Theoretical Analysis of Noise of Piston Knocking Cylinder Wall in Automotive Engines.docx

Theoretical Analysis of Noise of Piston Knocking Cylinder Wall in Automotive Engines.docx

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TheoreticalAnalysisofNoiseofPistonKnockingCylinderWallinAutomotiveEngines

TheoreticalAnalysisofNoiseofPistonKnockingCylinderWallinAutomotiveEngines

XIAOYun-kui（肖云魁），LIShi-yi（李世义），CAOYa-juan（曹亚娟），

YANGWan-cheng（杨万成），FENGHan-sheng（冯汉生）

（1．SchoolofMechatronicsEngineering，BeijingInstituteofTechnology,Beijing100081,China;

2．MilitaryTransportationInstituteofTianjin，Tianjin300161，China）

Abstract：

Basedontheloadingconditionsofengine，applyingdifferencemethodtosolvethehydrodynamiclu—bricationequationofpistonskirtmovement，theforceactingonpistonskirtandthemomentonwristpinwere

obtained．AcomputerprogramforsimulatingthepistonsecondordermotionwasconductedtocalculatethelateralmotionoftheupperpartandthebottompartofpistonskirtsoftheengineofautomotivemodelCA1091．Fromthesimulatedresult，themaximalimpactingphaseandthemaximalimpactingregionofthepistonwereobtained．Theresultcanbeusedfordesigningengine，diagnosingthenoiseofpistonknockingcylinderwallandexplainingmanypracticalfaultphenomenaintheory．

Keywords：

pistonknockingcylinderwall；secondordermotion；faultdiagnosis

CLCnumber：

TH165；TK428Documentcode：

AArticleID：

1004．0579（2005）03．0284—05

Thenoiseofpistonknockingcylinderwall（PKCW）isoneofthemostfamiliarfaultphenomenainautomotiveengines．Whenthepiston-to-cylinderclearanceisbigandtheengineisrunningattheidlespeedthemetalknockingnoisewithaclearrhythmcanbeheardfromtheengine．Whenthefaultcylinderisflameoutthemetalknockingnoiseweakensordisappears．ThemildnoiseofPKCWisallowedinnormalengines，buttheenginewithheavynoiseofPKCWmustberepairedimmediately．Otherwise，thepistonwouldbebrokentopieces.TherearemanywaystodiagnosethenoiseofPKCW.InthepastseveralyearsdiagnosingthefaultofPKCWdependedfullyonrepair-expert’sexperience．Nowadayspeoplecandiagnosethefaultbymeansofinstruments．

Commonly，therearetwowaystoworkoutthetransversemovingtrajectoryofpiston．OneistosolvethepredigestedReynoldsequationandtheotheristosolveReynoldsequationdirectly．Inthispaperwecalculatetheoilfilmthicknessbetweenpistonandcylinderwallbasedondifferentstructureparametersanddifferentoperationalconditionsofenginewiththesecondmethod．Thevaluableresultswereobtained．

1KineticsEquationsofPistonSystem

Ontheplanenormaltowristpinaxis，theforceandmomentactingonpistonwouldbringthetinytranslationsandtherotationswithintheconfinementofthecylinderclearance，whicharecalledthesecondordermotionsofthepistonsystem．Bothmotionscanbedefinedbytheeccentricdistanceetandeboreccentricityεtandεb．ItisshowninFig．1．

Ifthefrictionsofthewristpinbearing，pistonringgroovesandpistonringfacesareneglected，basedontheconditionsofboththeforceequilibriumofpistonskirtandthemomentequilibriumofwristpin，thefollowingequationscanbededuced：

=

（1）

（2）

（3）

whereFgisthepush-forcethattheburninggasincombustionchamberactsonpiston；Fcisthetransverseinertiaforceofpiston；Fpisthetransverseinertiaforcebetweenwristpinandthesmallerendofconnectingrod；F’cistheaxialinertiaforceofpiston；F’pistheaxialinertiaforcebetweenwristpinandthesmallerendofconnectingrod；Misthemomentonwristpinthatoilfilmactsonpistonskirt；Mcisthepistoninertiamomentonwristpin；Fistheforceofoilfilm；Cｐisthedistanceonx—axisbetweenpiston’saxisandwristpin’saxis；Cgisthedistanceonz—axisbetweenpistoncentroidandwristpincenter；aisthedistancebetweenthetopofpistonskirtandwristpincenter；bisthedistancebetweenthetopofpistonskirtandpistoncentroid；Listhelengthofpistonskirt；mpisisthepistonquality；mpinisthequalityofbothwristpinandsmallerendofrod；Ipisisthepistoninertiamomentonwristpincenter；e’’bisthesidewiseaccelerationofthebottomofpistonskirt；e’’tisthesidewiseaccelerationofthetopofpistonskirt；Ψistheangleincludedbetweenconnectingrodandcylinderboreaxis．

HydrodynamicLubricationEquationofCylinderSleeveandPiston：

Whenapplyingtheincompressibleliquidlubricantequationtoexpressthelubricatingeffectbetweenpistonskirtandcylinderbore，itcanbeexpressedas

（4）

wherePisthepressureofoilfilm；histheoilfilmthickness；isthed

ynamicalviscidityoflubricatingisthepistonradius；Uistheslidingspeedofpiston’sreciprocatingmotion（gainedfromcrankshaftandconnectingrodmechanism）；tisthetime．Becauseofet，eb<

（5）

wherecistheradialclearancebetweenpistonandcylinderwall；f（y）istheaxialshapefunctionofpistonskirt．

Thefollowingdimensionlessparametersareintroduced：

εtdenotestheeccentricityofthetopofpistonskirt；εbdenotestheeccentricityofthebottomofpistonskirt；ωistheanglespeedofcrankshaft，Eq．（4）canberewrittenas

；

；

；

；

；

；

；

（6）

Whereε’tandε’barethefirstderivativesofεtandεbwithrespecttotimet．WiththeboundaryconditionsEq．（7），thenumericalsolutionofPcanbeevaluatedbymeansofdifferencemethods．Thenthedimension1essoi1filmforceFandmomentMcanbeevaluatedbyintegralfortheareaofP>0．Theintegralfunctionsare

Toagivenpistonmechanism，λ，R，ωandUarefixedvalues．InEqs．（6）一（9），F’andM’dependontherelationsofε’t，ε’b，εtandεb．Therefore，

2AnalysisofPistonSecond0rderMotionResults

AccordingtotherealcylinderchamberpressureofthetypeoftheengineofautomotivemodelCA1091，thesecondordermotionofpistoniscomputedbythemethoddiscussedabove．Variouskindsofparametersareasfollows：

ThelengthofpistonskirtL=0．0755m，thepistonradiusR=0．05029m，thelengthofconnectingrodLl：

0．217m，theradiusofcrankshaftRl=0．05715m，thequalityofpiston，Mpis=0．82kg，thequalityofwristpin，Mpin=0．23kg．thepiston-to-cylinderclearancec=0.08，0.12，0.16mmrespectively，thedistanceonx-axisbetweenpiston’saxisandwristpin’saxisCp=0.001，0.002mrespectively，dynamicalviscidityoflubricantμ=0．016Pa·s，0.00745Pa·srespectively，rotaryspeedofcrankshaft，n=500，800，1600，2000，2400r／minrespectively．Selectingpartofgraphs，somevaluableresultsarediscussedasfollows：

①Fig．2andFig．3showεt，εb，ε’’tandε’’bcurvesonnorma1clearance0．08mmand0．24mmatdifferentspeeds．Themaximumsofε’’tandε’’boccurnearthetopdeadcenterofthecompressioncyclewhile；ε’’tandε’’bareapproachingzeroinotherplaces．Inordertoexpressclearlyx-axis．340°-380°Crankanglesareonlytaken．Atthenormalclearancethemaxima1dimensionlesstransversedisplacement（DTD）ofpistonskirtis0．22．Atclearance0．24mm．themaxima1DTDreaches0．99andthemaxima1dimensionlesstransverseacceleration（DTA）is8timesasbigasthatofthenormalcondition．ThusthenoiseofPKCWwil1bemoreobvio

②Fig．2andFig．3alsoshowthattheDTDandDTAofpistonattherotaryspeedsbelow1200r／minaremuchbiggerthanthoseattherotaryspeedsabove1600r／min．

Atlowerrotaryspeedsofengine，thesyntheticalactionoftheburninggaspressureandthereciprocatinginertiaforceaswellasthefrictionforcesmakespistonrapidlychangeitstransversemovingdirectionnearthetopdeadcenterofenginecompressivecycle．Withtherotaryspeedrising．thesyntheticalactionofvariousforcesmakespistontransversemovementweakenandresultsinthenoiseofPKCWdecreasingundertheconditionofabnormalmatchingclearance．

Fromtheanalyses①and②，wecanknowthatthesensorshouldbeplacedontheupperpartofcylinderbody，i．e．nearthetopdeadcenterofengine，andtherotaryspeedshouldbebelowthemiddlerangespeedofengine，ifweuseinstrumentstocheckthenoiseofPKCW．

③Fig．4showsεt，εb，ε’’tandε’’bcurvesatdifferentspeedsformatchingclearance0．24mmwiththecylinderflameout．FromFig．2andFig．3itcanbeknownthatwhenthecylinderisflameouttheDTDandtheDTAofpistondecreaseaccordingly．Intherangeoflowerandmiddlespeeds，themaximalDTDofpistonisonly0．4，whichcannotproducetheobviousnoiseofPKCW．ItindicatesthatwhentheloadofenginerisesthenoiseofPKCWbecomesbigger．Whenthecylinderisflameout，thenoiseofPKCWweakens．IfsomeonediagnosesthenoiseofPKCWbyhisexperience，whenhemakesthefaultcylinderflameoutandthenoiseofPKCWweakens．heisab1etojudgethefaultcylinderimmediate1y．

④ThedynamicalviscidityoflubricantsignificantlyaffectsthenoiseofPKCW．TheDTDofpistonwiththelubricantdynamica1viscidityof0.016Pa·sismuchsmallerthanthatofpistonwiththelubricantdynamicalviscidityof0.00745Pa·s．Withinthepropermatchingclearanceofpiston—to-cylinder．Thebiggerthedynamicalviscidityoflubricantis，thethickerthethicknessofoilfi1mwillbe,andthereisgreaterpossibilitytoavoidPKCW.Sincethedynamicalviscidityoflubricantdecreasesastemperatureincreases，ifthetemperatureistoohigh，thedynamicalviscidityoflubricantdecreasestempestuous1vandtheoilfilmofacertainthicknessbetweenpistonandcylindercannotbeformed，PKCWoccurseasily.

⑤PKCWmainlytakesplaceatthebottomofpistonskirt．EventhoughtheDTDonthetopofpistonskirtchangesgreatly，astheoilfilminthatpartisthickenough，thepossibilityofPKCWdecreases.ThereforealmostallthePKCWtakeplaceatthebottomofpistonskirt．

Themodernmotorcarenginehighestrotationalspeedmayreach4000~6000r/min,pistoneachsecondneedstowalkseveraldozenstravelingsc