Application of turbochargers in spark ignition passenger vehicles

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(1) . APPLICATION OF TURBOCHARGERS IN SPARK IGNITION PASSENGER VEHICLES By . Wallace William Bester Thesis presented at the University of Stellenbosch in partial fulfilment of the requirements for the degree of . Master of Science in Mechanical Engineering Department of Mechanical Engineering Stellenbosch University Private Bag X1, 7602 Matieland South Africa . Supervisor: Dr A. B. Taylor Co‐supervisor: Dr C. Scheffer April 2006 .

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(3) DECLARATIONȱ I,ȱtheȱundersigned,ȱherebyȱdeclareȱthatȱtheȱworkȱcontainedȱinȱthisȱthesisȱisȱmyȱ ownȱworkȱandȱthatȱIȱhaveȱnotȱpreviouslyȱinȱitsȱentiretyȱorȱinȱpartȱsubmittedȱitȱ atȱanyȱuniversityȱforȱaȱdegree.ȱ ȱ Signature:……………………………………………..ȱ ȱ Date:……………………………………………….…..ȱ ȱ ȱ. ȱ. i.

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(5) ABSTRACTȱ Theȱquestȱforȱhigherȱefficiencyȱofȱtheȱinternalȱcombustionȱengineȱwillȱalwaysȱ beȱ pursued.ȱ ȱ Increasinglyȱ stringentȱ emissionȱ regulationsȱ areȱ forcingȱ manufacturersȱ toȱ downsizeȱ onȱ engineȱ displacementȱ andȱ increaseȱ specificȱ power.ȱȱByȱaddingȱaȱturbocharger,ȱtheȱairflowȱthroughȱtheȱengineȱandȱhenceȱ theȱspecificȱpowerȱcanȱbeȱincreased.ȱȱȱ ȱ Theȱ advantagesȱ ofȱ aȱ smallȱ turbochargedȱ engineȱ overȱ aȱ naturallyȱ aspiratedȱ (NA)ȱ engineȱ ofȱ similarȱ powerȱ isȱ thatȱ itȱ isȱ lighter,ȱ havingȱ betterȱ partȱ loadȱ efficiencyȱ whenȱ operatingȱ atȱ theȱ sameȱ load,ȱ whileȱ producingȱ lessȱ emissions.ȱȱ Componentȱ sharing,ȱ increasedȱ productionȱ volumeȱ andȱ lowerȱ developmentȱ costsȱ areȱ furtherȱ possibleȱ advantagesȱ thatȱ turbochargingȱ couldȱ holdȱ forȱ theȱ manufacturer.ȱ ȱ TheȱobjectiveȱinȱthisȱstudyȱwasȱtoȱdetermineȱtheȱaccuracyȱwithȱwhichȱaȱoneȬ dimensionalȱ flowȱ simulationȱ packageȱ canȱ predictȱ theȱ performanceȱ ofȱ aȱ NAȱ andȱturbochargedȱengine.ȱȱTheȱimplicationsȱofȱaddingȱaȱturbochargerȱtoȱaȱNAȱ engineȱwereȱalsoȱinvestigated.ȱ ȱ Differentȱ exhaustȱ manifoldȱ conceptsȱ wereȱ evaluatedȱ forȱ theȱ turbochargedȱ engine.ȱȱAȱNAȱengineȱwasȱturbochargedȱandȱitsȱperformanceȱwasȱcomparedȱ toȱ theȱ simulatedȱ results.ȱ ȱ Simulationȱ predictedȱ theȱ actualȱ NAȱ engineȱ performanceȱtoȱwithinȱ6%ȱandȱtheȱactualȱturbochargedȱengineȱperformanceȱtoȱ withinȱ 9%ȱ whenȱ developingȱ theȱ sameȱ boostȱ pressure.ȱ ȱ Turbochargingȱ increasedȱ theȱ maximumȱ powerȱ byȱ 37%ȱ andȱ theȱ torqueȱ byȱ 48%.ȱ ȱ 82%ȱ ofȱ theȱ maximumȱ torqueȱ wasȱ availableȱ fromȱ 2000ȱrev/minȱ upȱ toȱ 5500ȱrev/min.ȱ ȱ Theȱ turbochargedȱengineȱcouldȱmatchȱtheȱfuelȱefficiencyȱofȱtheȱNAȱengineȱbothȱatȱ fullȱloadȱandȱpartȱload.ȱȱThusȱitȱisȱjustifiedȱthatȱaȱturbochargerȱcanȱbeȱusedȱtoȱ increaseȱ theȱ specificȱ powerȱ whileȱ stillȱ maintainingȱ partȱ loadȱ efficiency.ȱȱ Howeverȱ turbochargingȱ doesȱ increaseȱ theȱ mechanicalȱ loadsȱ onȱ theȱ engineȱ components,ȱtheȱextentȱofȱwhichȱwasȱquantified.ȱ ȱ ItȱwasȱfoundȱthatȱoneȬdimensionalȱanalysisȱisȱaȱvaluableȱtoolȱforȱtheȱuseȱinȱtheȱ applicationȱofȱturbochargersȱonȱSIȱengines.ȱ. ȱ. ii.

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(7) OPSOMMINGȱ Daarȱ salȱ altydȱ gestreefȱ wordȱ omȱ dieȱ effektiwiteitȱ vanȱ binnebrandenjinsȱ teȱ verhoog.ȱ ȱ Dieȱ toenemendeȱ strengȱ uitlaatgasȱ regulasiesȱ verpligȱ vervaardigersȱ omȱ enjinsȱ seȱ verplasingȱ teȱ verminder,ȱ maarȱ terselfdertydȱ dieȱ spesifiekeȱ kraguitsetȱ teȱ verhoog.ȱȱDieȱ toevoegingȱvanȱ’nȱ turboȬaanjaerȱkanȱ dieȱ lugvloeiȱ deurȱdieȱenjinȱvermeerderȱenȱdienooreenkomstigȱookȱdieȱkragȱuitset.ȱȱȱ ȱ Dieȱ voordeleȱ vanȱ ’nȱ kleinȱ turboȬaangejaagdeȱ (TA)ȱ enjinȱ teenoorȱ ’nȱ onaangejaagdeȱ(OA)ȱenjinȱmetȱgelykeȱwerkverrigtingȱisȱdatȱdieȱTAȱenjinȱligterȱ is,ȱ beterȱ deellasȱ effektiwiteitȱ hetȱ wannerȱ beideȱ byȱ dieselfdeȱ lasȱ toestandȱ opereerȱ terwylȱ ditȱ minderȱ emissiesȱ vrystel.ȱ ȱ Onderdeelȱ deling,ȱ verhoogdeȱ produksieȱ volumeȱ enȱ laerȱ ontwikkelingsȱ kosteȱ isȱ moontlikeȱ voordeleȱ watȱ turboȬaanjagingȱvirȱdieȱvervaardigersȱkanȱinhou.ȱ ȱ Dieȱ doelwitȱ wasȱ omȱ dieȱ akkuraatheidȱ teȱ bepaalȱ waarmeeȱ ’nȱ 1Ȭdimensioneleȱ vloeiȱanaliseȱpakketȱdieȱwerkverrigtingȱvanȱ’nȱOAȱenȱTAȱenjinȱvoorspelȱkanȱ word.ȱȱDieȱimplikasiesȱvanȱdieȱtoevoegingȱvanȱ’nȱturboȬaanjaerȱopȱ’nȱOAȱenjinȱ isȱookȱondersoek.ȱ ȱȱ Verskillendeȱ uitlaatȱ spruitstukȱ konsepteȱ isȱ geëvalueerȱ virȱ dieȱ TAȱ enjinȱ metȱ behulpȱvanȱdieȱ1Ȭdimensioneleȱsimulasieȱpakket.ȱȱ’nȱOAȱenjinȱisȱomgebouȱnaȱ ’nȱ TAȱ enjinȱ enȱ dieȱ gemeteȱ werkverrigtingȱ isȱ vergelykȱ metȱ dieȱ voorspeldeȱ resultate.ȱ ȱ Dieȱ simulasieȱ hetȱ dieȱ werkverrigtingȱ vanȱ dieȱ OAȱ enjinȱ metȱ ’nȱ akkuraatheidȱvanȱ6%ȱvoorspel.ȱȱDieȱwerkverrigtingȱvanȱdieȱTAȱenjinȱisȱmetȱ‘’nȱ akkuraatheidȱvanȱ9%ȱvoorspelȱwanneerȱdieselfdeȱaanjagingsȱdrukȱontwikkelȱ is.ȱ ȱ TurboȬaanjagingȱ hetȱ dieȱ maksimumȱ drywingȱ verhoogȱ metȱ 37%ȱ enȱ dieȱ wringkragȱmetȱ48%.ȱȱ82%ȱvanȱdieȱmaksimumȱwringkragȱisȱbeskikbaarȱvanafȱ 2000ȱopm.ȱtotȱbyȱ5500ȱopm.ȱȱDieȱTAȱenjinȱhetȱdieȱbrandstofȱverbruikȱvanȱdieȱ OAȱ enjinȱ geȬewenaarȱ beideȱ byȱ deelȱ lasȱ sowelȱ asȱ vollas.ȱ ȱ Dieȱ turboȬaanjaerȱ verhoogȱegterȱdieȱmeganieseȱbelastingȱopȱdieȱkomponenteȱenȱhierdieȱtoenameȱ isȱgekwantifiseer.ȱ ȱ Ditȱ isȱ bevindȱ datȱ 1Ȭdimensioneleȱ simulasieȱ ’nȱ nuttigeȱ hulpmiddelȱ isȱ inȱ dieȱ implementeringȱvanȱturboȬaanjaersȱopȱvonkontstekingsȱenjins.ȱȱȱ. ȱ. iii.

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(9) ACKNOWLEDGEMENTSȱ Iȱ wouldȱ likeȱ toȱ thankȱ theȱ followingȱ peopleȱ forȱ theirȱ involvementȱ inȱ theȱ completionȱofȱthisȱproject:ȱ x Drȱ Andrewȱ Taylor,ȱ myȱ supervisor,ȱ forȱ hisȱ guidanceȱ andȱ motivationȱthroughoutȱmyȱstudies;ȱ x Allȱ CAEȱ personnelȱ forȱ theirȱ helpȱ andȱ contributions,ȱ andȱ especiallyȱGerhardȱLourensȱforȱhisȱassistanceȱinȱtheȱlaboratory;ȱ x Antonȱ vanȱ denȱ Bergȱ atȱ SMDȱ forȱ hisȱ patienceȱ andȱ accuracyȱ inȱ manufacturingȱcomponentsȱneededȱforȱthisȱproject.ȱ ȱ ȱ. ȱ. iv.

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(11) TABLEȱOFȱCONTENTSȱ ȱ. DECLARATION ..................................................................................... i ABSTRACT .............................................................................................ii OPSOMMING.......................................................................................iii ACKNOWLEDGEMENTS.................................................................. iv TABLEȱOFȱCONTENTS ....................................................................... v LISTȱOFȱFIGURES...............................................................................vii LISTȱOFȱTABLES .................................................................................. xi LISTȱOFȱSYMBOLSȱANDȱABBREVIATIONS ..............................xii 1. INTRODUCTION............................................................................1 2. PROJECTȱOBJECTIVESȱANDȱOVERVIEW ..............................3 3. LITERATUREȱREVIEW ..................................................................7 3.1.. Supercharging.............................................................................................7. 3.2. Turbocharging ............................................................................................8 3.2.1. TurbochargerȱTheory........................................................................10 3.2.2. TurbochargingȱCIȱorȱSIȱengines ......................................................16 3.2.3. EnergyȱAvailableȱinȱtheȱExhaustȱGas.............................................18 3.2.4. ConstantȱPressureȱTurbocharging..................................................20 3.2.5. PulseȱTurbocharging ........................................................................22 3.2.6. PulseȱConvertersȱinȱTurbochargerȱApplications..........................26 3.3. EngineȱManagementȱSystems................................................................28 3.3.1. ElectronicȱThrottleȱControl ..............................................................29 3.3.2. TorqueȬBasedȱEngineȱManagement ...............................................29 3.3.3. BoostȱControl .....................................................................................31 3.4. EngineȱPerformanceȱSimulation ...........................................................33 3.4.1. FlowȱModelling .................................................................................34 3.4.2. CombustionȱModelling ....................................................................35 3.4.3. ModellingȱofȱCompressorsȱandȱTurbines......................................37. 4. ENGINEȱSIMULATION...............................................................40 4.1.. EngineȱSimulationȱModelȱ–ȱ1.6ȱlitreȱFordȱRocam..............................40. 4.2. EngineȱOptimisation ...............................................................................41 4.2.1. ModellingȱStrategyȱforȱWastegateȱControl ...................................43 4.2.2. ExhaustȱManifoldȱSimulation..........................................................45 4.2.3. ValveȱTimingȱOptimisation .............................................................46 4.3. ȱ. ExhaustȱManifoldȱConceptȱEvaluation................................................53 v.

(12) 5. EXPERIMENTALȱAPPARATUS.................................................58 5.1. ExhaustȱManifoldȱDesign.......................................................................58 5.1.1. Pulseȱinterference..............................................................................60 5.1.2. ExhaustȱManifold:ȱConceptȱ1 ..........................................................61 5.1.3. ExhaustȱManifold:ȱConceptȱ2 ..........................................................64 5.2. IntakeȱPipingȱDesign ..............................................................................64 5.2.1. PreȬCompressorȱPipe........................................................................65 5.2.2. PostȬCompressorȱPipe ......................................................................66 5.3.. VariableȱWastegateȱActuatorȱDesign ...................................................67. 5.4.. OilȱFeedȱandȱReturnȱlines ......................................................................70. 5.5.. FuelȱSystemȱUpgrade ..............................................................................72. 5.6.. ExhaustȱSystemȱUpgrade ........................................................................73. 5.7. ExperimentalȱSetȬup ................................................................................73 5.7.1. CombustionȱAnalysis .......................................................................74 5.7.2. PowerȱCorrection ..............................................................................77 5.7.3. ExhaustȱGasȱMeasurement ..............................................................78 5.7.4. EngineȱCalibration ............................................................................81. 6. RESEARCHȱRESULTS..................................................................82 6.1.. NAȱResults:ȱSimulationȱversusȱExperiments .....................................82. 6.2.. TurbochargedȱResults:ȱSimulationȱversusȱExperiments ..................90. 6.3. ComparisonȱofȱtheȱNAȱandȱTurbochargedȱResults.........................101 6.3.1. ComparisonȱofȱTurbochargedȱBoostȱSettings .............................101 6.3.2. ForceȱAnalysis .................................................................................105 6.3.3. EnergyȱBalance ................................................................................111 6.3.4. PerformanceȱComparison ..............................................................116 6.3.5. PartȬloadȱComparison ....................................................................129. 7. CONCLUSION .............................................................................137 8. RECOMMENDATIONS ............................................................140 9. REFERENCES ...............................................................................142 APPENDIXȱA OPTIMISATIONȱALGORITHMS .....................145 A.1.. NelderȬMeadȱAlgorithm .......................................................................145. A.2.. InitialȱValueȱScalingȱforȱOptimisation ..............................................146. APPENDIXȱB TURBOCHARGERȱOILȱFLOW ...........................147 APPENDIXȱC POWERȱCORRECTIONȱFACTORS...................148 APPENDIXȱD FORCEȱANALYSIS ...............................................149 ȱ ȱ. ȱ. vi.

(13) LISTȱOFȱFIGURESȱ ȱ Figureȱ2Ȭ1ȱȱEngineȱOutputȱTarget.............................................................................4 Figureȱ3Ȭ1ȱAutomotiveȱTurbochargerȱ(Venter,ȱ1999) ............................................8 Figureȱ3Ȭ2ȱComponentsȱofȱaȱRadialȱCompressorȱ(Sayers,ȱ1990) ........................11 Figureȱ3Ȭ3ȱhȬsȱDiagramȱforȱaȱRadialȱCompressorȱ(Watsonȱ&ȱJanota,ȱ1984) .....13 Figureȱ3Ȭ4ȱComponentsȱofȱaȱRadialȱTurbineȱ(Watsonȱ&ȱJanota,ȱ1984) .............15 Figureȱ3Ȭ5ȱhȬsȱdiagramȱforȱaȱradialȱturbineȱ(Watsonȱ&ȱJanota,ȱ1984)................15 Figureȱ 3Ȭ6ȱ Naturallyȱ Aspiratedȱ Idealȱ Limitedȱ Pressureȱ Cycleȱ (Watsonȱ &ȱ Janota,ȱ1984) .......................................................................................................18 Figureȱ 3Ȭ7ȱ Turbochargedȱ Idealȱ Pressureȱ Limitedȱ Cycleȱ (Watsonȱ &ȱ Janota,ȱ 1984) ....................................................................................................................19 Figureȱ3Ȭ8ȱSchematicȱofȱBirmannȱpulseȱconverterȱ(Watsonȱ&ȱJanota,ȱ1984) ....26 Figureȱ3Ȭ9ȱExhaustȱmanifoldȱwithȱpulseȱconverterȱ(Watsonȱ&ȱJanota,ȱ1984) ..28 Figureȱ3Ȭ10ȱComponentsȱofȱME7ȱ(Gerhardtȱetȱal.,ȱ1998)......................................30 Figureȱ3Ȭ11ȱConventionalȱBoostȱControlȱLayoutȱ(AudiȱAG,ȱ1998)....................32 Figureȱ3Ȭ12ȱTypicalȱElectronicȱBoostȱControlȱLayoutȱ(AudiȱAG,ȱ1998)............32 Figureȱ3Ȭ13ȱ3ȬwayȱSolenoidȱValveȱ(Normallyȱopen) ...........................................33 Figureȱ3Ȭ14ȱWiebeȱCombustionȱCurveȱShape.......................................................36 Figureȱ3Ȭ15ȱWiebeȱCumulativeȱCombustionȱCurveȱShape.................................37 Figureȱ4Ȭ1ȱSimulationȱmodel:ȱFordȱRSIȱTurbo......................................................41 Figureȱ 4Ȭ2ȱ Measuredȱ Rackȱ Travelȱ andȱ Calculatedȱ Wastegateȱ Areaȱ versusȱ BoostȱPressure ...................................................................................................43 Figureȱ4Ȭ3ȱWastegateȱAreaȱCalculation .................................................................44 Figureȱ4Ȭ4ȱSimulationȱModelȱofȱExhaustȱManifold:ȱConceptȱ1..........................45 Figureȱ4Ȭ5ȱSimulationȱModelȱofȱExhaustȱManifold:ȱConceptȱ2..........................46 Figureȱ4Ȭ6ȱEffectȱofȱOptimisedȱValveȱandȱWastegateȱSettingsȱonȱTorque........52 Figureȱ4Ȭ7ȱExhaustȱConceptȱEvaluation:ȱWastegateȱArea ..................................54 Figureȱ4Ȭ8ȱExhaustȱConceptȱEvaluation:ȱTorque..................................................54 Figureȱ4Ȭ9ȱExhaustȱConceptȱEvaluation:ȱVolumetricȱEfficiency ........................55 Figureȱ4Ȭ10ȱExhaustȱConceptȱEvaluation:ȱAirflow...............................................55 Figureȱ4Ȭ11ȱExhaustȱConceptȱEvaluation:ȱAverageȱResidualȱMass...................56 Figureȱ5Ȭ1ȱPositioningȱRig .......................................................................................59 Figureȱ5Ȭ2ȱFourȬcylinderȱengineȇsȱvalveȱtimingȱ(firingȱorderȱ1Ȭ3Ȭ4Ȭ2) ..............60 Figureȱ5Ȭ3ȱExhaustȱmanifold:ȱConceptȱ1,ȱCADȱmodel ........................................61 Figureȱ5Ȭ4ȱExhaustȱManifoldȱForceȱDiagram........................................................63 Figureȱ5Ȭ5ȱExhaustȱManifold:ȱConceptȱ2,ȱCADȱmodel ........................................64 Figureȱ5Ȭ6ȱPreȬCompressorȱPipe.............................................................................65 Figureȱ5Ȭ7ȱGuideȱVanesȱinȱaȱSharpȱBend...............................................................66 Figureȱ5Ȭ8ȱPostȬCompressorȱPipe ...........................................................................67 ȱ. vii.

(14) Figureȱ5Ȭ9ȱVariableȱWastegateȱActuator................................................................68 Figureȱ5Ȭ10ȱVWAȱDiaphragmȱTest .........................................................................69 Figureȱ5Ȭ11ȱOilȱReturnȱSchematic ...........................................................................71 Figureȱ5Ȭ12ȱLogPȬLogVȱofȱmotoredȱtest ................................................................75 Figureȱ 5Ȭ13ȱ Normalisedȱ Cumulativeȱ Heatȱ Releaseȱ (NAȱ engine,ȱ WOTȱ atȱ 4000ȱrev/min) .....................................................................................................77 Figureȱ5Ȭ14ȱThermocoupleȱSetȬupȱ(Ricardo,ȱ2002)...............................................78 Figureȱ5Ȭ15ȱExhaustȱPortȱversusȱDownstreamȱTemperatures............................80 Figureȱ6Ȭ1ȱNAȱSimulationȱvsȱExperiment:ȱTorque ..............................................83 Figureȱ6Ȭ2ȱNAȱSimulationȱvsȱExperiment:ȱVolumetricȱEfficiency.....................84 Figureȱ6Ȭ3ȱNAȱSimulationȱvsȱExperiment:ȱAirflow .............................................84 Figureȱ6Ȭ4ȱNAȱSimulationȱvsȱExperiment:ȱMaximumȱCombustionȱPressure..85 Figureȱ 6Ȭ5ȱ NAȱ Simulationȱ vsȱ Experiment:ȱ Motoredȱ inȬcylinderȱ Pressureȱ (1500ȱrev/min)....................................................................................................86 Figureȱ6Ȭ6ȱNAȱSimulationȱvsȱExperiment:ȱExhaustȱBackpressure ....................87 Figureȱ6Ȭ7ȱNAȱSimulationȱvsȱExperiment:ȱSpecificȱFuelȱConsumption............88 Figureȱ6Ȭ8ȱNAȱSimulationȱvsȱExperiment:ȱManifoldȱAbsoluteȱPressure..........88 Figureȱ6Ȭ9ȱNAȱSimulationȱvsȱExperiment:ȱIntakeȱManifoldȱAirȱTemperature 89 Figureȱ6Ȭ10ȱTurbochargedȱSimulationȱversusȱExperiment:ȱTorque ..................91 Figureȱ 6Ȭ11ȱ Turbochargedȱ Simulationȱ versusȱ Experiment:ȱ Absoluteȱ Boostȱ Pressure ..............................................................................................................91 Figureȱ6Ȭ12ȱTurbochargedȱSimulationȱvsȱExperiment:ȱVolumetricȱEfficiency 92 Figureȱ6Ȭ13ȱTurbochargedȱSimulationȱversusȱExperiment:ȱAirflow .................93 Figureȱ 6Ȭ14ȱ Turbochargedȱ Simulationȱ versusȱ Experiment:ȱ Max.ȱ Combustionȱ Pressure ..............................................................................................................94 Figureȱ6Ȭ15ȱTurbochargedȱSimulationȱversusȱExperiment:ȱSFC........................95 Figureȱ6Ȭ16ȱTurbochargedȱSimulationȱversusȱExperiment:ȱMAP......................95 Figureȱ6Ȭ17ȱTurbochargedȱSimulationȱversusȱExperiment:ȱIntakeȱManifoldȱAirȱ Temperature ......................................................................................................96 Figureȱ6Ȭ18ȱTurbochargedȱSimulationȱversusȱExperiment:ȱCompressorȱOutletȱ Temperature ......................................................................................................97 Figureȱ6Ȭ19ȱTurbochargedȱSimulationȱversusȱExperiment:ȱWastegateȱArea ...98 Figureȱ6Ȭ20ȱTurbochargedȱ Simulationȱversusȱ Experiment:ȱ WastegateȱAreaȱ asȱ functionȱofȱmassȱflow .......................................................................................99 Figureȱ 6Ȭ21ȱ Turbochargedȱ Simulationȱ versusȱ Experiment:ȱ Turbineȱ Pressureȱ Ratio ....................................................................................................................99 Figureȱ 6Ȭ22ȱ Turbochargedȱ Simulationȱ versusȱ Experiment:ȱ Turbineȱ Inletȱ Temperature ....................................................................................................100 Figureȱ6Ȭ23ȱTurbochargedȱBoostȱSettings:ȱBoostȱPressure................................102 Figureȱ6Ȭ24ȱTurbochargedȱBoostȱSettings:ȱLambda ...........................................103 Figureȱ6Ȭ25ȱTurbochargedȱBoostȱSettings:ȱIgnitionȱTiming..............................103 ȱ. viii.

(15) Figureȱ6Ȭ26ȱTurbochargedȱBoostȱSettings:ȱCompressorȱOperatingȱPoints .....104 Figureȱ6Ȭ27ȱTurbochargedȱBoostȱSettings:ȱTorque .............................................105 Figureȱ6Ȭ28ȱPistonȱVelocityȱandȱAccelerationȱCorrelation................................106 Figureȱ 6Ȭ29ȱ SmallȬEndȱ Bearingȱ Forceȱ Comparison:ȱ ȱ Analyticalȱ versusȱ Simulation ........................................................................................................107 Figureȱ6Ȭ30ȱBigȬEndȱBearingȱForceȱComparison:ȱAnalyticalȱversusȱSimulation ............................................................................................................................107 Figureȱ6Ȭ31ȱAnalyticallyȱDeterminedȱBearingȱForces........................................108 Figureȱ6Ȭ32ȱNAȱversusȱTurbochargedȱResults:ȱGasȱForceȱonȱPiston...............109 Figureȱ6Ȭ33ȱNAȱversusȱTurbochargedȱResults:ȱSmallȬendȱBearingȱForces.....110 Figureȱ6Ȭ34ȱNAȱversusȱTurbochargedȱResults:ȱBigȬendȱBearingȱForces.........110 Figureȱ6Ȭ35ȱExtrapolationȱofȱSpecificȱHeat..........................................................112 Figureȱ6Ȭ36ȱNAȱversusȱTurbochargedȱResults:ȱHeatȱRejection ........................113 Figureȱ6Ȭ37ȱNAȱversusȱTurbochargedȱResults:ȱOilȱTemperature ....................114 Figureȱ6Ȭ38ȱNAȱengine:ȱEnergyȱBalanceȱatȱWOT ...............................................115 Figureȱ6Ȭ39ȱTurbochargedȱEngine:ȱEnergyȱBalanceȱatȱWOT ............................115 Figureȱ6Ȭ40ȱNAȱEngineȱTorque .............................................................................116 Figureȱ6Ȭ41ȱNAȱversusȱTurbochargedȱResults:ȱTorqueȱandȱPower.................117 Figureȱ6Ȭ42ȱNAȱversusȱTurbochargedȱResults:ȱIgnitionȱTiming......................118 Figureȱ6Ȭ43ȱNAȱversusȱTurbochargedȱResults:ȱLambda ...................................118 Figureȱ6Ȭ44ȱNAȱversusȱTurbochargedȱResults:ȱSFC...........................................119 Figureȱ6Ȭ45ȱNAȱversusȱTurbochargedȱResults:ȱMAPȱandȱTMAP....................120 Figureȱ6Ȭ46ȱNAȱversusȱTurbochargedȱResults:ȱWastegateȱArea......................120 Figureȱ6Ȭ47ȱNAȱversusȱTurbochargedȱResults:ȱIntakeȱManifoldȱAirȱDensity121 Figureȱ 6Ȭ48ȱ NAȱ versusȱ Turbochargedȱ Results:ȱ Ambientȱ andȱ Intakeȱ Manifoldȱ AirȱTemperature..............................................................................................122 Figureȱ6Ȭ49ȱNAȱversusȱTurbochargedȱResults:ȱAirflow....................................122 Figureȱ6Ȭ50ȱNAȱversusȱTurbochargedȱResults:ȱVolumetricȱEfficiency ...........123 Figureȱ6Ȭ51ȱNAȱversusȱTurbochargedȱResults:ȱExhaustȱManifoldȱPressure..123 Figureȱ 6Ȭ52ȱ NAȱ versusȱ Turbochargedȱ Results:ȱ Pressureȱ Differenceȱ (Intakeȱ ManifoldȱȬȱExhaustȱManifold).......................................................................124 Figureȱ6Ȭ53ȱNAȱversusȱTurbochargedȱResults:ȱExhaustȱBackpressure...........125 Figureȱ6Ȭ54ȱNAȱversusȱTurbochargedȱResults:ȱExhaustȱManifoldȱTemperature ............................................................................................................................126 Figureȱ 6Ȭ55ȱ NAȱ versusȱ Turbochargedȱ Results:ȱ Fuelȱ consumptionȱ vsȱ Powerȱ Output...............................................................................................................127 Figureȱ6Ȭ56ȱNAȱversusȱTurbochargedȱResults:ȱSparkȱAdvanceȱandȱLambda127 Figureȱ6Ȭ57ȱNAȱversusȱTurbochargedȱResults:ȱBurnȱDuration........................128 Figureȱ6Ȭ58ȱNAȱversusȱTurbochargedȱResults:ȱ50%ȱBurnȱPoint ......................128 Figureȱ 6Ȭ59ȱ NAȱ versusȱ Turbochargedȱ Results:ȱ Maximumȱ Combustionȱ Pressure ............................................................................................................129 ȱ. ix.

(16) Figureȱ6Ȭ60ȱNAȱversusȱTurbochargedȱResults:ȱPartȱloadȱSFC .........................130 Figureȱ6Ȭ61ȱNAȱversusȱTurbochargedȱResults:ȱPartȱloadȱSparkȱAdvance .....131 Figureȱ6Ȭ62ȱNAȱversusȱTurbochargedȱResults:ȱPartȱloadȱLambda ..................131 Figureȱ 6Ȭ63ȱ NAȱ versusȱ Turbochargedȱ Results:ȱ Partȱ loadȱ Energyȱ Balanceȱ atȱ 2500ȱrev/min ....................................................................................................133 Figureȱ6Ȭ64ȱNAȱversusȱTurbochargedȱResults:ȱPartȱloadȱFuelȱFlow ...............133 Figureȱ6Ȭ65ȱRequiredȱEngineȱPower ....................................................................134 Figureȱ6Ȭ66ȱ2.0LȱNAȱversusȱTurbochargedȱSFCȱatȱ4000ȱrev/min.....................135 Figureȱ6Ȭ67ȱ2.0LȱNAȱversusȱTurbochargedȱSFCȱatȱ2500ȱrev/min.....................136 FigureȱBȬ1ȱK03ȱOilȱFlowȱSpecificationȱ(Kühnle,ȱKopp,ȱKausch,ȱ1994)............147 FigureȱDȬ1ȱPistonȬCrank:ȱFreeȱBodyȱDiagram....................................................149 ȱ ȱ. ȱ. x.

(17) LISTȱOFȱTABLESȱ ȱ Tableȱ2Ȭ1ȱTestȱEngineȱSpecifications ........................................................................3 Tableȱ4Ȭ1ȱInitialȱValuesȱforȱFullȱFactorialȱValveȱOptimisation...........................49 Tableȱ4Ȭ2ȱFullȱFactorialȱResults ...............................................................................49 Tableȱ4Ȭ3ȱSimplexȱOptimisationȱResultsȱ(30ȱiterations).......................................52 Tableȱ5Ȭ1ȱMaterialȱPropertiesȱofȱMildȱSteelȱatȱHighȱTemperaturesȱ(BritishȱIronȱ andȱSteelȱResearchȱAssociationȱMetallurgy,ȱ1953).......................................62 Tableȱ6Ȭ1ȱEstimatedȱFuelȱSaving...........................................................................136 TableȱCȬ1ȱECEȬStandardȱReferenceȱConditions .................................................148 ȱ ȱ. ȱ. xi.

(18) LISTȱOFȱSYMBOLSȱANDȱABBREVIATIONSȱ AFRȱ ȱ ATDCȱȱ BDCȱ ȱ BMEPȱȱ CAȱ ȱ CADȱ ȱ CFDȱ ȱ CIȱ ȱ CRȱ ȱ ECUȱ ȱ EGRȱȱ ȱ EMSȱ ȱ ETAȱȱ ȱ ETCȱ ȱ EVCȱ ȱ EVOȱ ȱ IDȱ ȱ IVCȱ ȱ IVOȱ ȱ KLSAȱ ȱ MAPȱ ȱ MBTȱ ȱ NAȱ ȱ ODȱ ȱ OEMȱ ȱ PLCȱ ȱ SFCȱ ȱ SIȱ ȱ TDCȱ ȱ VWAȱ ȱ WOTȱ ȱ. ȱ. AirȱFuelȱRatioȱ AfterȱTopȱDeadȱCentreȱ BottomȱDeadȱCentreȱ BrakeȱMeanȱEffectiveȱPressureȱ CrankȱAngleȱ ComputerȬaidedȱDesignȱ ComputationalȱFluidȱDynamicsȱ CompressionȱIgnitionȱ CompressionȱRatioȱ ElectronicȱControlȱUnitȱ ExhaustȱGasȱRecirculationȱ EngineȱManagementȱSystemȱ EngineȱTestȱAutomationȱ ElectronicȱThrottleȱControlȱ ExhaustȱValveȱClosureȱ ExhaustȱValveȱOpeningȱ InsideȱDiameterȱ IntakeȱValveȱClosureȱ IntakeȱValveȱOpeningȱ KnockȱLimitedȱSparkȱAdvanceȱ ManifoldȱAbsoluteȱPressureȱ MostȱBeneficialȱTimingȱ NaturallyȱAspiratedȱ OutsideȱDiameterȱ OriginalȱEquipmentȱManufacturerȱ ProgrammableȱLogicȱControllerȱ SpecificȱFuelȱConsumptionȱ SparkȱIgnitionȱ TopȱDeadȱCentreȱ VariableȱWastegateȱActuatorȱ WideȬOpenȱThrottleȱ. xii.

(19) ȱ. 1. INTRODUCTIONȱ Turbochargedȱ sparkȱ ignitionȱ (SI)ȱ enginesȱ haveȱ beenȱ aroundȱ sinceȱ theȱ 1970s,ȱ butȱ theirȱ popularityȱ outsideȱ theȱ motorsportȱ sectorȱ hasȱ beenȱ smallȱ untilȱ theȱ recentȱadvancesȱinȱengineȱcontrol.ȱȱTheȱlackȱofȱpopularityȱcouldȱpartlyȱbeȱdueȱ toȱ theȱ drivabilityȱ issuesȱ associatedȱ withȱ earlyȱ turbochargedȱ engines.ȱ ȱ Theȱ engine’sȱresponseȱtoȱaȱsuddenȱincreaseȱinȱdriver’sȱdemandȱwasȱdelayedȱdueȱ toȱturbochargerȱlag.ȱȱTheȱlagȱwasȱthenȱusuallyȱfollowedȱbyȱaȱrapidȱincreaseȱofȱ powerȱwhichȱresultedȱinȱlossȱofȱtractionȱandȱpossibleȱlossȱofȱcontrolȱoverȱtheȱ car.ȱ ȱ Theȱ advancesȱ andȱ developmentsȱ madeȱ inȱ theȱ electronicȱ controlȱ andȱ managementȱ ofȱ internalȱ combustionȱ enginesȱ madeȱ itȱ possibleȱ toȱ overcomeȱ mostȱofȱtheseȱdrivabilityȱlimitations.ȱȱPassengerȱvehiclesȱwithȱturbochargedȱSIȱ enginesȱ areȱ nowȱ becomingȱ moreȱ common.ȱ ȱ Audi,ȱ Volvoȱ andȱ VWȱ allȱ offerȱ differentȱ passengerȱ vehicleȱ modelsȱ withȱ turbochargedȱ SIȱ engines.ȱ ȱ Inȱ theȱ performanceȱ sectorȱ Mitsubishi,ȱ Porsche,ȱ andȱ Subaruȱ offerȱ turbochargedȱ enginesȱwhereasȱMercedesȱoffersȱsuperchargedȱandȱturbochargedȱengines.ȱȱInȱ theȱquestȱforȱmoreȱefficientȱengines,ȱturbochargedȱenginesȱwillȱmostȱprobablyȱ increaseȱinȱpopularity.ȱ ȱ Theȱoperatingȱprincipleȱofȱaȱturbochargerȱisȱtoȱuseȱenergyȱrecoveredȱfromȱtheȱ exhaustȱgasesȱtoȱforceȱmoreȱairȱintoȱtheȱcombustionȱchamber.ȱȱThisȱincreasesȱ theȱamountȱofȱoxygenȱinȱtheȱcombustionȱchamberȱandȱhenceȱmoreȱfuelȱcanȱbeȱ burned.ȱȱIfȱmoreȱfuelȱcanȱbeȱburned,ȱmoreȱpowerȱcanȱbeȱproduced.ȱȱThereforeȱ aȱ turbochargedȱ engineȱ canȱ produceȱ moreȱ powerȱ thanȱ aȱ similarȬsizeȱ NAȱ engine.ȱ ȱ Itȱ isȱ claimedȱthatȱ theȱ displacementȱ ofȱ aȱ turbochargedȱ engineȱ canȱ beȱ reducedȱbyȱupȱtoȱ40%ȱrelativeȱtoȱaȱNAȱengine,ȱwithoutȱcompromisingȱpowerȱ output.ȱ ȱ Thusȱ theȱ turbochargedȱ engineȱ couldȱ beȱ smaller,ȱ lighterȱ andȱ moreȱ fuelȬefficientȱasȱwellȱasȱproduceȱlessȱemissions.ȱȱThereforeȱthisȱisȱanȱattractiveȱ optionȱ forȱ manufacturersȱ whoȱ needȱ toȱ lowerȱ theirȱ fleetȱ averageȱ fuelȱ consumption,ȱbutȱalsoȱforȱthoseȱwhoȱmustȱmeetȱ emissionȱstandardsȱwithoutȱ compromisingȱperformance.ȱ ȱ Engineȱ simulationȱ andȱ performanceȱ predictionȱ areȱ playingȱ anȱ increasinglyȱ importantȱ roleȱ inȱ engineȱ development.ȱ ȱ Withȱ engineȱ simulationȱ andȱ performanceȱpredictionȱmuchȱiterationȱinȱtheȱdevelopmentȱphaseȱcanȱnowȱbeȱ doneȱinȱsimulation,ȱwhichȱnotȱonlyȱcostsȱlessȱthanȱactualȱtestingȱbutȱalsoȱleadsȱ toȱ fasterȱ developmentȱ times.ȱ ȱ Thereȱ areȱ aȱ numberȱ ofȱ engineȱ simulationȱ packagesȱ availableȱ onȱ theȱ marketȱ todayȱ rangingȱ fromȱ packagesȱ toȱ simulateȱ combustion,ȱengineȱandȱdrivelineȱdynamics,ȱcontrolȱsystems,ȱcoolingȱsystemȱ andȱ theȱ valveȱ train,ȱ toȱ packagesȱ whichȱ combineȱ someȱ orȱ allȱ theȱ aboveȱ intoȱ one.ȱ 1ȱ.

(20) ȱ ȱ Ifȱ aȱ oneȬdimensionalȱ (1ȬD)ȱ flowȱ simulationȱ couldȱ beȱ usedȱ toȱ replicateȱ andȱ predictȱ theȱ complicatedȱ threeȬdimensionalȱ (3ȬD)ȱ flowȱ foundȱ inȱ reality,ȱ itȱ wouldȱ significantlyȱ reduceȱ theȱ computationȱ time.ȱ ȱ Theȱ simplerȱ aȱ simulationȱ package,ȱtheȱfasterȱitȱwouldȱyieldȱresults.ȱȱShorteningȱsimulationȱtimeȱwouldȱ enableȱ moreȱ iterationȱ inȱ aȱ specificȱ timeȱ frame,ȱ enablingȱ aȱ higherȱ levelȱ ofȱ optimisationȱ andȱ inȱ theȱ end,ȱ aȱ betterȱ product.ȱ ȱ Simplifyingȱ theȱ simulation,ȱ certainȱassumptionsȱmustȱbeȱmade,ȱcausingȱinaccuracies.ȱȱCertainȱprocessesȱinȱ theȱinternalȱcombustionȱengineȱsuchȱasȱflowȱthroughȱaȱcompressorȱorȱturbineȱ areȱ difficultȱ toȱ predictȱ withȱ 1ȬDȱ simulationȱ only.ȱ ȱ Thusȱ complexȱ 3ȬDȱ computationalȱfluidȱdynamicsȱ(CFD)ȱmayȱbeȱnecessaryȱtoȱaccuratelyȱsimulateȱ theȱreality.ȱȱByȱusingȱ3ȬDȱCFDȱonlyȱforȱcertainȱcomplexȱprocessesȱratherȱthanȱ forȱ theȱ wholeȱ engineȱ model,ȱ itȱ wouldȱ beȱ possibleȱ toȱ retainȱ aȱ highȱ degreeȱ ofȱ accuracyȱwhileȱnotȱcompromisingȱexcessivelyȱonȱcomputationȱtime.ȱ ȱ Theȱresearchȱquestionsȱthatȱareȱaddressedȱinȱthisȱprojectȱareȱfirstlyȱtoȱascertainȱ whatȱtheȱimplicationsȱwouldȱbeȱofȱaddingȱaȱturbochargerȱtoȱaȱNAȱengineȱand,ȱ secondly,ȱ toȱ determineȱ whetherȱ theȱ performanceȱ ofȱ aȱ turbochargedȱ engineȱ canȱbeȱpredictedȱaccuratelyȱbyȱusingȱ1ȬDȱflowȱsimulation.ȱ ȱ. 2ȱ.

(21) ȱ. 2. PROJECTȱOBJECTIVESȱANDȱOVERVIEWȱ Theȱobjectiveȱofȱtheȱprojectȱisȱtoȱaddressȱtheȱfollowingȱtwoȱresearchȱquestions:ȱ x WhatȱisȱtheȱimplicationȱofȱaddingȱaȱturbochargerȱtoȱaȱNAȱengine;ȱȱ x Canȱ1ȬDȱsimulationȱpredictȱtheȱperformanceȱofȱaȱturbochargedȱengineȱ accurately?ȱ Inȱorderȱtoȱaddressȱtheȱaboveȱquestions,ȱaȱstandardȱNAȱengineȱwasȱconvertedȱ toȱaȱturbochargedȱengineȱandȱaȱsimulationȱmodelȱofȱeachȱengineȱwasȱusedȱforȱ comparativeȱ purposes.ȱ ȱ Aȱ 1.6ȱlitreȱ Fordȱ Rocamȱ engineȱ wasȱ chosenȱ forȱ theȱ project.ȱȱTheȱmaximumȱpowerȱoutputȱtargetȱwasȱsetȱasȱ100ȱkWȱandȱaȱtorqueȱ curveȱasȱflatȱasȱpossibleȱforȱaȱwideȱasȱpossibleȱengineȱspeedȱrange.ȱȱTheȱtargetȱ speedȱ rangeȱ wasȱ setȱ asȱ 2000ȱrev/minȱ upȱ toȱ 5000ȱrev/min.ȱ ȱ Theȱ engineȱ specificationsȱandȱoutputȱtargetsȱareȱrepresentedȱinȱTableȱ2Ȭ1ȱandȱFigureȱ2Ȭ1.ȱ Tableȱ2Ȭ1ȱTestȱEngineȱSpecificationsȱ Specification:ȱ Standardȱ EngineȱSizeȱ[cc]ȱ No.ȱofȱCylindersȱ ValvesȱPerȱCylinderȱ CompressionȱRatioȱ BoreȱxȱStrokeȱ[mm]ȱ MaxȱPowerȱ[kW]ȱ Engineȱ Speedȱ @ȱ Maxȱ Powerȱ [rev/min]ȱ Max.ȱTorqueȱ[Nȉm]ȱ Engineȱ Speedȱ @ȱ MaxȱTorqueȱ [rev/min]ȱ FuelȱInjectionȱ FuelȱInjectorsȱ FuelȱPressureȱRegulatorȱ Turbochargerȱ Intercoolerȱ FuelȱOctaneȱ ExhaustȱManifoldȱ ExhaustȱSystemȱ ȱ. Targetȱ 1594ȱ 4ȱ 2ȱ 9.48ȱ 82ȱxȱ75.48ȱ. 70ȱ. 100ȱ 5500ȱ. 137ȱ. 174ȱ. 2500ȱ. 2000Ȭ5000ȱ. Yesȱ BOSCHȱ110ȱg/minȱ BOSCHȱ160ȱg/minȱ BOSCHȱ2.7ȱbarȱ BOSCHȱ3.0ȱbarȱ Noȱ Yesȱ Noȱ 95ȱ 102.6ȱ STDȱCastȱironȱ Customȱ STDȱ(35ȱmmȱID)ȱ Customȱ(51ȱmmȱID)ȱ. 3ȱ.

(22) ȱ TargetȱTorque. Std.ȱPower. TargetȱPower. 200.0. 100.0. 180.0. 80.0. 160.0. 60.0. 140.0. 40.0. 120.0. 20.0. 100.0. 0.0. 1000. 2000. 3000 4000 5000 EngineȱSpeedȱ[rev/min]. Figureȱ2Ȭ1ȱȱEngineȱOutputȱTargetȱ. Powerȱ[kW]. Torqueȱ[N.m]. Std.ȱTorque. 6000. ȱ. Inȱ orderȱ toȱ minimiseȱ costȱ andȱ time,ȱ theȱ modificationsȱ toȱ theȱ engineȱ wereȱ limited.ȱ ȱ Theȱ completeȱ packageȱ wasȱ alsoȱ requiredȱ toȱ fitȱ inȱ theȱ originalȱ car’sȱ engineȱ bayȱ withoutȱ anyȱ modificationsȱ toȱ it.ȱ ȱ Thisȱ posedȱ aȱ veryȱ challengingȱ packagingȱexerciseȱsinceȱtheȱtransverselyȱmountedȱengineȱisȱofȱtheȱcrossȱflowȱ typeȱwithȱtheȱexhaustȬsideȱofȱtheȱengineȱcloseȱtoȱtheȱfirewall.ȱ ȱ Theȱ modificationsȱ includedȱ theȱ designȱ andȱ manufactureȱ ofȱ anȱ exhaustȱ manifoldȱtoȱaccommodateȱtheȱturbocharger.ȱȱOilȱandȱwaterȱwereȱsuppliedȱtoȱ theȱ turbochargerȱ andȱ pipesȱ wereȱ madeȱ toȱ connectȱ theȱ airȱ filterȱ toȱ theȱ compressorȱandȱtheȱcompressorȱtoȱtheȱintakeȱmanifold.ȱȱDueȱtoȱtheȱincreasedȱ airflow,ȱtheȱexhaustȱhadȱtoȱbeȱreplacedȱbyȱaȱlargerȱdiameterȱfreeȬflowȱexhaustȱ toȱkeepȱtheȱexhaustȱbackpressureȱwithinȱreasonableȱlimits.ȱȱTheȱstandardȱfuelȱ injectorsȱ wereȱ replacedȱ withȱ injectorsȱ thatȱ wouldȱ beȱ capableȱ toȱ supplyȱ theȱ increasedȱamountȱofȱfuel.ȱȱTheȱfuelȱpressureȱregulatorȱwasȱalsoȱchangedȱsinceȱ theȱhigherȱflowȱinjectorsȱrequiredȱaȱhigherȱfuelȱpressure.ȱ ȱ. 4ȱ.

(23) ȱ Theȱ intakeȱ manifoldȱ pressureȱ sensorȱ hadȱ toȱ beȱ replacedȱ withȱ aȱ sensorȱ thatȱ wouldȱ beȱ ableȱ toȱ measureȱ pressuresȱ aboveȱ atmosphericȱ pressure.ȱ ȱ Aȱ knockȱ sensorȱ wasȱ addedȱ toȱ theȱ engineȱ controlȱ unitȱ (ECU)ȱ toȱ enableȱ itȱ toȱ retardȱ theȱ ignitionȱtimingȱinȱtheȱeventȱofȱknock.ȱȱHighȱoctaneȱfuelȱwouldȱbeȱusedȱduringȱ testingȱtoȱreduceȱtheȱlikelihoodȱthatȱknockȱwouldȱoccur.ȱȱTheȱtimeȱframeȱandȱ budgetȱ ofȱ theȱ projectȱ didȱ notȱ allowȱ forȱ engineȱ failure,ȱ thusȱ theȱ useȱ ofȱ highȱ octaneȱ fuelȱ wasȱ aȱ precautionaryȱ measureȱ andȱ notȱ aȱ technicalȱ requirement.ȱȱ Theȱ fuellingȱ andȱ timingȱ mapsȱ ofȱ theȱ ECUȱ wereȱ adjustedȱ forȱ maximumȱ performanceȱatȱfullȱload.ȱȱAtȱpartȱloadȱtheȱairȱfuelȱratioȱwasȱkeptȱtheȱsameȱforȱ bothȱenginesȱasȱfarȱasȱpossibleȱ(limitedȱbyȱexhaustȱportȱtemperature),ȱbutȱtheȱ ignitionȱtimingȱwasȱoptimisedȱforȱmaximumȱpowerȱoutput.ȱ ȱ Theȱfirstȱlimitationȱonȱengineȱchangeȱwasȱthatȱtheȱcompressionȱratioȱ(CR)ȱofȱ theȱ engineȱ wouldȱ notȱ beȱ reduced.ȱ ȱ Itȱ wasȱ notȱ envisagedȱ thatȱ highȱ boostȱ pressuresȱ wouldȱ beȱ neededȱ toȱ developȱ theȱ targetȱ output,ȱ thusȱ reducingȱ theȱ CRȱwasȱnotȱaȱrequirement.ȱȱKnockȱwouldȱhaveȱbeenȱaȱlimitation,ȱbutȱbyȱusingȱ theȱhighȱoctaneȱfuelȱthisȱshouldȱbeȱovercome.ȱReducingȱCRȱseverelyȱimpairsȱ theȱ efficiencyȱ ofȱ theȱ engine.ȱ ȱ Sinceȱ theȱ aimȱ isȱ toȱ improveȱ engineȱ efficiency,ȱ reducingȱtheȱCRȱwouldȱcontradictȱtheȱinitialȱintention.ȱȱȱ ȱ Theȱ secondȱ limitationȱ wasȱ thatȱ theȱ valveȱ timingȱ wouldȱ notȱ beȱ altered.ȱ ȱ Theȱ resultȱwouldȱbeȱnonȬoptimalȱ valveȱtimingȱ forȱtheȱturbochargedȱengine.ȱȱ Theȱ valveȱ timingȱ isȱ aȱ criticalȱ partȱ ofȱ theȱ gasȱ exchangeȱ mechanismȱ andȱ directlyȱ influencesȱ theȱ breathingȱ characteristicsȱ ofȱ anȱ engine.ȱ ȱ Optimisingȱ theȱ valveȱ timingȱ couldȱ benefitȱ lowȬendȱ torqueȱ orȱ highȬendȱ power,ȱ orȱ aȱ compromiseȱ betweenȱ theseȱ twoȱ extremes.ȱ ȱ Developingȱ camshaftsȱ andȱ camȱ profilesȱ isȱ aȱ scienceȱoutsideȱtheȱscopeȱofȱthisȱproject.ȱȱValveȱtimingȱoptimisationȱwasȱdoneȱ withȱtheȱsimulationȱinȱorderȱtoȱdemonstrateȱitsȱadvantage.ȱȱȱ ȱ Thirdlyȱanȱintercoolerȱwouldȱnotȱbeȱused.ȱȱAnȱintercoolerȱwouldȱhaveȱaȱtwoȬ foldȱ benefit.ȱ ȱ Itȱ wouldȱ reduceȱ theȱ intakeȱ chargeȱ temperature,ȱ reducingȱ theȱ chancesȱ ofȱ knockȱ andȱ byȱ loweringȱ theȱ temperatureȱ itȱ wouldȱ effectivelyȱ increaseȱ theȱ densityȱ ofȱ theȱ air,ȱ whileȱ usingȱ theȱ sameȱ boostȱ pressure.ȱ ȱ Sinceȱ highȱ boostȱ pressureȱ wouldȱ notȱ beȱ required,ȱ anȱ intercoolerȱ wouldȱ addȱ unnecessaryȱcostȱandȱcomplexityȱtoȱtheȱsystem.ȱȱDueȱtoȱtheȱspaceȱlimitationsȱ onȱ passengerȱ vehicles,ȱ packagingȱ ofȱ theȱ intercoolerȱ wouldȱ alsoȱ poseȱ aȱ veryȱ challengingȱexercise.ȱ ȱ. 5ȱ.

(24) ȱ Theȱ fourthȱ limitationȱ wasȱ thatȱ onlyȱ mechanicalȱ boostȱ controlȱ wouldȱ beȱ utilised.ȱ ȱ Thisȱ isȱ aȱ majorȱ simplificationȱ sinceȱ theȱ mechanicalȱ wastegateȱ respondsȱdirectlyȱtoȱtheȱboostȱpressure.ȱȱUsingȱelectronicȱboostȱcontrolȱwouldȱ furtherȱ aidȱ theȱ abilityȱ toȱ developȱ aȱ flatȱ torqueȱ curveȱ butȱ theȱ complexityȱ andȱ timeȱneededȱtoȱimplementȱsuchȱaȱsystemȱwouldȱbeȱbeyondȱtheȱscopeȱofȱthisȱ project.ȱ ȱ Howeverȱ aȱ mechanicalȱ wastegateȱ actuatorȱ wasȱ developedȱ whichȱ wouldȱfacilitateȱindependentȱadjustmentȱofȱtheȱspringȱstiffnessȱandȱtheȱpresetȱ compressionȱofȱtheȱspring.ȱ ȱ Thisȱ concludesȱ theȱ objectivesȱ andȱ overviewȱ andȱ definesȱ theȱ frameworkȱ inȱ whichȱthisȱprojectȱwasȱexecuted.ȱ ȱ. 6ȱ.

(25) ȱ. 3. LITERATUREȱREVIEWȱ Thisȱ projectȱ wasȱ concernedȱ withȱ theȱ turbochargingȱ ofȱ aȱ fourȬstrokeȱ petrolȱ engine.ȱȱTurbochargedȱfourȬstrokeȱdieselȱenginesȱwillȱalsoȱbeȱdiscussedȱbrieflyȱ andȱ differencesȱ willȱ beȱ highlighted.ȱ ȱ Theȱ discussion,ȱ however,ȱ omitsȱ twoȬ strokeȱenginesȱdueȱtoȱtheirȱdifferentȱgasȱexchangeȱprocesses.ȱ ȱ. 3.1. Superchargingȱ Superchargingȱ canȱ beȱ definedȱ asȱ theȱ introductionȱ ofȱ airȱ (orȱ air/fuelȱ mixture)ȱ intoȱanȱengineȱcylinderȱatȱaȱdensityȱgreaterȱthanȱambientȱdensity.ȱȱThisȱallowsȱ aȱ proportionalȱ increaseȱ inȱ theȱ fuelȱ thatȱ canȱ beȱ burnedȱ andȱ henceȱ raisesȱ theȱ potentialȱ powerȱ output.ȱ ȱ Theȱ principalȱ objectiveȱ ofȱ superchargingȱ isȱ toȱ increaseȱ powerȱ output,ȱ notȱ toȱ improveȱ efficiency,ȱ althoughȱ efficiencyȱ mayȱ benefit.ȱȱȱ ȱ Variousȱ methodsȱ ofȱ superchargingȱ areȱ available.ȱ ȱ Theseȱ methodsȱ canȱ beȱ classifiedȱintoȱtwoȱcategories.ȱȱTheȱfirstȱcategoryȱusesȱaȱcompressorȱdrivenȱbyȱ theȱengineȱoutputȱshaftȱtoȱcompressȱtheȱairȱtoȱaȱdensityȱgreaterȱthanȱambientȱ density.ȱ ȱ Theȱ compressorȱ canȱ beȱ anyȱ positiveȱ displacementȱ pumpȱ suchȱ asȱ aȱ RootsȬtypeȱ blower,ȱ aȱ centrifugalȱ compressorȱ orȱ aȱ vaneȬtypeȱ blower.ȱ ȱ Theȱ speedȱ ofȱ theȱ superchargerȱ isȱ proportionalȱ toȱ theȱ engineȱ speed,ȱ thusȱ atȱ lowȱ engineȱ speedsȱ theȱ centrifugalȱ compressorȱ mightȱ beȱ ineffectiveȱ becauseȱ theȱ outputȱpressureȱvariesȱapproximatelyȱasȱtheȱsquareȱofȱtheȱimpellerȱspeed.ȱ ȱ Theȱ secondȱ category,ȱ knownȱ asȱ turbocharging,ȱ usesȱ theȱ energyȱ availableȱ inȱ theȱexhaustȱgasȱtoȱcompressȱtheȱchargedȱairȱ(orȱair/fuelȱmixture).ȱȱTheȱenergyȱ isȱ recoveredȱ byȱ expandingȱ theȱ highȬpressureȱ exhaustȱ gasȱ inȱ aȱ turbine.ȱ ȱ Thisȱ energyȱ isȱ thenȱ usedȱ toȱ driveȱ theȱ compressor.ȱ ȱ Inȱ bigȱ dieselȱ enginesȱ axialȱ turbinesȱ andȱ compressorsȱ mayȱ beȱ used,ȱ whileȱ radialȱ compressorsȱ andȱ turbinesȱ areȱ moreȱ commonȱ inȱ mediumȬsizeȱ andȱ smallȱ engines.ȱȱ Turbochargingȱwillȱbeȱdiscussedȱinȱmoreȱdetailȱinȱtheȱnextȱsection.ȱ ȱ Theȱ mainȱ advantageȱ ofȱ turbochargingȱ asȱ opposedȱ toȱ superchargingȱ isȱ thatȱ turbochargingȱ usesȱ theȱ energyȱ inȱ theȱ hotȱ exhaustȱ gasȱ thatȱ wouldȱ haveȱ beenȱ lost.ȱ ȱ Superchargingȱ usesȱ powerȱ fromȱ theȱ engine’sȱ crankshaftȱ andȱ thusȱ lessȱ powerȱisȱavailableȱforȱpropulsion.ȱȱȱ ȱ. 7ȱ.

(26) ȱ. 3.2. Turbochargingȱ Theȱ authorȱ acknowledgesȱ thatȱ theȱ basisȱ ofȱ theȱ theoryȱ representedȱ inȱ thisȱ sectionȱwasȱextractedȱfromȱWatsonȱandȱJanotaȱ(1984)ȱandȱSayersȱ(1990).ȱ ȱ Theȱ exhaustȬdrivenȱ turbochargerȱ wasȱ inventedȱ byȱ aȱ Swissȱ engineerȱ namedȱ Buchi,ȱ whoȱ fittedȱ hisȱ creationȱ toȱ aȱ dieselȱ engineȱ backȱ inȱ 1909.ȱ ȱ However,ȱ heȱ onlyȱachievedȱsuccessȱmanyȱyearsȱlaterȱ(aroundȱ1925).ȱȱItȱtookȱaȱlongȱtimeȱforȱ turbochargersȱ toȱ becomeȱ established,ȱ butȱ itȱ isȱ nowȱ recognisedȱ thatȱ theirȱ characteristicsȱ areȱ particularlyȱ suitedȱ toȱ theȱ dieselȱ engine,ȱ theȱ reasonȱ beingȱ thatȱ onlyȱ airȱ isȱ compressed,ȱ andȱ noȱ throttlingȱ isȱ used.ȱ ȱ Asȱ aȱ result,ȱ turbochargedȱ dieselȱ enginesȱ areȱ becomingȱ recognisedȱ asȱ suitable,ȱ evenȱ desirable,ȱforȱprivateȱcarsȱasȱwellȱasȱforȱcommercialȱvehicles.ȱȱȱ ȱ Aȱtypicalȱturbochargerȱconsistsȱofȱaȱradialȱturbine,ȱwhichȱrecoversȱtheȱenergyȱ fromȱ theȱ hotȱ exhaustȱ gases.ȱ ȱ Theȱ turbineȱ isȱ coupledȱ toȱ aȱ radialȱ compressor,ȱ whichȱ increasesȱ theȱ pressureȱ inȱ theȱ intakeȱ manifold.ȱ ȱ Betweenȱ theȱ twoȱ isȱ aȱ wideȱ supportingȱ bearing,ȱ usuallyȱ inȱ theȱ formȱ ofȱ aȱ freeȬfloatingȱ journalȱ bearing,ȱ becauseȱ anȱ ordinaryȱ rollerȱ bearingȱ wouldȱ notȱ surviveȱ theȱ highȱ rotationalȱ speedȱ (upȱ toȱ 250ȱ000ȱrev/min)ȱ ofȱ whichȱ aȱ smallȱ turbineȱ isȱ capable.ȱȱ Figureȱ3Ȭ1ȱshowsȱaȱtypicalȱturbochargerȱusedȱinȱautomotiveȱapplications.ȱ ȱ. Figureȱ3Ȭ1ȱAutomotiveȱTurbochargerȱ(Venter,ȱ1999)ȱ. 8ȱ. ȱ.

(27) ȱ Thereȱhaveȱbeenȱconcernsȱthatȱtheȱincreasedȱexhaustȱbackpressureȱcausedȱbyȱ theȱ turbineȱ isȱ aȱ disadvantage,ȱ butȱ analysisȱ refutesȱ thisȱ statement.ȱ ȱ Whenȱ theȱ exhaustȱvalveȱfirstȱopens,ȱtheȱpressureȱinsideȱtheȱcylinderȱisȱveryȱmuchȱhigherȱ thanȱtheȱpressureȱinȱtheȱexhaustȱmanifold.ȱȱAsȱtheȱcylinderȱpressureȱdrops,ȱaȱ stageȱ isȱ reachedȱ whereȱ theȱ ascendingȱ pistonȱ hasȱ toȱ driveȱ outȱ theȱ gases,ȱ becauseȱtheȱpressureȱinȱtheȱexhaustȱsystemȱisȱhigherȱdueȱtoȱtheȱturbine.ȱȱThisȱ higherȱ backpressureȱ couldȱ alsoȱ increaseȱ theȱ amountȱ ofȱ residualȱ exhaustȱ gasȱ insideȱtheȱcombustionȱchamber.ȱ ȱ Thisȱ representsȱ aȱ lossȱ ofȱ energy.ȱ ȱ However,ȱ whenȱ theȱ inletȱ valveȱ opens,ȱ theȱ extraȱ pressureȱ createdȱ byȱ theȱ compressorȱ suppliesȱ extraȱ energyȱ toȱ forceȱ theȱ pistonȱ downȱ onȱ theȱ intakeȱ stroke,ȱ whichȱ representsȱ aȱ netȱ gainȱ inȱ energy.ȱȱ Duringȱtheȱperiodȱofȱvalveȱoverlap,ȱtheȱextraȱpressureȱinȱtheȱintakeȱmanifoldȱ mayȱ evenȱ helpȱ scavengeȱ theȱ residualȱ exhaustȱ gasesȱ outȱ ofȱ theȱ clearanceȱ volume,ȱ representingȱ aȱ furtherȱ gainȱ inȱ energy.ȱ ȱ Allȱ thisȱ presupposesȱ aȱ wellȬ designedȱ system,ȱ withȱ theȱ turbochargerȱ beingȱ efficientȱ enoughȱ toȱ raiseȱ theȱ boostȱpressureȱaboveȱtheȱexhaustȱpressureȱofȱtheȱengine.ȱȱActualȱtemperatureȱ measurementsȱ atȱ fullȱ powerȱ haveȱ shownȱ aȱ significantȱ dropȱ inȱ exhaustȱ gasȱ temperatureȱ acrossȱ theȱ turbine,ȱ whichȱ isȱ aȱ measureȱ ofȱ theȱ energyȱ removed.ȱȱ Thisȱ energyȱ wouldȱ haveȱ goneȱ toȱ wasteȱ ifȱ theȱ turbochargerȱ wereȱ notȱ there.ȱȱ Thereȱareȱcurrentlyȱtwoȱwaysȱofȱutilisingȱtheȱhighȱpressureȱofȱtheȱgasȱinsideȱ theȱ combustionȱ chamberȱ atȱ theȱ momentȱ ofȱ valveȱ opening,ȱ namely:ȱ constantȱ pressureȱ turbochargingȱ andȱ pulseȱ turbocharging,ȱ eachȱ ofȱ whichȱ hasȱ itsȱ ownȱ meritsȱandȱwillȱbeȱdiscussedȱinȱlaterȱsectionsȱ(3.2.4ȱandȱ3.2.5).ȱ ȱ Maximumȱ allowableȱ boostȱ onȱ Compressionȱ Ignitionȱ (CI)ȱ enginesȱ dependsȱ onlyȱ onȱ theȱ mechanicalȱ strengthȱ ofȱ theȱ engine,ȱ becauseȱ theyȱ haveȱ noȱ knockȱ limitations.ȱȱOnȱSIȱenginesȱtheȱboostȱpressureȱisȱlimitedȱbyȱknockȱ(selfȬignitionȱ ofȱ theȱ endȬgasȱ underȱ highȱ temperatureȱ andȱ pressure).ȱ ȱ Thus,ȱ ifȱ theȱ boostȱ pressureȱ isȱ highȱ onȱ SIȱ engines,ȱ theȱ CRȱ mustȱ beȱ sufficientlyȱ low,ȱ highȬoctaneȱ fuelȱ mustȱ beȱ usedȱ orȱ theȱ ignitionȱ timingȱ mustȱ beȱ retarded.ȱ ȱ Theȱ differenceȱ betweenȱCIȱandȱSIȱcombustionȱareȱdiscussedȱinȱmoreȱdetailȱinȱsectionȱ3.2.2.ȱ ȱ. 9ȱ.

(28) ȱ 3.2.1.. TurbochargerȱTheoryȱ. Theȱ operatingȱ characteristicsȱ ofȱ turbomachinesȱ suchȱ asȱ compressorsȱ andȱ turbinesȱ areȱ completelyȱ differentȱ fromȱ thoseȱ ofȱ theȱ reciprocatingȱ internalȱ combustionȱengine.ȱȱThusȱmatchingȱtheseȱtwoȱcompletelyȱdifferentȱmachinesȱ toȱ operateȱ togetherȱ isȱ anȱ optimisationȱ problemȱ withȱ manyȱ parameters.ȱ ȱ Theȱ basicȱ theoryȱ ofȱ turbomachinesȱ willȱ beȱ brieflyȱ reviewedȱ toȱ highlightȱ certainȱ keyȱaspectsȱthatȱmustȱbeȱborneȱinȱmindȱwhenȱcombiningȱturbomachinesȱwithȱ reciprocatingȱinternalȱcombustionȱengines.ȱȱ ȱ Theȱ mostȱ commonȱ turbochargerȱ assemblyȱ usedȱ inȱ theȱ automotiveȱ industryȱ consistsȱofȱaȱradialȱcompressorȱcoupledȱtoȱaȱradialȱturbine.ȱȱTheȱbearingsȱareȱ generallyȱ ofȱ theȱ plainȱ journalȱ bearingȱ type;ȱ however,ȱ forȱ racingȱ applicationsȱ ceramicȱballȱbearingsȱareȱbeingȱusedȱmoreȱfrequently.ȱȱOnȱbigȱenginesȱsuchȱasȱ thoseȱusedȱforȱrailȱandȱmarineȱapplications,ȱwhereȱtheȱoperatingȱrangeȱisȱveryȱ narrowȱ andȱ operationȱ isȱ mostlyȱ steadyȱ state,ȱ anȱ axialȱ turbineȱ coupledȱ toȱ aȱ radialȱ compressorȱ isȱ theȱ mostȱ commonȱ configuration.ȱ ȱ Axialȱ turbinesȱ areȱ preferredȱforȱtheirȱsuperiorȱefficiencyȱtoȱthoseȱofȱaȱradialȱturbine,ȱbutȱaȱradialȱ turbine’sȱ operatingȱ rangeȱ isȱ muchȱ wider.ȱ ȱ Thisȱ makesȱ radialȱ turbinesȱ moreȱ suitableȱforȱautomotiveȱapplications,ȱwhereȱtheȱoperatingȱrangeȱisȱveryȱwide.ȱȱ Radialȱ compressorsȱ alsoȱ haveȱ aȱ muchȱ widerȱ operatingȱ rangeȱ andȱ areȱ thusȱ moreȱ widelyȱ usedȱ thanȱ axialȱ compressorsȱ inȱ turbochargerȱ applications.ȱȱ Radialȱcompressorsȱareȱlimitedȱtoȱaȱpressureȱratioȱofȱaboutȱ3.5,ȱbecauseȱhigherȱ pressureȱ ratiosȱ willȱ causeȱ supersonicȱ flowȱ andȱ causeȱ shockwavesȱ toȱ formȱ atȱ theȱcompressorȱinlet.ȱȱThisȱwillȱcauseȱaȱrapidȱdeteriorationȱinȱtheȱcompressorȱ efficiency.ȱȱ ȱ Beforeȱdiscussingȱtheȱworkingȱandȱcharacteristicsȱofȱturbomachines,ȱpressureȱ andȱ temperatureȱ measurementsȱ willȱ beȱ revisitedȱ andȱ theirȱ significanceȱ discussed.ȱ ȱ 3.2.1.1. TotalȱandȱStaticȱPressureȱandȱTemperatureȱ Theȱ staticȱ pressureȱ (P1)ȱ ofȱ aȱ fluidȱ flowingȱ inȱ aȱ ductȱ isȱ thatȱ measuredȱ atȱ theȱ surfaceȱofȱtheȱwall.ȱȱTheȱtotalȱorȱstagnationȱpressureȱ(P01)ȱisȱtheȱpressureȱthatȱ willȱbeȱmeasuredȱinȱtheȱstreamȱifȱtheȱfluidȱwereȱbroughtȱtoȱrestȱisentropically.ȱȱȱ ThusȱP01ȱcanȱbeȱrelatedȱtoȱP1ȱasȱinȱEqȱ3Ȭ1.ȱ. P01. §T · P1 ¨¨ 01 ¸¸ © T1 ¹. J (J 1). Eqȱ3Ȭ1ȱ. ȱ. ȱ. 10ȱ.

(29) ȱ Whereȱgammaȱ(J)ȱrepresentsȱtheȱpolytropicȱcoefficientȱ(ratioȱofȱspecificȱheats).ȱȱ Similarlyȱ theȱ staticȱ temperatureȱ (T1)ȱ isȱ theȱ freeȱ streamȱ temperatureȱ andȱ theȱ totalȱ (orȱ stagnation)ȱ temperatureȱ (T01)ȱ isȱ theȱ temperatureȱ thatȱ willȱ beȱ measuredȱifȱtheȱgasȱwereȱ broughtȱ toȱrest.ȱȱForȱaȱ perfectȱgasȱitȱcanȱbeȱshownȱ thatȱEqȱ3Ȭ2ȱholds.ȱ 2. T01. 1 C1 T1 ȱ 2 cp. Eqȱ3Ȭ2ȱ. WhereȱC1ȱisȱtheȱvelocityȱofȱtheȱgasȱandȱcpȱtheȱspecificȱheatȱatȱconstantȱpressure.ȱ ȱ 3.2.1.2. TheȱRadialȱCompressorȱȱ Figureȱ 3Ȭ2ȱ showsȱ theȱ threeȱ importantȱ partsȱ ofȱ aȱ radialȱ compressor:ȱ impeller,ȱ diffuserȱ ringȱ andȱ voluteȱ casing.ȱ ȱ Inȱ someȱ applicationsȱ thereȱ mightȱ beȱ aȱ diffuserȱringȱincluded.ȱȱTheȱdiffuserȱringȱisȱoptionalȱandȱmayȱorȱmayȱnotȱbeȱ presentȱdependingȱonȱsize,ȱuseȱandȱcostȱofȱtheȱcompressor.ȱȱȱ ȱ Theȱ impellerȱ isȱ aȱ solidȱ rotatingȱ discȱ withȱ curvedȱ bladesȱ standingȱ outȱ axiallyȱ fromȱtheȱfaceȱofȱtheȱdisc.ȱȱInȱmostȱturbochargerȱapplicationsȱtheȱbladeȱtipsȱareȱ leftȱopenȱandȱtheȱcasingȱofȱtheȱcompressorȱitselfȱformsȱtheȱsolidȱouterȱwallȱofȱ theȱbladeȱpassages.ȱȱInȱsomeȱcasesȱtheȱbladeȱtipsȱmayȱbeȱcoveredȱwithȱanotherȱ flatȱdiscȱtoȱgiveȱshroudedȱblades.ȱȱTheȱadvantageȱofȱtheȱshroudedȱbladeȱisȱthatȱ noȱleakageȱcanȱtakeȱplaceȱfromȱoneȱpassageȱtoȱtheȱnext.ȱȱTheȱdisadvantageȱofȱ havingȱ shroudedȱ bladesȱ isȱ extraȱ weightȱ andȱ aȱ moreȱ complicatedȱ manufacturingȱ process.ȱ ȱ Inȱ turbochargerȱ applicationsȱ whereȱ veryȱ highȱ rotationalȱspeedsȱareȱrequired,ȱtheȱdisadvantageȱofȱleakageȱisȱmoreȱthanȱoffsetȱ byȱtheȱreducedȱweightȱofȱtheȱimpeller.ȱȱȱ. Figureȱ3Ȭ2ȱComponentsȱofȱaȱRadialȱCompressorȱ(Sayers,ȱ1990)ȱ 11ȱ. ȱ.

(30) ȱ Asȱtheȱimpellerȱrotates,ȱtheȱfluidȱ(air)ȱthatȱisȱdrawnȱintoȱtheȱbladeȱpassagesȱatȱ theȱimpellerȱinletȱisȱacceleratedȱasȱitȱisȱforcedȱradiallyȱoutwards.ȱȱInȱthisȱway,ȱ theȱstaticȱpressureȱatȱtheȱoutletȱradiusȱisȱmuchȱhigherȱthanȱatȱtheȱinletȱradius.ȱȱ Theȱfluidȱhasȱaȱveryȱhighȱvelocityȱatȱtheȱouterȱradiusȱofȱtheȱimpellerȱand,ȱtoȱ recoverȱthisȱkineticȱenergyȱbyȱchangingȱitȱtoȱpressureȱenergy,ȱdiffuserȱbladesȱ mountedȱonȱtheȱdiffuserȱringȱmayȱbeȱused.ȱȱTheȱstationaryȱbladeȱpassagesȱsoȱ formedȱ haveȱ anȱ increasingȱ crossȬsectionalȱ areaȱ asȱ theȱ fluidȱ movesȱ throughȱ them,ȱtheȱkineticȱenergyȱofȱtheȱfluidȱbeingȱreduced,ȱwhileȱtheȱpressureȱenergyȱ isȱfurtherȱincreased.ȱȱVanelessȱdiffuserȱpassagesȱmayȱalsoȱbeȱutilised.ȱȱȱ ȱ Finally,ȱtheȱfluidȱmovesȱfromȱtheȱdiffuserȱbladesȱintoȱtheȱvoluteȱcasing,ȱwhichȱ collectsȱitȱandȱconveysȱitȱtoȱtheȱcompressorȱoutlet.ȱȱAsȱtheȱfluidȱmovesȱalongȱ theȱ voluteȱ casing,ȱ furtherȱ pressureȱ recoveryȱ occurs.ȱ ȱ Sometimesȱ onlyȱ theȱ voluteȱcasingȱexistsȱwithoutȱtheȱdiffuser.ȱȱȱ ȱ Thisȱprocessȱcanȱbeȱplottedȱonȱanȱenthalpyȱversusȱentropyȱdiagramȱasȱshownȱ inȱ Figureȱ 3Ȭ3,ȱ soȱ thatȱ anyȱ departuresȱ fromȱ isentropicȱ compressionȱ canȱ beȱ shown.ȱȱStationȱ01ȱrepresentsȱambientȱpressureȱofȱtheȱair.ȱȱAccelerationȱofȱtheȱ fluidȱ inȱ theȱ inletȱ causesȱ aȱ pressureȱ dropȱ fromȱ P01ȱ toȱ P1ȱ (orȱ P00ȱ toȱ P1ȱ whenȱ consideringȱlossesȱinȱtheȱinlet),ȱtheȱchangeȱinȱenthalpyȱbeingȱequivalentȱtoȱtheȱ increaseȱ inȱ kineticȱ energyȱ (C12/2).ȱ ȱ Isentropicȱ compressionȱ toȱ theȱ deliveryȱ stagnationȱpressureȱP05sȱisȱshownȱbyȱtheȱverticalȱlineȱ01Ȭ05s.ȱȱEnergyȱtransferȱ toȱtheȱfluidȱtakesȱplaceȱinȱtheȱimpellerȱandȱtheȱlineȱ1Ȭ2ȱindicatesȱthisȱprocess.ȱȱ Theȱ correspondingȱ isentropicȱ processȱ isȱ shownȱ byȱ 1Ȭ2s.ȱ ȱ Ifȱ theȱ totalȱ kineticȱ energyȱ ofȱ theȱ fluidȱ leavingȱ theȱ impellerȱ (C22/2)ȱ wereȱ convertedȱ toȱ pressure,ȱ isentropically,ȱ theȱ deliveryȱ pressureȱ wouldȱ beȱ P02ȱ (pointȱ 02).ȱ ȱ Sinceȱ theȱ diffusionȱ processȱ isȱ notȱ accomplishedȱ isentropicallyȱ (2Ȭ5),ȱ andȱ someȱ kineticȱ energyȱremainsȱatȱtheȱdiffuserȱexitȱ(velocityȱC5),ȱtheȱstaticȱdeliveryȱpressureȱatȱ pointȱ5ȱisȱP5.ȱ. 12ȱ.

(31) ȱ. Figureȱ3Ȭ3ȱhȬsȱDiagramȱforȱaȱRadialȱCompressorȱ(Watsonȱ&ȱJanota,ȱ1984)ȱ. ȱ. Thisȱ describesȱ theȱ basicȱ workingȱ ofȱ aȱ radialȱ compressor.ȱ ȱ Forȱ moreȱ detailedȱ analysesȱandȱliteratureȱonȱcompressorȱdesignȱtheȱreaderȱisȱreferredȱtoȱSayersȱ (1990)ȱorȱWatsonȱandȱJanotaȱ(1984).ȱ ȱ 3.2.1.3. CompressorȱEfficiencyȱ Theȱ efficiencyȱ ofȱ theȱ radialȱ compressorȱ canȱ beȱ definedȱ asȱ theȱ workȱ requiredȱ forȱ idealȱ adiabaticȱ compressionȱ dividedȱ byȱ theȱ actualȱ workȱ requiredȱ toȱ achieveȱtheȱsameȱpressureȱratio.ȱȱFromȱtheȱsecondȱlawȱofȱthermodynamicsȱitȱisȱ clearȱthatȱthisȱdefinitionȱisȱequivalentȱtoȱEqȱ3Ȭ3.ȱ. Kc. isentropic work ȱ actual work. Eqȱ3Ȭ3ȱ. Fromȱtheȱfirstȱlawȱofȱthermodynamics,ȱassumingȱthatȱtheȱheatȱtransferȱrateȱtoȱ andȱfromȱtheȱcompressorȱcanȱbeȱneglectedȱasȱwellȱasȱtheȱchangeȱinȱpotentialȱ energy,ȱEqȱ3Ȭ3ȱcanȱbeȱrewrittenȱinȱtheȱfollowingȱform:ȱ. K cTT. h02 s h01 ȱ h02 h01. Eqȱ3Ȭ4ȱ. Assumingȱthatȱairȱisȱaȱperfectȱgas,ȱthusȱcpȱisȱconstant.ȱ. K cTT. T02 s T01 ȱ T02 T01. Eqȱ3Ȭ5ȱ. TheȱexpressionsȱareȱforȱtotalȬtoȬtotalȱisentropicȱefficiency.ȱȱȱ ȱ. 13ȱ.

(32) ȱ Anȱ evaluationȱ basedȱ onȱ Eqȱ 3Ȭ5ȱ assumesȱ thatȱ allȱ theȱ kineticȱ energyȱ atȱ theȱ compressorȱoutletȱcanȱbeȱused.ȱȱThisȱisȱtrueȱinȱtheȱcaseȱofȱaȱgasȱturbine,ȱsinceȱ theȱ velocityȱ atȱ theȱ compressorȱ deliveryȱ isȱ maintainedȱ atȱ theȱ combustionȱ chamber.ȱȱHowever,ȱtheȱcompressorȱofȱaȱturbochargerȱmustȱsupplyȱairȱviaȱaȱ relativelyȱ largeȱ inletȱ manifoldȱ toȱ theȱ cylinders.ȱ ȱ Henceȱ theȱ engineȱ willȱ onlyȱ ‘feel’ȱtheȱstaticȱpressureȱatȱtheȱcompressorȱdeliveryȱandȱisȱunlikelyȱtoȱbenefitȱ fromȱ theȱ kineticȱ energyȱ atȱ theȱ compressorȱ outlet.ȱ ȱ Thusȱ aȱ turbochargerȱ compressorȱ shouldȱ beȱ designedȱ forȱ highȱ kineticȱ toȱ potentialȱ energyȱ conversionȱbeforeȱtheȱoutletȱduct.ȱ ȱ Sinceȱ theȱ engineȱ benefitsȱ littleȱ fromȱ theȱ kineticȱ energyȱ ofȱ theȱ airȱ leavingȱ theȱ compressor,ȱ aȱ moreȱ realisticȱ definitionȱ ofȱ theȱ compressorȱ efficiencyȱ isȱ basedȱ onȱstaticȱdeliveryȱtemperatureȱasȱinȱEqȱ3Ȭ6,ȱwhereȱTSȱdenotesȱtotalȬtoȬstatic.ȱ. K cTS. T2 s T01 ȱ T02 T01. Eqȱ3Ȭ6ȱ. ȱ ItȱisȱcommonȱpracticeȱforȱmanufacturersȱtoȱquoteȱtotalȬtoȬtotalȱefficienciesȱforȱ turbochargerȱcompressors,ȱandȱquiteȱoftenȱthoseȱareȱquotedȱwithoutȱdeclaringȱ theȱbasisȱonȱwhichȱtheȱefficiencyȱvaluesȱareȱcalculated.ȱ ȱ 3.2.1.4. TheȱRadialȱTurbineȱ Theȱradialȱflowȱturbineȱconsistsȱofȱaȱscrollȱorȱinletȱcasing,ȱaȱsetȱofȱinletȱnozzlesȱ (sometimesȱomitted)ȱfollowedȱbyȱaȱshortȱvanelessȱgapȱandȱtheȱturbineȱwheelȱ itselfȱ(Figureȱ3Ȭ4).ȱȱMostȱsmallȱturbochargers’ȱturbinesȱuseȱaȱvanelessȱcasing;ȱ theȱnozzleȱisȱthenȱinȱtheȱformȱofȱaȱslotȱrunningȱallȱtheȱwayȱbetweenȱtheȱscrollȱ andȱturbineȱwheel.ȱȱAȱvanelessȱcasingȱcanȱbeȱusedȱtoȱimproveȱflowȱrangeȱatȱ someȱ penaltyȱ inȱ peakȱ performance,ȱ whileȱ alsoȱ reducingȱ cost.ȱ ȱ However,ȱ consideringȱtheȱmoreȱconventionalȱtypeȱwithȱnozzles,ȱtheȱfunctionȱofȱtheȱinletȱ casingȱ isȱ purelyȱ toȱ deliverȱ aȱ uniformȱ flowȱ ofȱ inletȱ gasȱ toȱ theȱ nozzleȱ entries.ȱȱ Theȱnozzlesȱaccelerateȱtheȱflow,ȱreducingȱpressureȱandȱincreasingȱtheȱkineticȱ energy.ȱ ȱ Aȱ shortȱ vanelessȱ spaceȱ preventsȱ theȱ rotorȱ andȱ nozzleȱ bladesȱ fromȱ touchingȱandȱallowsȱwakesȱcomingȱoffȱtheȱtrailingȱedgeȱofȱtheȱnozzleȱbladesȱ toȱ mixȱ out.ȱ ȱ Energyȱ transferȱ occursȱ solelyȱ inȱ theȱ impeller,ȱ whichȱ shouldȱ beȱ designedȱforȱminimumȱkineticȱenergyȱatȱtheȱexit.ȱ. 14ȱ.

(33) ȱ. Figureȱ3Ȭ4ȱComponentsȱofȱaȱRadialȱTurbineȱ(Watsonȱ&ȱJanota,ȱ1984)ȱ. ȱ. Theȱ flowȱ processȱ throughȱ theȱ turbineȱ mayȱ beȱ plottedȱ onȱ anȱ enthalpyȱ versusȱ entropyȱ diagramȱ asȱ shownȱ inȱ Figureȱ 3Ȭ5.ȱ ȱ Stationȱ 01ȱ refersȱ toȱ stagnationȱ conditionsȱatȱtheȱentryȱtoȱtheȱcasing.ȱȱTheȱgasȱwillȱalreadyȱhaveȱaȱsignificantȱ velocityȱ(C1),ȱhenceȱtheȱstagnationȱpressureȱisȱP01.ȱȱTheȱinletȱnozzlesȱaccelerateȱ theȱ flowȱ fromȱ stationȱ 1ȱ toȱ 2.ȱ ȱ Ifȱ thisȱ processȱ wereȱ isentropic,ȱ theȱ endȱ pointȱ wouldȱ beȱ 2s.ȱȱ Energyȱtransferȱoccursȱ inȱ theȱ rotor,ȱ betweenȱ stationȱ4ȱ andȱ 5ȱ (4ȱ andȱ 5sȱ ifȱisentropic)ȱ downȱtoȱtheȱexitȱpressureȱ P5.ȱȱTheȱstagnationȱP05ȱwillȱbeȱ higherȱthanȱP5ȱsinceȱtheȱexitȱvelocityȱwillȱremainȱsignificant.ȱȱStationȱ3ȱisȱtheȱ nozzleȱexitȱorȱtheȱnozzleȱthroat,ȱdenotedȱasȱstationȱ2.ȱ. Figureȱ3Ȭ5ȱhȬsȱdiagramȱforȱaȱradialȱturbineȱ(Watsonȱ&ȱJanota,ȱ1984)ȱ. 15ȱ. ȱ.

(34) ȱ 3.2.1.5. TurbineȱEfficiencyȱ Theȱ isentropicȱ efficiencyȱ ofȱ aȱ turbineȱ mayȱ beȱ definedȱ asȱ theȱ actualȱ workȱ outputȱ dividedȱ byȱ thatȱ obtainedȱ fromȱ reversibleȱ adiabaticȱ (isentropic)ȱ expansionȱbetweenȱtheȱsameȱtwoȱpressures.ȱȱȱ. Kt. actual work ȱ isentropic work. Eqȱ3Ȭ7ȱ. Assumingȱaȱ perfectȱgasȱ(cpȱ =ȱconstant)ȱ andȱfollowingȱ theȱsameȱ reasoningȱ asȱ withȱ compressors,ȱ itȱ canȱ beȱ shownȱ thatȱ Eqȱ 3Ȭ7ȱ canȱ beȱ expressedȱ inȱ termsȱ ofȱ temperaturesȱasȱinȱEqȱ3Ȭ8.ȱ. K tTT. T03 T04 ȱ T03 T04 s. Eqȱ3Ȭ8ȱ. Theȱ totalȬtoȬtotalȱ efficiencyȱ givenȱ inȱ Eqȱ 3Ȭ8ȱ assumesȱ thatȱ theȱ kineticȱ energyȱ leavingȱ theȱ turbineȱ exitȱ canȱ beȱ harnessed.ȱ ȱ Inȱ mostȱ applicationsȱ thisȱ isȱ notȱ possible.ȱ ȱ Theȱ energyȱ leavingȱ theȱ turbineȱ exitȱ goesȱ toȱ wasteȱ throughȱ theȱ exhaustȱ pipe.ȱ ȱ Thusȱ aȱ moreȱ relevantȱ isentropicȱ efficiencyȱ couldȱ beȱ basedȱ onȱ theȱ staticȱ exitȱ temperature.ȱ ȱ Theȱ totalȬtoȬstaticȱ isentropicȱ efficiencyȱ wouldȱ beȱ definedȱ asȱ theȱ actualȱ workȱ outputȱ dividedȱ byȱ isentropicȱ expansionȱ betweenȱ theȱstagnationȱinletȱandȱstaticȱoutletȱpressures.ȱ. K tTS. T03 T04 ȱ T03 T4 s. Eqȱ3Ȭ9ȱ. ȱ 3.2.2.. TurbochargingȱCIȱorȱSIȱenginesȱ. Today,ȱ turbochargedȱ CIȱ enginesȱ areȱ moreȱ commonȱ thanȱ turbochargedȱ SIȱ engines.ȱȱThereȱareȱsoundȱreasonsȱforȱthis,ȱbothȱeconomicȱandȱtechnical.ȱȱTheȱ principalȱ reasonsȱ stemȱ fromȱ theȱ differenceȱ betweenȱ theȱ combustionȱ andȱ controlȱsystemsȱofȱSIȱandȱCIȱengines.ȱȱTheȱSIȱengineȱuseȱaȱcarburettorȱorȱfuelȱ injectionȱ systemȱ toȱ mixȱ airȱ andȱ fuelȱ inȱ theȱ inletȱ manifoldȱ soȱ thatȱ aȱ homogeneousȱ mixtureȱ isȱ compressedȱ inȱ theȱ cylinder.ȱ ȱ Aȱ sparkȱ isȱ usedȱ toȱ controlȱ theȱ initiationȱ ofȱ combustion,ȱ whichȱ thenȱ spreadsȱ throughoutȱ theȱ mixture.ȱȱItȱfollowsȱthatȱtheȱmixtureȱtemperatureȱduringȱcompressionȱmustȱbeȱ keptȱbelowȱtheȱselfȬignitionȱtemperatureȱofȱtheȱfuel.ȱȱȱ ȱ. 16ȱ.

(35) ȱ Onceȱcombustionȱhasȱstarted,ȱitȱtakesȱtimeȱforȱtheȱflameȱfrontȱtoȱmoveȱacrossȱ theȱ combustionȱ chamberȱ burningȱ theȱ fuel.ȱ ȱ Duringȱ thisȱ time,ȱ theȱ unȬburntȱ endȬgasȱ (furthestȱ fromȱ theȱ sparkplug)ȱ isȱ heatedȱ byȱ furtherȱ compressionȱ andȱ radiationȱ fromȱ theȱ flameȱ front.ȱ ȱ Ifȱ itȱ reachesȱ theȱ selfȬignitionȱ temperatureȱ beforeȱ theȱ flameȱ frontȱ arrives,ȱ aȱ largeȱ quantityȱ ofȱ mixtureȱ mayȱ burnȱ veryȱ rapidly,ȱ producingȱ severeȱ pressureȱ wavesȱ inȱ theȱ combustionȱ chamber.ȱ ȱ Thisȱ situationȱisȱcommonlyȱreferredȱtoȱasȱknockȱandȱmayȱresultȱinȱsevereȱcylinderȱ headȱandȱpistonȱdamage.ȱȱLoweringȱtheȱCR,ȱusingȱfuelȱwithȱaȱhigherȱoctaneȱ numberȱorȱretardingȱtheȱignitionȱtimingȱareȱwaysȱtoȱpreventȱtheȱoccurrenceȱofȱ knock.ȱȱ ȱ Inȱ theȱ CIȱ engineȱ cylinder,ȱ airȱ aloneȱ isȱ compressed.ȱ ȱ Fuelȱ isȱ injectedȱ directlyȱ intoȱ theȱ combustionȱ chamberȱ fromȱ anȱ injector,ȱ onlyȱ whenȱ combustionȱ isȱ required.ȱ ȱ Thisȱ fuelȱ vaporisesȱ andȱ mixesȱ withȱ theȱ air,ȱ itȱ selfȬignitesȱ and,ȱ inȱ contrastȱtoȱSIȱcombustion,ȱitȱfollowsȱthatȱinȱaȱCIȱengineȱtheȱCRȱmustȱbeȱhighȱ enoughȱforȱtheȱairȱtemperatureȱduringȱcompressionȱtoȱexceedȱtheȱselfȬignitionȱ temperatureȱofȱtheȱfuel.ȱȱBecauseȱinjectionȱtakesȱtime,ȱonlyȱsomeȱofȱtheȱfuelȱisȱ inȱtheȱcombustionȱchamberȱwhenȱignitionȱstarts.ȱȱSinceȱmuchȱofȱtheȱfuelȱhasȱ notȱ fullyȱ vaporisedȱ andȱ mixedȱ withȱ theȱ air,ȱ theȱ initialȱ rateȱ ofȱ combustionȱ isȱ notȱsufficientȱtoȱinitiateȱdestructiveȱpressureȱwavesȱasȱinȱtheȱcaseȱofȱknockingȱ inȱaȱSIȱengine,ȱandȱthusȱdoesȱnotȱleadȱtoȱengineȱdamage.ȱ ȱ TheȱmaximumȱCRȱofȱtheȱSIȱengine,ȱbutȱnotȱtheȱCIȱengine,ȱisȱthereforeȱlimitedȱ byȱ theȱ ignitionȱ propertiesȱ ofȱ theȱ fuel.ȱ ȱ Theȱ minimumȱ CRȱ isȱ limitedȱ byȱ theȱ resultingȱ lowȱ overallȱ engineȱ efficiency.ȱ ȱ Turbochargingȱ resultsȱ inȱ notȱ onlyȱ aȱ higherȱcompressionȱpressure,ȱbutȱalsoȱaȱhigherȱtemperature.ȱȱȱ ȱ Unlessȱ theȱ CRȱ ofȱ aȱ SIȱ engineȱ isȱ reducedȱ theȱ temperatureȱ atȱ theȱ endȱ ofȱ compressionȱstrokeȱmayȱbeȱtooȱhighȱandȱtheȱengineȱmayȱknock.ȱȱTheȱengineȱ mayȱ remainȱ knockȱ freeȱ underȱ mildȱ boostȱ –ȱ butȱ onlyȱ becauseȱ thereȱ isȱ aȱ sufficientlyȱ safeȱ knockȬfreeȱ margin.ȱ ȱ Thusȱ theȱ potentialȱ powerȱ outputȱ ofȱ aȱ turbochargedȱ SIȱ engineȱ isȱ limited.ȱ ȱ Theȱ CIȱ engineȱ hasȱ noȱ suchȱ limitȱ andȱ canȱ thereforeȱuseȱaȱmuchȱhigherȱboostȱpressure.ȱ ȱ SIȱ enginesȱ costȱ substantiallyȱ lessȱ toȱ produceȱ thanȱ CIȱ enginesȱ ofȱ equivalentȱ powerȱ output,ȱ primarilyȱ asȱ aȱ resultȱ ofȱ higherȱ operatingȱ speedsȱ andȱ cheaperȱ fuelȱ injectionȱ system.ȱ ȱ Theȱ costȱ ofȱ theȱ turbochargerȱ onȱ aȱ CIȱ engineȱ isȱ moreȱ thanȱ offsetȱ byȱ theȱ reducedȱ engineȱ sizeȱ requiredȱ forȱ aȱ specificȱ powerȱ outputȱ (withȱtheȱexceptionȱofȱveryȱsmallȱengines).ȱȱThisȱsituationȱwillȱrarelyȱoccurȱinȱ theȱcaseȱofȱaȱSIȱengine.ȱ ȱ 17ȱ.

(36) ȱ 3.2.3.. EnergyȱAvailableȱinȱtheȱExhaustȱGasȱ. Figureȱ 3Ȭ6ȱ showsȱ theȱ idealȱ limitedȱ pressureȱ engineȱ cycleȱ inȱ termsȱ ofȱ aȱ pressure/volumeȱdiagramȱforȱaȱnaturallyȱaspiratedȱengine.ȱȱSuperimposedȱisȱaȱ lineȱ representingȱ isentropicȱ expansionȱ fromȱ pointȱ 5,ȱ atȱ whichȱ theȱ exhaustȱ valveȱopens,ȱdownȱtoȱtheȱambientȱpressureȱ(Pa),ȱwhichȱcouldȱbeȱobtainedȱbyȱ furtherȱexpansionȱifȱtheȱpistonȱwereȱallowedȱtoȱmoveȱtoȱpointȱ6.ȱȱTheȱshadedȱ areaȱ1Ȭ5Ȭ6ȱrepresentsȱtheȱmaximumȱtheoreticalȱenergyȱthatȱcouldȱbeȱextractedȱ fromȱtheȱexhaustȱsystem;ȱthisȱisȱcalledȱtheȱblowȬdownȱenergy.ȱ. ȱ Figureȱ 3Ȭ6ȱ Naturallyȱ Aspiratedȱ Idealȱ Limitedȱ Pressureȱ Cycleȱ (Watsonȱ &ȱ Janota,ȱ1984)ȱ Considerȱnowȱtheȱturbochargedȱengine;ȱtheȱidealȱfourȬstrokeȱpressure/volumeȱ diagramȱwouldȱappearȱasȱshownȱinȱFigureȱ3Ȭ7,ȱwhereȱP1ȱisȱtheȱturbochargingȱ orȱboostȱpressureȱandȱP7ȱisȱtheȱexhaustȱmanifoldȱpressure.ȱȱProcessȱ12Ȭ1ȱisȱtheȱ inductionȱstroke,ȱduringȱwhichȱfreshȱairȱatȱtheȱcompressorȱdeliveryȱpressureȱ entersȱtheȱcylinder.ȱȱProcessȱ5Ȭ1Ȭ13Ȭ11ȱrepresentsȱtheȱexhaustȱprocess.ȱȱWhenȱ theȱexhaustȱvalveȱfirstȱopensȱ(pointȱ5)ȱsomeȱofȱtheȱgasȱinȱtheȱcylinderȱescapesȱ toȱ theȱ exhaustȱ manifoldȱ expandingȱ alongȱ 5Ȭ7,ȱ ifȱ theȱ expansionȱ isȱ isentropic.ȱȱ ThusȱtheȱremainingȱgasȱinȱtheȱcylinderȱisȱatȱP7,ȱwhenȱtheȱpistonȱmovesȱtowardȱ topȱ deadȱcentreȱ(TDC),ȱ displacingȱtheȱcylinderȱcontentsȱthroughȱ theȱexhaustȱ valveȱ againstȱ theȱ backpressureȱ P7.ȱ ȱ Atȱ theȱ endȱ ofȱ theȱ exhaustȱ strokeȱ theȱ cylinderȱ retainsȱ aȱ volumeȱ (Vcl,ȱ clearanceȱ volume)ȱ ofȱ residualȱ combustionȱ products,ȱwhichȱforȱsimplicityȱcanȱbeȱassumedȱtoȱremainȱthere.ȱȱTheȱareaȱ7Ȭ8Ȭ 10Ȭ11ȱ willȱ representȱ theȱ maximumȱ possibleȱ energyȱ thatȱ couldȱ beȱ extractedȱ duringȱtheȱexpulsionȱstroke,ȱwhereȱ7Ȭ8ȱrepresentsȱisentropicȱexpansionȱdownȱ toȱtheȱambientȱpressure.ȱ. 18ȱ.

(37) ȱ. ȱ Figureȱ 3Ȭ7ȱ Turbochargedȱ Idealȱ Pressureȱ Limitedȱ Cycleȱ (Watsonȱ &ȱ Janota,ȱ 1984)ȱ ThereȱareȱtwoȱdistinctȱareasȱinȱFigureȱ3Ȭ7ȱrepresentingȱenergyȱavailableȱfromȱ theȱexhaustȱgas,ȱtheȱblowȬdownȱenergyȱ(areaȱ5Ȭ8Ȭ9)ȱandȱtheȱworkȱdoneȱbyȱtheȱ pistonȱ(areaȱ13Ȭ9Ȭ10Ȭ11).ȱȱTheȱmaximumȱpossibleȱenergyȱavailableȱtoȱdriveȱtheȱ turbochargerȱturbineȱwillȱclearlyȱbeȱtheȱsumȱofȱtheseȱtwoȱareas.ȱȱAlthoughȱtheȱ energyȱ associatedȱ withȱ oneȱ areaȱ isȱ easierȱ toȱ harnessȱ thanȱ theȱ other,ȱ itȱ isȱ difficultȱtoȱdeviseȱaȱsystemȱthatȱwillȱharnessȱallȱtheȱenergy.ȱȱToȱharnessȱallȱtheȱ energy;ȱ theȱ turbineȱ inletȱ pressureȱ mustȱ riseȱ instantaneouslyȱ toȱ P5ȱ whenȱ theȱ exhaustȱ valveȱ opens,ȱ followedȱ byȱ isentropicȱ expansionȱ ofȱ theȱ exhaustȱ gasȱ throughȱP7ȱtoȱtheȱambientȱpressureȱ(P8=Pa).ȱȱDuringȱtheȱdisplacementȱpartȱofȱ theȱexhaustȱprocessȱ(expulsionȱstroke)ȱtheȱturbineȱinletȱpressureȱmustȱbeȱheldȱ atȱP7.ȱȱSuchȱaȱseriesȱofȱprocessesȱisȱimpractical.ȱ ȱ Considerȱtheȱsimplerȱprocessȱinȱwhichȱaȱlargeȱchamberȱisȱfittedȱbetweenȱtheȱ engineȱandȱtheȱturbineȱinlet,ȱinȱorderȱtoȱdampȱoutȱtheȱpulsatingȱexhaustȱgasȱ flow.ȱ ȱ Byȱ formingȱ aȱ restrictionȱ toȱ flow,ȱ theȱ turbineȱ mayȱ maintainȱ itsȱ inletȱ pressureȱatȱP7ȱforȱtheȱwholeȱcycle.ȱȱTheȱavailableȱworkȱatȱtheȱturbineȱwillȱthenȱ beȱgivenȱbyȱareaȱ7Ȭ8Ȭ10Ȭ11.ȱȱThisȱisȱtheȱidealȱconstantȱpressureȱturbochargingȱ system.ȱ ȱ Nextȱ considerȱ anȱ alternativeȱ system,ȱ inȱ whichȱ aȱ turbineȱ wheelȱ isȱ placedȱdirectlyȱdownstreamȱofȱtheȱengineȱcloseȱtoȱtheȱexhaustȱvalve.ȱȱIfȱthereȱ wereȱ noȱ lossesȱ inȱ theȱ port,ȱ theȱ gasȱ wouldȱ expandȱ directlyȱ outȱ throughȱ theȱ turbineȱaloneȱlineȱ5Ȭ6Ȭ7Ȭ8,ȱassumingȱisentropicȱexpansion.ȱȱIfȱtheȱturbineȱareaȱ wereȱsufficientlyȱlarge,ȱbothȱcylinderȱandȱturbineȱinletȱpressuresȱwouldȱdropȱ toȱ P9ȱ beforeȱ theȱ pistonȱ hasȱ movedȱ significantlyȱ upȱ theȱ bore.ȱ ȱ Henceȱ theȱ availableȱ energyȱ atȱ theȱ turbineȱ wouldȱ beȱ givenȱ byȱ areaȱ 5Ȭ8Ȭ9.ȱ ȱ Thisȱ canȱ beȱ consideredȱ theȱ idealȱ pulseȱ turbochargingȱ system.ȱ ȱ Theȱ systemsȱ commonlyȱ referredȱtoȱasȱ‘constantȱpressureȱturbocharging’ȱandȱ‘pulseȱturbocharging’ȱareȱ basedȱ onȱ theȱ aboveȱ principles,ȱ butȱ inȱ practiceȱ theyȱ differȱ fromȱ theȱ idealȱ theoreticalȱcycles.ȱȱȱ. 19ȱ.

(38) ȱ ȱ 3.2.4.. ConstantȱPressureȱTurbochargingȱ. Withȱ constantȱ pressureȱ turbocharging,ȱ theȱ exhaustȱ portsȱ fromȱ allȱ cylindersȱ willȱ beȱ connectedȱ toȱ aȱ singleȱ exhaustȱ manifold,ȱ whoseȱ volumeȱ willȱ beȱ sufficientlyȱlargeȱtoȱdampȱdownȱtheȱunsteadyȱflow,ȱcausedȱbyȱtheȱblowȬdownȱ andȱ expulsion,ȱ fromȱ eachȱ cylinderȱ inȱ turn.ȱ ȱ Onlyȱ oneȱ turbochargerȱ needȱ beȱ used,ȱwithȱaȱsingleȱentry.ȱȱWhenȱtheȱexhaustȱvalveȱofȱaȱcylinderȱopens,ȱtheȱgasȱ expandsȱ downȱ toȱ theȱ (constant)ȱ pressureȱ inȱ theȱ exhaustȱ manifoldȱ withoutȱ doingȱ anyȱ usefulȱ work.ȱ ȱ However,ȱ notȱ allȱ ofȱ theȱ blowȬdownȱ energyȱ isȱ lost.ȱȱ Fromȱtheȱlawȱofȱconservationȱofȱenergy,ȱtheȱonlyȱenergyȱactuallyȱlostȱbetweenȱ cylinderȱ andȱ turbineȱ willȱ beȱ dueȱ toȱ heatȱ transfer.ȱ ȱ Withȱ aȱ wellȬinsulatedȱ manifold,ȱthisȱlossȱwillȱbeȱveryȱsmallȱandȱcanȱbeȱneglected.ȱȱȱ ȱ Considerȱ whatȱ happensȱ toȱ theȱ exhaustȱ gasȱ leavingȱ theȱ cylinder,ȱ expandingȱ downȱintoȱtheȱexhaustȱmanifoldȱandȱthenȱflowingȱthroughȱtheȱturbine.ȱȱAtȱtheȱ momentȱofȱexhaustȱvalveȱopening,ȱtheȱcylinderȱpressureȱwillȱbeȱmuchȱhigherȱ thanȱ theȱ exhaustȱ manifoldȱ pressure.ȱ ȱ Duringȱ earlyȱ stagesȱ ofȱ valveȱ openingȱ (whenȱtheȱthroatȱareaȱofȱtheȱvalveȱisȱveryȱsmall)ȱtheȱpressureȱratioȱacrossȱtheȱ valveȱ orȱ portȱ willȱ beȱ aboveȱ theȱ chokedȱ value.ȱ ȱ Henceȱ theȱ gasȱ flowȱ willȱ accelerateȱtoȱsonicȱvelocityȱinȱtheȱthroatȱfollowedȱbyȱaȱshockȱwaveȱatȱtheȱvalveȱ throatȱ andȱ suddenȱ expansionȱ toȱ theȱ exhaustȱ manifoldȱ pressure.ȱ ȱ Dueȱ toȱ turbulentȱmixingȱandȱthrottling,ȱnoȱpressureȱrecoveryȱoccurs.ȱȱTheȱstagnationȱ enthalpyȱ remainsȱ unchangedȱ andȱ henceȱ theȱ flowȱ fromȱ valveȱ toȱ turbineȱ isȱ accompaniedȱbyȱanȱincreaseȱinȱentropy.ȱ ȱ Asȱ theȱ valveȱ continuesȱ toȱ open,ȱ theȱ cylinderȱ pressureȱ willȱ fallȱ andȱ flowȱ throughȱ theȱ valveȱ becomesȱ subsonic.ȱ ȱ Theȱ flowȱ willȱ continueȱ toȱ accelerateȱ throughȱtheȱvalveȱthroatȱandȱexpandȱtoȱtheȱpressureȱinȱtheȱexhaustȱmanifold.ȱȱ Theȱ energyȱ availableȱ toȱ doȱ usefulȱ workȱ inȱ theȱ turbineȱ isȱ givenȱ byȱ theȱ isentropicȱ enthalpyȱ changeȱ acrossȱ theȱ turbine,ȱ whereasȱ theȱ actualȱ energyȱ recoveredȱisȱgivenȱbyȱtheȱenthalpyȱchangeȱacrossȱtheȱturbine.ȱȱClearlyȱitȱisȱtheȱ lackȱ ofȱ recoveryȱ ofȱ theȱ kineticȱ energyȱ leavingȱ theȱ valveȱ throatȱ andȱ theȱ throttlingȱ lossesȱ thatȱ leadȱ toȱ poorȱ exhaustȱ gasȱ energyȱ utilisationȱ withȱ theȱ constantȱpressureȱsystem.ȱ ȱ. 20ȱ.

(39) ȱ Theȱ volumeȱ ofȱ theȱ exhaustȱ manifoldȱ shouldȱ beȱ sufficientȱ toȱ dampȱ pressureȱ pulsationsȱdownȱtoȱaȱlowȱlevel.ȱȱThusȱtheȱvolumeȱrequiredȱwillȱdependȱonȱtheȱ cylinderȱreleaseȱpressureȱandȱfrequencyȱofȱtheȱexhaustȱgasȱpulsationsȱcomingȱ fromȱeachȱcylinderȱinȱturn.ȱȱPulseȱamplitudeȱwillȱbeȱaȱfunctionȱofȱengineȱload,ȱ theȱ timingȱ atȱ whichȱ theȱ exhaustȱ valveȱ opens,ȱ turbineȱ areaȱ andȱ exhaustȱ manifoldȱ volume.ȱȱ Frequencyȱwillȱbeȱdependentȱ onȱ theȱ numberȱofȱcylindersȱ andȱengineȱspeed.ȱȱTheȱeffectȱofȱengineȱspeedȱwillȱbeȱlessȱsignificant,ȱsinceȱtheȱ durationȱofȱtheȱexhaustȱprocessȱfromȱeachȱcylinderȱwillȱbeȱrelativelyȱconstantȱ inȱtermsȱofȱcrankȱangle,ȱratherȱthanȱtime,ȱandȱaȱsuitableȱturbineȱareaȱwillȱbeȱ chosenȱatȱtheȱoperatingȱspeedȱandȱload.ȱȱȱ ȱ Ifȱtheȱexhaustȱmanifoldȱisȱnotȱsufficientlyȱlarge,ȱtheȱblowȬdownȱorȱfirstȱpartȱofȱ theȱ exhaustȱ pulseȱ fromȱ theȱ cylinderȱ willȱ raiseȱ theȱ generalȱ pressureȱ inȱ theȱ manifold.ȱȱIfȱtheȱengineȱhasȱmoreȱthanȱthreeȱcylinders,ȱitȱisȱinevitableȱthatȱatȱ theȱ momentȱ whenȱ theȱ blowȬdownȱ pulseȱ fromȱ oneȱ cylinderȱ arrivesȱ inȱ theȱ manifold,ȱ anotherȱ cylinderȱ isȱ nearingȱ theȱ endȱ ofȱ itsȱ exhaustȱ process.ȱ ȱ Theȱ pressureȱ inȱ theȱ latterȱ cylinderȱ willȱ beȱ low,ȱ henceȱ anyȱ increaseȱ inȱ exhaustȱ manifoldȱ pressureȱ willȱ impedeȱ orȱ evenȱ reverseȱ itsȱ exhaustȱ processes.ȱ ȱ Thisȱ willȱbeȱparticularlyȱimportantȱwhereȱtheȱcylinderȱhasȱbothȱintakeȱandȱexhaustȱ valvesȱ partiallyȱopenȱ (valveȱoverlap)ȱandȱisȱrelyingȱonȱaȱthroughȬflowȱ ofȱairȱ forȱscavengingȱofȱtheȱburntȱcombustionȱproducts.ȱ ȱ Theȱconstantȱpressureȱsystemȱhasȱsomeȱadvantagesȱandȱdisadvantages:ȱȱȱ x Conditionsȱ atȱ turbineȱ entryȱ areȱ steady,ȱ thusȱ lossesȱ inȱ theȱ turbineȱ thatȱ resultȱfromȱunsteadyȱflowȱareȱabsent;ȱ x Aȱ singleȬentryȱ turbineȱ mayȱ beȱ used,ȱ eliminatingȱ ‘endȬofȬsector’ȱ lossesȱȱ (lossesȱassociatedȱwithȱflowȱfromȱoneȱturbineȱnozzleȱtoȱanother);ȱ x Useȱ ofȱ aȱ singleȱ turbochargerȱ impliesȱ aȱ largerȱ turbochargerȱ andȱ largerȱ machinesȱhaveȱhigherȱefficienciesȱthanȱsmallerȱones;ȱ x Aȱ turbineȱ designedȱ forȱ constantȱ pressureȱ operationȱ mayȱ haveȱ highȱ degreeȱ ofȱ reaction,ȱ coupledȱ withȱ anȱ exhaustȱ diffuser,ȱ bringingȱ additionalȱgainsȱinȱefficiency;ȱ x Fromȱ aȱ practicalȱ pointȱ ofȱ view,ȱ theȱ exhaustȱ manifoldȱ isȱ simpleȱ toȱ construct,ȱ butȱ isȱ ratherȱ bulky,ȱ particularlyȱ relativeȱ toȱ smallȱ enginesȱ withȱfewȱcylinders;ȱ x Transientȱ responseȱ ofȱ aȱ constantȱ pressureȱ systemȱ isȱ poor.ȱ ȱ Dueȱ toȱ theȱ largeȱ volumeȱ ofȱ gasȱ inȱ theȱ exhaustȱ manifold,ȱ theȱ pressureȱ isȱ slowȱ toȱ rise,ȱ resultingȱ inȱ poorȱ engineȱ responseȱ andȱ makingȱ itȱ unsuitableȱ forȱ applicationsȱwithȱfrequentȱloadȱorȱspeedȱchanges.ȱ ȱ. 21ȱ.

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