProcesses
*Engineering Tables and Data, known affectionately as HLT after the initials of its authors, has been the primary reference for generations of Oxford University engineering students.
Behaviourofsteam-watermixturesSaturatedTableinHLTWhenwaterandsteamareinequilibriumwitheachother,wesaythatthesteamissaturated.Tofindthepropertiesofsuchsteam–watermixturesweusethesaturatedtableinHLTonpages54–65.Pages54–55aretabulatedasafunctionoftemperatureandpages56–65asafunctionofpressure.
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SuperheatedsteamSuperheatedsteaminHLT
PolytropicprocessesPolytropicequations
Polytropicprocessesonp-vdiagram
whereniscalledthepolytropicindex.Figure6.14showstheformofthis