GAI Hong-wu(盖洪武), CHENG Ying(程颖), YAO Xiu-gong(姚秀功)

(School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, China)

Influence of the cylinder head structure of a diesel engine on fatigue strength

GAI Hong-wu(盖洪武), CHENG Ying(程颖), YAO Xiu-gong(姚秀功)

(School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, China)

Using finite element method, influence of diesel cylinder head structure on fatigue strength is investigated. A simplified head model with function characteristics is built for thermal-mechanical simulation. From the simulation results, the influence of valve bridge structure and roof transition fillet dimension on fatigue strength are obtained. And a new valve bridge structure which can effectively improve the fatigue life is proposed.

cylinder head; structural feature; fatigue; conceptual design; thermal-mechanical

Because of the poor working environment and complex load situation, the diesel engine cylinder head is prone to fatigue failure. The future development of automobile industry require high power density and lightweight structure[1], which makes the fatigue problems of cylinder head even worse. Cylinder heads are more often considered as the power-limiting component in the conception of new engine designs[2]. In recent years, lots of studies have been carried out in the field of the cylinder head fatigue with tremendous progress. But most of these studies focus on some specific engines. For the ever-changing forms of cylinder head structure, the research results are not transferable, because of the particularity of structures.

In this paper, the locations of the cylinder head which are prone to fatigue are summarized and classified. Further researches were carried out aiming at the most dangerous parts of cylinder head. The structures of cylinder heads are diverse and tanglesome. Based on the analysis of structural characteristics of the cylinder head, its structure characteristics are divided into seven parts in accordance with the functions. And the simplified geometric model of cylinder head that can achieve the basic function has been built. Then, for the purpose of investigating the structure modification of fatigue-prone locations influence on fatigue life, the structures are changed and thermo-mechanical simulations are carried out on the simplified model.

1 Fatigue-prone positions

According to analysis of the fatigue failure instances of cylinder head and relevant literature, the fatigue-prone locations of diesel engine cylinder head are summarized as follows:

①F1 flame deck valve bridge region[3];

②F2 the connection of roof and valve guide wall[4];

③F3 the connections of flame deck and exhaust or inlet[5];

④F4 the injector hole[6];

⑤F5 valve seat boring, near to the valve bridge;

⑥F6 valve guide boss, within the exhaust guide boss itself;

⑦F7 fatigue-prone areas of cooling water jacket surface[7].

Among these fatigue-prone positions, valve bridges and the roof are the most likely to fatigue failure.

2 Analysis of structure characteristics

Cylinder head structure is extremely complex, and wide variation. But from the perspective of the basic functions and working principle, most cylinder heads have common structural features.

As a fixed part, cylinder heads must have a basic frame structure to carry the other functional structures mounted on it. Secondly, as an internal combustion engine，which output power depending on the combustion of mixed gas, cylinder head need channels to provide air and fuel to the combustion chamber and channel to exhaust. Therefore, the corresponding cylinder head must have the gas circulation structure and fuel injector mounting structure. For completing the gas exchange process in accordance with the valve timing, cylinder heads must have valve train installation structure. Since the combustion process releases large amounts of heat, a cooling system is needed to take the heat away to prevent the cylinder head over heating.

In addition, in order to enhance the structural strength and manufacture easy, the cylinder head must have general process and strengthen structure. Therefore, the cylinder head can be divided into the following seven parts: basic framework; gas circulation structure; valve train mounting structure; injector mounting structure; cooling system; sealing structure; process and strengthening structure. Concrete structural analysis is presented in Fig.1, the gray blocks are parts which prone to damage.

Fig.1 Diesel engine cylinder head structure analysis

3 Conceptual design of geometric

In modern diesel engine, four-valve technology is a mature technology among the multi-valve technology, and is widely used in heavy vehicles and special vehicles. Research shows that when the boost pressure reaches or exceeds five, the combustion efficiency of engine with spiral inlet will be significantly reduced and the influence of helical port in cylinder on the combustion efficiency will be weakened dramatically[4]. There are many advantages for the overall layout of vertical mid-injector, including improvements of the uniformity of the cylinder head structure, the symmetry of the load and the interchangeability of related parts. As a result, a simplified model with 4-valve, tangential inlet, vertical mid-injector cylinder head is built as the research object. The process of establishment the simplified cylinder head model and the simplified model is shown in Fig.2 and Fig.3.

Fig.2 Process of establishment the simplified model

Fig.3 Simplified geometrical model

4 Finite element analysis

4.1 FE model

The simplified model is a symmetrical structure. In order to reduce the grid size, a half model is taken for calculation. The details are described as follows:

①The model size and mesh density: the FF model is meshed with second-order terta elements, and the critical areas are locally refined, such as F1, F2 and F3.

②The material property definition: elastoplasticity.

③The timescale of the analysis: steady state.

Fig.4 Assembly CAE model

④The contact definition: small sliding.

The assembly CAE model is shown in Fig.4.

4.2 Thermodynamic calculation

The surface heat transfer coefficient of water jacket and the temperature of coolant are obtained from the computational fluid dynamic calculation; the temperature and heat transfer coefficient of the high temperature gas are obtained by GT-power, an engine performance simulation software, and the thermal boundary conditions of combustion chamber are shown in Fig.5a[8]. And then, the thermal boundary conditions are applied in Abaqus to get the cylinder head temperature field. Fig.5b shows the temperature distribution of cylinder head.

Fig.5 Thermodynamic calculation

4.3 Mechanical calculations

Under the condition of assembly with bolts, valve seat rings, valve guides and the block, the mechanical calculations of head was achieved by using finite element analysis software Abaqus. The following four most typical operating cases were analyzed in steady state. The simulation results are shown in Fig.6.

Fig.6 Mechanical calculation results and the location of the investigated nodes

Case 1: the assembly conditions, the preload of the bolts, the interference assembly force of valve seats and valve guides are loaded to the CAE model.

Case 2: apply the peak firing pressure to cylinder head on the basis of case 1.

Case 3: superimpose the temperature field obtained in the thermodynamic analysis onto the head, on the basis of case 1.

Case 4: superimpose the firing pressure onto case 3.

5 Result analysis

In order to study the affects of mechanical load and thermal load in the fatigue-prone locations, the equivalent stresses of the six locations in the aforementioned four cases are extracted. The ratio of the thermal stress (σth)andthetotalstress(σt)ateachlocationisinvestigated.

Loadingconditionsoffatigue-pronelocationsindifferentoperatingcasesareshowedinFig.7.FromFig.7,wecanseethat①InF1,F4andF5,thermalloadisthemainfactorwiththeratiosofthermalstresstototalstressbeing75%, 89%and65%; ②InF2andF3,mechanicalloadisthemainfactor,withtheratiosofthermalstresstototalstressbeing4%and20%; ③InF6,itisinfluencedbybothmechanicalloadandthermalloadwiththeratioof44%.

Fig.7 Loading conditions of fatigue critical locations

6 Influenceofstructure

Accordingtotheaboveoutcomes,thefailureofF1andF2arequiterepresentative.Therefore,thenextcontentfocusonthetwocriticallocations.

6.1F1area

ThefailureofF1isthetypicallowcyclethermalmechanicalfatiguecausedbythestart-stop-operation.Weinterestintheeffectsofstructuredesignonfatiguewithoutprecisefatiguelifeprediction,ignoringcreepandoxidation.Thethermo-mechanicalrestraintratio(RT)[1,3,9],definedasratioofthemechanicalstrainrange(Δεm)tothethermalstrainrange(Δεth)inahot-coldcycleisusedtoqualifytheTMFproblemofvalvebridge.

(1)

whereΔεp=plasticstrainrange,Δεe=elasticstrainrange, α=thermalexpansioncoefficient, Tmax=maximumtemperatureofthermalcycle, Tmin=minimumtemperature.

Theequationdefinesthelifetimeatanylocationtogetherwiththemaximumtemperatureofthecycleatthatlocation[10].Therefore,ifthetemperatureleverofthehotconditionremainsunchanged,therestraintratioisproportionaltothelifetime[3].

Thefatigueatthevalvebridgeisthetypicalthermalmechanicalfatigue.Themaximumstressattheflamedeckoccursattheminimumcross-sectionofthevalvebridgebetweentheintakevalveandtheexhaustvalve.Thusthislocationisselectedasthetargetplaceinspectingthethermalmechanicalfatigue.

TherearefivevariantsofvalvebridgesasshowninFig.8.Theoriginaldesignisthemostcommonflatdesign.Fig.9showstheinfluenceofh(thethicknessofthedeck)ontherestraintratioatthevalvebridgeofflatform.Inengineeringapplication,thefirstandtheseconddesignarerelativelycommon.Fortheseconddesign,itinvestigatestheimpactofthedischargegroovedepthtontherestraintratioasdepictedinFig.10.Theresultscanbesummarizedasfollows.

Fig.8 Different structure of valve bridge

Fig.9 Influence of h on restraint ratio

Fig.10 Influence of t on restraint ratio

①Thereisanoptimalvalueofthickness,wheretherestraintratiohasminimumvalue.AccordingtoEq.(1),ononehandtheincreasingofh,isnotconducivetocoolingofthevalvebridges,whichincreasesthethermalstrain,ontheotherhandtheenhancementstructuralstrengthreducesthemechanicalstrain.Wecandeducethattherestraintratioreduces.Butthecalculationresultsreversedthisdeduction.Thereisnotasimplelinearrelationshipbetweenthethicknessofvalveandtherestraintratio.

②Therestraintratioattheminimumcross-sectionforthefirstdesignislarger.Itgoesagainstimprovingthefatiguestrength.

③Fromthestandpointofloweringtherestraintratio,theseconddesigncan’tincreasethestructure’slifebecausetherestraintratiowillriseasthedepth(t)ofthedischargelaunderincreases.

④Therestraintratiodecreasesalongedgesofthedischargegroovewhileitincreasesinthemiddle.

Thethirdandforthstructuredesignsareachievedfromtheaboveoutcomesinordertoreducepeakvalueoftherestraintratioatthevalvebridgewhenhremainsunchanged.Therestraintratioincreaseatthebottomofgroove.

ThesimulationresultsareshowedinFig.11.Althoughthethirddesignleadstherestraintratioattheminimumcross-sectiondown,thepeakvalueofthewholecurveincreases.Theresultoftheforthdesignisoptimistic,thepeakvalueoftherestraintratioreduced4%.

Fig.11 Restraint ratio of design 3 and 4

6.2F2area

TheloadingconditionsofF2mainlydependonthemechanicalloadings.Thetemperatureeffectsarequitesmall.Thelifetimecanberemarkedbythefatiguestrength.Hence,thelifetime(N)ofthiscriticalpointsispredictedaccordingto[11]

(2)

whereεaisthestrainamplitude, EistheYoung’smodulus, σ′fisthefatiguestrengthcoefficient, σmisthemeanstress, bisthefatiguestrengthexponent, cisthefatigueductilityexponent.

Theeffectsofthefilletsizewhichbetweenthevalveguideinstallationwallandtheheadroofwerealsoinvestigated,withtheresultsshowninFig.12.FromFig.12,thelifetimeofF2increaseslogarithmicallyasthefilletsizerises.Forthisreason,thetransitionfilletshouldbeaslargeaspossibleinthedesignperiod.

Fig.12 Lifetime of F2

7 Conclusion

Thecylinderheadstructuresaredividedintosevenpartsbaseonthestructurecharacteristicsanalysis.Thesimplifiedmodelwiththebasicfunctionalstructuresandfatigue-pronelocationsisbuilt.Thethermo-mechanicalcalculationsarecarriedoutwiththismodel.Thefollowingconclusionscanbedrawnfromthecalculationresults:thermalloadisthedominatefactoratF1,F4andF5;mechanicalloadisthedominatefactoratF2andF3;itisinfluencedbybothmechanicalloadandthermalloadatF6.Theinfluencesofthestructureformsofthetwotypicalfatigue-prone(F1,F2)locationsareinvestigated.Fromtheresults,wecanseehowthethicknessofthedeck,dischargegroovedeptinfluenceonthelifetimeofF1,andthefilletsizeinfluenceonF2.Anewoptimumproposalaboutthevalvebridgeisputforward.

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(Edited by Cai Jianying)

2013- 03- 06

Supported by the National Basic Research Program of China (613570303)

TK 422 Document code: A Article ID: 1004- 0579(2014)02- 0184- 06

E-mail: chengy@bit.edu.cn