Fundamental Research On Thermal Reliability Engineering Design For Cylinder Head

During the internal combustion engine operating procedure, the cylinder head has to withstand thermal loads and peak pressure loads generated by gas combustion. Substantial change of internal combustion engine working conditions can easily lead to low cycle fatigue of cylinder head parts while the failure regions are mostly concentrated in such places like bridge zone, injector hole which has higher thermal stress. The key to cylinder head reliability design research is to extend the fatigue life of these regions in order to meet real application requirements by optimizing some factors such as local structure, materials, etc. However, there are several problems in the cylinder head reliability design process.During the numerical analysis procedure, high time cost, lack of systematic design basis&low efficiency of optimization are all thorny issues. In order to solve these problems, the structure of cylinder head model was simplified and parameterized in this dissertation. Moreover, a modeling-solving-post-processing automatic analysis platform was established; this platform was used to accomplish some research on cylinder head thermal state and focus on the influence of burning intensity, cooling flow, materials, water cavity structures. On this basis, the amount of numerical calculation was reduced while the efficiency of global optimization was improved. A cylinder head thermal state approximate model.was established; combined with genetic algorithm, this model was used to accomplish global automatic optimization and carry out the application in cylinder head thermal reliability optimal design. The following works were completed in this dissertation:(1) Research on fluid-solid numerical simulation parameterized model of cylinder head. First, the gas and cooling boundary conditions were obtained from1D combustion and cooling system models, after that the fluid-solid coupling numerical simulation method was used to conduct the whole engine simulation and get the whole cooling water jacket flow field, solid temperature field and stress field. Based on the whole machine calculation results and cylinder head structural characteristics, considering both calculation accuracy and time cost, the cylinder head structure was simplified and a parametric model was established. The complete engine temperature field calculation results and the actual cylinder head temperature measurement data were used to validate the model. The accuracy of stress solver based on the finite volume method and time cost between complete and parametric model have been analyzed. The results showed that the cylinder head parametric model could significantly reduce calculation cost while remain a good accuracy.(2) Research on numerical simulation influence of cylinder head thermal state. A modeling-solving-post-processing automatic analysis platform has been established. Based on this platform and cylinder head parametric model, the orthogonal design method was used to research the influence of several factors such as burning intensity, materials, coolant flow, water cavity structure on cylinder head bridge zone temperature&stress variation. The sensitivity of these factors were classified and compared at last. The results showed that the influence patterns of burning intensity, materials, coolant flow were approximately linear. As for the influence of structure factors, take water cavity structure as an example, the temperature of exhaust valve bridge zone is mainly affected by the height and width of water cavity structure in this place while the stress is affected by the whole structure of water cavity structure in bridge zone, which indicates that the influence of water cavity structure towards bridge zone temperature and stress field are presented to have local and global characteristics separately. The influence patterns of these factors and their sensitivities obtained in this research could be used to provide evidence in the subsequent cylinder head thermal state approximate model establishing process.(3) Research on the influence of mechanical load and material plasticity on the cylinder head thermal-mechanical coupled stress field. A thermal-mechanical coupled model including cylinder head, bolts and fake body was established. The influence of bolt preload and peak pressure on the thermal-mechanical coupled stress field on cylinder head plate, while the influence of gray cast iron material plasticity setting on the thermal-mechanical coupled stress and strain calculation results was also studied. The results showed that bolt preload, peak pressure and material plasticity will lead to a slight increase of cylinder head bridge zone strain amplitude. During low cycle fatigue prediction of cylinder head in order to improve the accuracy of cylinder head reliability prediction, the above coupling effect should be considered.(4) Research on the approximate model of cylinder head thermal state. In the establishing process, the cylinder head plate thickness, gas temperature inside cylinder and heat transfer coefficient was used as boundary condition; the water cavity structure of cylinder head bridge zone and coolant water flow were used as design variables; the optimal Latin hypercube method was used to design sampling points. Based on first-order response surface method, second-order response surface method and RBF neural network (RBF) method, several approximate models focused on cylinder head bridge zone temperature, stress, water cavity heat transfer&resistance were established. The accuracy of all the models based on various methods was compared. After comprehensive consideration including accuracy and model complexity, the second-order response surface method was chosen to establish all index approximate models to substitute the parametric model in numerical simulation process. Moreover, low-cycle thermal fatigue theory was used to construct a calculation function based on the life and damage of each nose area in approximate models, which was also the theoretical foundation of subsequent optimize design.(5) The application research based on cylinder head thermal reliability structure optimal design method. Focused on the cylinder head optimal design after the increasement of internal combustion engine specific power, based on cylinder head thermal state approximate model while water cavity resistance and heat transfer rate of cylinder head were used as constraints, the damage of cylinder head bridge zone was considered as objective function to conduct global optimization combined with Multi-island genetic algorithm. The optimal value of design variables were obtained after optimal calculation. The optimization efficiency between simulation model and approximate model was also compared. The whole engine thermal-mechanical coupling and fatigue calculation results were used to verify this optimization design. The results showed that the fatigue life of cylinder head bridge zones was significantly increased in this optimization design. The global optimal design solution can be quickly obtained by this optimize algorithm combined with approximate model; the optimization efficiency can be improved at the same time.