《發(fā)動機原理》雙語教案

Chapter5CombustioninCompression-ignitionEngines

Key:CombustionphenomenoninCIengines,C.I.enginecombustionchamber.

Difficultpoints:Improvefuelsprayandatomization.

Section1Fuelsprayandatomization

5.1.1Fuelinjectionsystem

1.Injectionsystemoverview

Neartheendofthecompressionstroke,liquidfuelisinjectedasoneormorejets.Theinjectorreceivesfuelatveryhighpressuresinordertoproducerapidinjection,withhighvelocityjet(s)ofsmallcross-sectionalarea.

Thefueljetsentrainairandbreakupintodroplets;thisprovidesrapidmixingwhichisessentialifthecombustionistooccursufficientlyfast.Therewillbelargevariationsinfuel/airmixturesonbothalargeandsmallscalewithinthecombustionchamber.

2.Traditionalinjectionpumps

Thefuelcanbemeteredatahighpressure,asinthecommonrailsystem,oratalowpressure,asinthejerkpumpsystem.

Thejerkpumpsystemaretwoprincipaltypes:in-linepumpsandrotaryordistributorpumps.

3.Fuelinjectornozzles

Nozzleshaveaneedlethatclosesunderaspringloadwhentheyarenotspraying.Theneedle-openingisdeterminedbythespringloadandtheprojectedareaoftheneedle.

5.1.2Fuelsprayandatomization

1.Overallspraystructure

Fromgeometricalshapeandatomizationqualitytwoaspectsdescribingfuelbeamcharacteristics.

Differentcombustionchamberhasdifferentdemandsonfuelbeams.

2.Atomization

Underdieselengineinjectionconditions,thefueljetusuallyformsacone-shapedsprayatthenozzleexit.Throughtwostepsofwind-inducedbreakupregime,thedropletsbecomemuchsmallerthanthenozzlediameter.

3.Dropletsizedistribution

Dropsizedistributioninthefuelspaydependsoninjectionparametersandtheairandfuelproperties.

AnappropriateandcommonlyusedmeandiameteristheSautermeandiameter:

While,

Section2CombustionphenomenoninCIengines

5.2.1FourphasesofCIenginecombustion

Figureshowsthepressurediagramforacompressionignitionengine

(i)Ignitiondelay,AB.Afterinjectionthereisinitiallynoapparentdeviationfromtheunfiredcycle.Duringthisperiodthefuelisbreakingupintodropletsbeingvaporized,andmixingwithair.Chemicalreactionswillbestarting,albeitslowly.

(ii)Rapidoruncontrolledcombustion,BC.Averyrapidriseinpressurecausedbyignitionofthefuel/airmixturepreparedduringtheignitiondelayperiod.

(iii)Controlledcombustion,CD.Combustionoccursataratedeterminedbythepreparationoffreshair/fuelmixture.

(iv)Finalcombustion,D.Aswithcontrolledcombustiontherateofcombustionisgovernedbydiffusionuntilallthefuelorairisutilized.

5.2.2Delayperiod(orignitionlag)inCIengines

Effectsofvariousfactorsondelayperiod:

1)Fuelproperties

2)Intaketemperature

3)Compressionratio

4)Enginespeed

5)Typesofcombustionchamber

6)Injectoradvance

5.2.3Detonationinthedieselengine(dieselknock)

Theprimarycauseofdieselknockisduetoaprolongeddelayperiodforthefollowingfactors:

1)Alowdesigncompressionratio

2)Alowcombustionpressure

3)Poorfuelignitionquality

4)Apoorlyatomizedfuelspray

5)Aninadequateinjectorneedlespringload

6)Averylowairintaketemperature

Section3C.I.enginecombustionchamber

Despitethevarietyofshapes，allthecombustionchambersareclaimedtogiveequallygoodperformanceintermsoffueleconomy，powerandemissions，whenproperlydeveloped．

Themostimportantairmotionindirectinjectiondieselenginesisswirl，theorderedrotationofairaboutthecylinderaxis．

5.3.1Directinjection(DI)chambers

1)Inductionswirl

2)Compressionswirl

3)Combustioninduceswirl

5.3.2Indirectinjection(IDI)chambers

Indirectinjectionsystemshaveadividedcombustionchamber,withsomeformofpre-chamberinwhichthefuelisinjected,andamainchamberwiththepistonandvalves.Thepurposeofadividedcombustionchamberistospeedupthecombustionprocess,inordertoincreasetheengineoutputbyincreasingenginespeed.

Therearetwoprincipalclassesofthiscombustionsystem;pre-combustion

chamberandswirlchamber.Pre-combustionchambersrelyonturbulencetoincreasecombustionspeedandswirlchambers(whichstrictlyarealsopre-combustionchambers)relyonanorderedairmotiontoraisecombustionspeed.

5.3.3Theeffectsofoperatingfactorsonthecombustionprocess

Fuelcharacteristics

Fuelsupplyadvancedangle

Injectionrule

Qualityoffuelatomize

Airmotionincylinderchamber

Compressiontemperatureandpressure

Speed

Load

Fuelcharacteristics

Thefuelcetanenumberdirectlyaffectsthelengthofignitiondelay

Viscosityofdieseloilinfluencefuelsprayandatomization

Fuelsupplyadvancedangle

Fuelsupplyadvanceangleistoolarge,fuelinjectsintoairwithrelativelylowtemperatureandpressure,ignitiondelayperiodgrow,runningroughly

Fuelsupplyadvanceangleistoosmall,thefuelcannotcombustnearthetopdeadcenterintime,adversetotheeconomyandpowerperformance,particleemissionswillincrease

Foreachkindofcondition,allhaveabestfuelsupplyadvanceangle

Fuelinjectionrule

Inordertopreventthedieselengineworkroughlyandreducefinalburning,idealfuelinjectionlawshouldbe"slowafterthefirsturgent",namelytheinitialstageisslow,fuelinjectionrateisincreasedsharplyafterfire,theendoffuelinjectionneedbreakquickly.

Qualityoffuelatomize

Improvementoffuelatomizationfinenesscanpromotefuelevaporation,acceleratemixtureformation,improvecombustionrate.Buttoomuchatomizationoffuelinjectedinignitiondelayperiodcanalsoleadtothepressureistoohigh,runningroughly

Airmotionincombustionchamber

Properorganizationofairmovementincombustionchambercanenhancefuelevaporationrate,shortentheignitiondelay.Iftheswirlistoostrong,acoolinglosswillincrease.Itwillleadtothegrowthofignitiondelay,meanwhile,organizationofintakeswirlconsumesmoreenergy,lowerthevolumeefficiency

Compressiontemperatureandpressure

Highcompressiontemperaturecanspeedupthefuelevaporationandoxidation

Pressureincreases,themixturedensityincrease,shorte