| Piston is one of the core components of marine diesel,the reliability of which is closely related to the reliability of the diesel.In the current trend of high performance,high reliability,low fuel consumption and low emission for the marine diesel engine.It is an important part to improve the reliability of the high-strength piston when Pe is larger than 2.5MPa and Pe·Cm is 28-32(MPa·m/s).In this paper,the research works of combustion analysis in cylinder,oscillation analysis in cooling chamber,thermal analysis of the piston by coupling the fluid results,thermal mechanical coupling strength analysis of the piston by coupling the thermal results and structural stress were carried out,the validity of simulation methods and the accuracy of these results were verified by experiments,a method which can predict the temperature and strength of high-strengthed piston accuracy was proposed,the studies focus on the design problems of high-strengthed piston,which include the thermal load control,strength,stiffness,fatigue and so on.The main research aspects as below are contained in this thesis.1.Based on the physical disciplines including fluid,thermal,mechanical which were used in piston design,the models of combustion in cylinder,working process of diesel,oscillation in cooling chamber,finite element model for thermal and mechanical were established respectively,the mapping method between the different physical results was studied to ensure the effectiveness of date flow between different models.The full simulation model was established in the combustion analysis,the lubrication flow model and the transit CFD model of piston cooling chamber were established in the oscillation cooling analysis,the multi-body contact model was established in finite element analysis.The research focused on the experiential method and multiphysics coupled method to obtain the thermal boundary condition of piston,proposed the method of obtain the thermal boundary condition for the piston analysis,studied the influence factors which including the contact boundary type and setting,and proposed the setting method which can fit in with the real operation condition for the piston analysis.2.The moving mesh technique was used to disperse the calculation domain in fluid analysis.The temperature and HTC on the walls were obtained by analyzing the combustion in cylinder and oscillation in cooling chamber,the thermal boundary condition of finite element analysis was obtained from CFD using the mapping method.Based on the working process model,the mean temperature and HTC were solved,the distribution of HTC along the radius was obtained by experience formula which was applied on the top surface of piston by discrete tool,and also,the cooling boundary condition of the piston was obtained by experience formula.3.The study of the piston thermal load was carried out by finite element method based on the different boundary conditions which were obtained by these two ways.Furthermore,the calculation results were verified by experiments using hardnessplus.The results showed that the accuracy of experiential method cannot meet the thermal analysis requirements of the high strengthed piston in design phase because of its restrictions of using conditions.CFD-CAE coupled method was more accuracy than experiential method;but which is restricted by the input date and calculate scale,the accuracy cannot be ensured at the initial stage of design and cannot be used widely at the optimization stage.So a new method to determine the thermal boundary condition by using the experiential method which was corrected by CFD-CAE method was proposed,it can not only improve the accuracy of calculation but also can be widely used to verify the effectiveness of the optimal design at the optimization stage.4.Based on the high cycle fatigue strength of material,the high temperature mechanical properties of piston material were researched by testing method;the strength performance of this material was obtained and can be used as the input date for strength analysis.The thermal strength,deformation and fatigue of the piston under working condition were studied using thermal-mechanical coupled analysis method based on the thermal results.The accuracy of the thermal-mechanical analysis results can be verified indirectly by the results of normal strength test.According to the stress distribution characteristics and the structure of the piston,the influence of local structural on piston strength was researched,which include the hole of connecting bolt,the contact surface of head and skirt and the pin boss.Furthermore,the local structural and the load transfer path of the piston was optimized,the results indicated that the overall strength and the anti fatigue properties of high strength piston were improved effectively.5.Based on the fatigue test procedure,a rapid fatigue test of the piston skirt and the real machine durability test were carried out.By the three sample pieces of skirt and the real piston inspection,the reliability of the piston was verified.These research results have been applied in the high strength piston design for a new diesel engine;the designed piston has stable performance and good operating condition. |
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