| Piston is one of the most typical and important components in the reciprocating equipment.The lubricating characteristics of the liner/piston system can affect the consumption of oil,leakage,vibration characteristics,wear and frictional energy consumption of mechanical equipment directly.With good tribological performance,it can not only reduce power loss and prolong engine life,but also reduce the engine vibration and harmful emissions effectively.It is an effective way to improve its operational reliability and economy with the application of surface texture to improve the lubrication characteristics of the piston/cylinder.In order to realize the engineering and application of surface texture design,we must first find the design basis from the successful cases abroad and further grasp the surface texture optimization design method.Therefore,it is of great significance and broad application to explore the mechanism and design rule of surface texture.In this thesis,the piston/cylinder of reciprocating equipment is taken as the research object,and Caterpillar swash plate pump and Ford Motor Company's engine are used as the targets,the hydrodynamic lubrication model and mixed lubrication model were established.Considering the geometric parameters of the friction pair,operating conditions,surface texture size,depth,shape,bottom shape,texture density,distribution,type,and so on,the mechanism of the surface texture was explored,and the Influence on piston lubrication characteristics was studied.And artificial intelligence was used to optimize the surface texture design parameters.Aiming at the hydrodynamic lubrication state,the Caterpillar's swash plate pump was used as the research object,and the method to solve the transient Reynolds equations with variable calculation domain and moving boundary was proposed.The unified lubrication model of the piston/cylinder with different structures was established.Compared with the literature results,the accuracy of the model was verified.Comparing with the tribological characteristics of two typical swash plate pumps,the effects of assembly clearance and liner length on the minimum oil film thickness,average fluid pressure,fluid friction,and piston tilt angle were investigated.It was found that the longer cylinder liner is conducive to the safe and stable operation of the swash plate pump,but at the same time it will lead to a higher friction and a greater power loss.Therefore,it is necessary to investigate the surface texture design method of the piston to reduce the friction force and power loss.Based on the transient hydrodynamic lubrication model with variable computational domain and moving boundary,the effect of the texture type,location,size,and depth on the lubrication characteristics of the piston is obtained.The characteristics of the grooves and dimples were analyzed.The stepped multiple groove design and combination design scheme were proposed.The Combine design can create larger load carrying capacity compared with the Step-Multiple-grooves design.As to the manufacture and machining accuracy,the StepMultiple-grooves design is preferred.The piston/cylinder friction force was reduced about 13%.For the mixed lubrication state,the piston ring/cylinder of Ford's internal combustion engine was studied.According to the test data of the topography and the material characteristic parameters,the contact theory was used to establish the contact model of the asperity considering the real surface topography.The tribological properties of the piston ring/cylinder liner were obtained by the mixed lubrication model.The accuracy of the calculation program was verified by the comparison with experimental data.Through the initial experimental research carried out by the company,the design method of the surface texture of the piston ring/cylinder liner was preliminarily discussed.The results showed that the surface texture is a means to improve the lubrication characteristics of the piston ring/cylinder liner and the influence is not absolute.The beneficial effects depend on whether the design scheme is reasonable and whether the design parameters are optimized.Therefore,considering the surface texture geometric parameters and operating conditions comprehensively,it is very important to systematically study the surface texture design of the piston ring/cylinder liner.Based on the mixed lubrication model with real-surface roughness,the mechanism of the surface texture of the piston ring was explored,and the surface texture design of the piston ring and cylinder liner were studied respectively.The general rule of surface texture design was obtained.It was found that when the surface texture is located in the middle of the piston ring,the bearing capacity drops sharply and the film thickness decreased.When the groove is far from the center of the piston ring,there is almost no effects on the lubrication performance of piston ring;There is an optimal value of texture depth to maximum minimum film thickness and decrease the friction force significantly Selecting a reasonable design parameter can significantly reduce the power loss.The surface design of the piston ring/cylinder liner was combined to reduce the friction and power loss about 8%.Considering the geometric parameters of the friction pair,operating conditions and surface texture design parameters,at least 8 degrees of freedom will be introduced,thus the complexity of the system will increase significantly,and the optimal design cannot be found only by parametric analysis.Therefore,artificial intelligence was tried to optimize the surface texture design parameters.The mapping relationship between piston geometry parameters,operating conditions and lubrication characteristics was established with Kriging model.The accuracy of the model was verified by the comparison with simulation results.Based on this,the relationship between the different groove positions,widths,depths and the minimum film thickness and maximum friction force were established.The optimal groove design parameters were developed with the above prediction model. |
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