The Simulation Research Based On Multi-field Coupling Analysis And Fatigue Life Prediction For The Diesel Engine Cylinder

The cylinder head is one of the key parts for the diesel engine. When it is working, thecylinder bears high temperature and pressure. Besides, because of its complex geometricshape and flow situation in water jacket, heat stress and mechanical stress distribution is notuniform, it is easy to produce the stress concentration phenomenon which can cause seriousfatigue damage. The paper takes a diesel engine as the research object, based on some theoriesof finite element analysis such as the fluid dynamics theory, heat transfer theory and so on.Multi-physics field coupling analysis was taken on the the model which is combined bycylinder head, engine body, bolts, cylinder cushion, and other parts using nonlinear contactanalysis method. In order to calculate the stress and strain field results in the workingconditions such as the prestressing force, explosion pressure, heat load and the thermalmechanical coupling conditions. Then the paper explored the influence on the cylinder headfatigue strength cased by mechanical stress and thermal stress, found out the main stressconcentration areas. At last, the paper put forward some structure improvement measuresbased on the results above to forecast the fatigue life of the cylinder head using fatigueanalysis software FE-safe.The paper built both the finite element assembly model and fluid analysis model afterof the simplification for the cylinder head, engine body, water jacket and cylinder cushionwith the specialized processing software Hypermesh. And then I put the mesh to the Fluiddynamics software CFX, took the directly coupled heat transfer analysis the based on thetheory of disperse integrally and solve integrally. The analysis results showed that watervelocity is reasonable, and can basically meet with cylinder head cooling requirements. Butthere are also some shortcomings in the water inlets, especially in the area near the two exhausting channels, the water velocity is too slow, this also explains the cause of the hightemperature in exhausting channels.Then the paper mapped the cylinder head temperature made in flow analysis to theanalysis model, calculated the stress field in the four working conditions such as theprestressing force, explosion pressure, heat load and the thermal mechanical couplingconditions, The results indicated that the stress concentration is mainly distributed close to thebolt support convex platforms when it bears bolt prestressing force and explosion pressure.We can conclude that stress concentration occurs in Mechanical load condition. Besides, onthe thermal stress and heat coupling conditions, stress concentrated areas are almost the same,which are mainly distributed in the cylinder water channel floor , nose bridge surface betweentwo exhaust ports in fire floor, near the water-jacket fillets between close to cylinder boltholes in cylinder base floor. so the results confirms that thermal stress takes a key role in theseregions..This paper puts forward five improvements of cylinder head such as the change ofbottom thickness, inlet diameter, processing discharge groove which can influence thecylinder head temperature distribution and cooling liquid cooling effect. Then analyzed theheat transfer effect and stress distribution in various working conditions, through thecomparison with the original model, we found that the cooling fluid flow more reasonable,and cylinder head temperature has reduce. After the improvement of processing dischargegroove, the highest temperature raised because of the increase of heated area. In addition, nodoubt that the mechanical load had increased after the thinning improvement of cylinderbaseboard, but heat load stress and strain reduced significantly to the contrary. When thediameter of cooling hole increased, the intake surface temperature dropped significantly,while the highest temperature increased on the contrary. It is mainly because that most of thecooling water has gone to the output directly through the cooling hole, which reduced thecooling effect in the region of exhaust channel.Finally, the paper exported stress results into the fatigue analysis software FE-safe tocalculate the cylinder head fatigue life of the optimal case. The results show that high stress and strain are located in the regions such as the nose bridge surfaces both in fire floor andbetween two exhaust ports and bolt support convex platforms, where we should make aspecial consideration in diesel engine design. By comparison, the paper also found that thefatigue cyclic numbers and service life have both increased after improvement. Through thecomparison with the original model, we find that pretightening force plays an important rolein the mechanical load conditions; while the thermal stress has a key effect on the fatigue lifein the bottom floor of cylinder head.