NUMERICAL ANALYSIS OF THE THERMAL STRESSES OF A PETROL ENGINE PISTON WITH DIFFERENT MATERIALS

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1 The Irqi Journl For Mechnicl And Mteril Engineering, Vol.8, No.3, 2008 NUMERICAL ANALYSIS OF THE THERMAL STRESSES OF A PETROL ENGINE PISTON WITH DIFFERENT MATERIALS Dr.Njim A.Sd * ; Dr. Hithm R. Aed Ali ** ; Dr. Hyder Shkir Audll *** * Mteril Eng.Dept./Bylon University ** Mech. Eng.Dept./Bylon University *** Mech. Eng.Dept./ Qdisiy University ABSTRACT In this pper numericl nlysis is used to nlyze the stresses due to therml cycle with different luminum lloy of piston.finite element method ws used to evlute the coupling field (therml stress) on the piston.ansys5.4 Finite element code is used to crry out the modeling process to determine the coupling stress.two models with three dimensions re creted.the first is used to evlute the temperture distriution through the piston volume, nd the second is used to evlute the therml stress distriution due to het grdient nd the mteril different. The result show the mximum rnge of tempertures is 4.3 C nd increses with decresing of mteril therml conductivity.therml stress is concentrted on the piston edges nd depends on the mteril types. الخالصه: تم خالل ھذا البحث استخدام التحليل العددي لالجھادات الناتجه من دورة الحراره التي يتعرض لھا مكبس محرك بنزين مصنع م ن س بائك مختلف ه م ن االلومني وم.ت م اس تخدام طريق ه العناص رالمحدده لح ساب توزي ع االجھ ادات المرافقه للحراره على المكبس.استخدم نظام التحليل ANSYS5.4 لتنفيذ عملية النمذجه.تم بناء نموذجين ثالثي ة االبعاد االول لحساب توزيع درجة الحرارة خالل المكبس اما الثاني فيستخدم لحساب توزيع االجھ ادات الحراري ه الناتجه من االنحدار الحراري واختالف مادة المكبس. اوضحت النتائج ان اقصى مدى للحراره ھو (C 4.3) ويزداد مع نقصان التوصليه الحراريه للماده. االجھ اد الحراري يتركز على حافات المكبس ويعتمد على نوع الماده. INTRODUCTION High pek pressure (hpp) opertion hs importnt mechnicl design. Consequences, notly incresed component stresses due to the higher pressure nd therml lods [1][2]. There re mny studies to the het trnsfer in diesel, engines components ex, A.P. Kleemn et l. [1], in their work the mgnitude nd origins of locl of sptil nd temporl surfce het flux vritions in diesel engine hve een investigted using computtionl fluid dynmics (CFD) simultion code nd experimentl mesurement, s prt of the CFD explortions two diffrct wll function models of flow nd therml oundry lyers hve een used. The single-cylinder Hpp proto types DI diesel engine developed in their project is sed on the OM500 series heving-duty four vlve truck engine with chrcteriztion[3], s shown in tle(1). Thoms Gross [4], in his pper develops engines vlue with improved het trnsfer for lrge diesel engines. Finite element nlysis of therml operting chrcteristics of thermo vlue is crried out y using ANSYS FE code. 3-Dimension model of one qurter of vlve with cylinder heds is creted nd implemented ANSYS codes. The level of efficiency is incresed with incresing of pressures nd tempertures, this, in turn, requires improved het trnsfer from comustion chmer to the cooling medium y ll cylinder hed components, which must therefore lso e more nd more resistnt to het. In the re of gs exchnge component, this prolem ws 249

2 Dr.Njim, The Irqi Journl For Mechnicl And Mteril Engineering, Vol.8, No.3, 2008 solved y using new mteril nd new comintions of mterils s well s y using lterntive geometry [4,5]. In present work 3-Dimensionl FEM model of engine piston is creted nd implemented y using FE ANSYS 5.4 code to study the rnge of stresses distriution within one cycle of opertion for petrol engine ws mde of different lloys. Boundry Conditions: Temperture nd pressure clcultion of fresh chrge (ir & fuel mixture): The temperture nd pressure of fresh chrge in compression, expnsion nd comustion strokes re clculted depending on the first lw of thermodynmic [8], which consists ll equtions to clculte the temperture nd pressure s function of crnk ngle. In this reserch, the compression rtio is given (8) while the temperture nd the pressure t the eginning of the stroke re ssumed s 25 C nd 1 r respectively. After some clcultion [8], the pek pressure nd temperture my e reched to 64 r nd 3600 C Het trnsfer conditions: Three of the hottest points re round the sprk plug, the exhust vlve nd port, nd the fce of the piston. Not only re these plces exposed to the high-temperture comustion gses, ut they re difficult plces to cool. The piston fce is difficult to cool ecuse it is seprted from the wter Jcket or outer finned cooling surfces. During comustion pek gs tempertures on the order of ( k) occur within the cylinders, nd effective het trnsfer is needed to keep the cylinder wlls nd the piston from over heting. The piston sors convective nd rdition het trnsfer from the high tempertures gses nd losses the het to the cylinder wll, rings nd the luricting oil such s: Convective nd rdition het trnsfer from the high temperture gses to the piston fce: convective het trnsfer on the piston fce from the comustion gses: The mthemticl formuls of the convection het trnsfer in the IC engine re: q = h g A pf (T g -T pf ) Where: h g = het trnsfer coefficient y convection A pf = cross sectionl re of the piston fce T g = gs temperture t the pek comustion T pf = initil temperture of the piston fce The verge vlue of the convection het trnsfer coefficient cn e clculted from the knowledge of Nusselt numer vlue. The Nusselt numer for the inside of the comustion chmer cn e defined using the following reltion (Dittus - Boelter eqution)[8]. Nu=h g B / k g = Re 0.8 Pr 0.3 Where: B = ore of cylinder K g = therml conductivity of cylinder gs Pr = Prndtl numer Re = Reynolds numer 250

3 Dr.Njim, The Irqi Journl For Mechnicl And Mteril Engineering, Vol.8, No.3, 2008 Re = [(m + m f )B] / A pf µ g Where: m = mss flow rte of ir into the cylinder m f = mss flow rte of fuel into the cylinder A pf = re of piston fce µ g = dynmic viscosity of gs in the cylinder Rdition het trnsfer etween comustion gses nd cylinder wlls nd piston fce The mthemticl formuls of the rdition het trnsfer in the IC engine re: q = Q/A = [σ (T g 4 -T pf 4 )] / [[(1-ε g ) / ε g ]+[1/F 1-2 ]+[(1-ε pf ) / ε pf ]] Where: T g = gs temperture T pf = piston fce temperture σ = Stefn Boltzmnn constnt ε g = emissivity of gs ε pf = emissivity of piston fce mteril F 1-2 = view fctor etween gs nd piston fce Even though gs temperture re very high, rdition to the wlls only mounts to out 10% of the totl het trnsfer in SI engines. This is due to the poor emitting properties of gses, which emit only t specific wvelengths. N 2 nd O 2, which mke up the mjority of the gses efore comustion, rdite very little, while the CO 2 nd H 2 O of the products do contriute more to rdition het trnsfer. Het trnsfer from the piston to the rings: Het is trnsferred from the piston to the rings y the conductive het trnsfer ecuse of the temperture grdient etween the piston nd the rings such s Q ring = (T pc T i ) k ring / x ring Where: T pc = Temperture of contct re etween the piston nd the rings T i = Temperture of luricting oil sprys ove the rings K ring = Therml conductivity of the rings x ring = thickness of the ring Het trnsfer from the piston to the cylinder wlls: There re droplets of luricting oil etween the piston nd the cylinder wll with smll thickness ( S) which cuses conductive het trnsfer from the piston to the inner wlls of comustion chmer through the luricting oil lyer s follow: Q = (T wp T w ) k oil / S Where: Q = Conductive het trnsfer etween the piston nd the cylinder wlls directly T wp = Temperture t the outer surfce of the piston T w = Temperture of the cylinder wlls K oil = Therml conductivity of the luricting oil S = Clernce etween the piston nd the cylinder wlls (equl to the luricting oil lyer thickness) 251

4 Dr.Njim, The Irqi Journl For Mechnicl And Mteril Engineering, Vol.8, No.3, 2008 Het trnsfer from the piston to the luricting oil: Convective het trnsfer is occurred from the inner fce of the piston to the luricting oil s the following reltion. Q oil = h oil (T ip T oil ) Where: h oil = het trnsfer coefficient y convection T ip = Temperture of inner fce of the piston T oil = Luricting oil temperture Nu oil = h oil D i / k oil = Re 0.9 Pr 0.3 Re = ρ oil u D i / µ oil Where: ρ oil = density of the luricting oil µ oil = dynmic viscosity of the luricting oil u = SN/30 S = length of the cylinder stroke N = rpm Finite Element Modeling FE simultion of engine piston ws performed with ANSYS 5.4 code, using widely employed su-models for the het trnsfer, stress in elstic, plstic regions nd mteril properties. As fllows: 1. Geometry su-model: This su-model includes 3-D dimensions Geometry of the engine piston with rings (mm) dimensions, s show in Fig.(1). 2. Mteril su-model: this su-model includes the mteril properties of the engine piston nd ring in the tle (2): the mteril properties re vrying with silicon content. 3. Meshing genertion: three dimensions coupling element of 3-D het trnsfer (Conduction) nd 3-D-stress nlysis re used for modeling of het trnsfer nd stress nlysis re (solid 70 nd solid 98) with mesh size s shown in Fig. (2,,). 4. Prmeter study: Temperture (vrition) nd mteril of piston re studied through the simultion. Result nd discussion: Numericl nlysis of petrol engine show the distriution of temperture on the piston ody during one cycle of opertion for the piston ws mde of different AL-Si lloy [7]..One of the importnt fetures of this lloy is wer resistnce [7], ecuse of the second phse of silicon in this lloy, ut it suffer from decresing the therml conductivity nd expnsion with compred with AL element nd tht leds to increse the temperture grdient rnge in the piston ody s shown in the figures (3,4,5 ). The temperture grdient increses due to decrese of therml conductivity with incresing of the silicon content in AL -lloy.incresing of temperture grdient leds to increse the therml stress in the piston lloy s shown in the figures (6,7,8. ) which re shown the stress contour within piston ody,lso, the numericl nlysis show the distriution of the temperture with piston volume. From the previous figures we note the mximum temperture is concentrted on the top fce of the piston. Tempertures vlues decrese with incresing the distnce from top fce of 252

5 Dr.Njim, The Irqi Journl For Mechnicl And Mteril Engineering, Vol.8, No.3, 2008 piston, nd tht mens the numericl modl is ctive to descrie the temperture distriution nd temperture grdient in piston volume. Fig.s (3.4.5,)show temperture distriution in the rings mteril, lso the figures (6,7,8 ) show the therml stress in these rings. The temperture nd stress distriution is differ from tht in the piston mteril nd tht ecuse of the different in the mteril conductivity, where the ring ws mde of the cst iron.from the numericl results we cn conclude those models re ctive in nlysis temperture nd stress in these components. Tle (1) Hpp prototype engine geometricl informtion nd opertion conditions [3] Bore Stroke Connecting rode length 130mm 150mm 273mm Compression rtio Boost pressure Boost temp. Engine speed r 315 K(42 C) 1420 R.P.M Tle (2) mteril properties of model component Components Piston (lloy1) Piston (lloy1) Piston (lloy1) Rings 2.5% Si 8% Si 12% Si (cst iron) E (GP) K (Wlm.k) cm 12.6 cm Fig.(1) : Geometry Su-model 253

6 Dr.Njim, The Irqi Journl For Mechnicl And Mteril Engineering, Vol.8, No.3, 2008 Fig (2) Element Genertion Fig (3) temperture distriution, piston (lloy1) -rings Fig (4) temperture distriution, piston (lloy2) -rings 254

7 Dr.Njim, The Irqi Journl For Mechnicl And Mteril Engineering, Vol.8, No.3, 2008 Fig (5) temperture distriution, piston (lloy3) -rings Fig (6) stress distriution, piston (lloy1) -rings Fig (7) stress distriution, piston (lloy2) -rings 255

8 Dr.Njim, The Irqi Journl For Mechnicl And Mteril Engineering, Vol.8, No.3, 2008 Fig(8) stress distriution, piston (lloy3) -rings References A.P. Kleemnnt etl., "Het trnsfer in diesel engines: A CFD evolution study" the Fifth Interntionl symposium on Dignostics nd modeling of comustion in Internl comustion Engineering July 1-4, G. Bormn nd K. Nishiwki, "Internl Comustion engine het trnsfer" progress in energy nd Comustion Science High Fuel. Efficient diesel engine with significntly incresed pek pressure (HEDE), periodic progress Report, 12 months, contct No. BRPR-CT , project No, BE , August 19970July presenttion R. Gregg Bruce. "Modern mteril nd mnufcturing processes", prentice hll, second Edition Pt L. Mngonon, "The principle of mteril selection for engineering design " prentice hll R.A.Benson,et. l. " I internl comustion engines " vol.1, 2, pergmon press Willrd "Engineering fundmentls of the internl comustion engine" New jersey 256