| As automotive power systems move towards low-carbon,high-speed,and diversified development,the demands for power and durability of core components such as efficient engines,transmissions,and drive motors become more strict.The lubrication and low-friction design of key components in the powertrain system has always been the key to improving its performance and quality.Due to differences in working conditions and lubrication needs,the lubrication and friction problems of key friction pairs in the powertrain system are complex and diverse,with the four typical lubrication modes of boundary lubrication,mixed lubrication,elastohydrodynamic lubrication(EHL),and hydrodynamic lubrication transitioning during the working process.Therefore,developing lubrication and friction reduction methods and innovative designs under complex working conditions and multiple lubrication modes has become the research hotspot and difficulty.Based on the "surface micro-topography controlling" technology of key friction pairs,and aiming at the diversity of actual surface surface topography characteristics and lubrication conditions in the engineering powertrain system,the lubrication theoretical model of textured friction pairs is constructed in this dissertation.Through simulating calculations,the mechanism of surface texture under different lubrication modes is clarified,and the transition law of the Stribeck curve for friction coefficient is analyzed by combining basic friction experiments,forming the optimized design method for surface texture in multiple lubrication modes.Taking the specific engine and transmission clutch of the hybrid powertrain system as the object,the differential surface textures and functional designs are carried out based on corresponding key friction pair working conditions,for improving the lubrication and friction properties,and enhancing the overall performance of the powertrain system.The main research results are as follows:(1)Aiming at the four typical lubrication modes divided by lubrication condition in the working process of key friction pairs in powertrain,a theoretical lubrication model of textured friction pair for multi-mode lubrication is build,which provides the theoretical basis for the simulation and calculation of the tribological performance of surface textures under multiple lubrication modes.Based on the Reynolds equation,the pressure and shear flow factors are added to consider the influence of surface roughness,and the contact factor is introduced to consider the effect of micro asperity contact,the lubrication model of textured friction pair with surface contact is established.By combining the finite difference method the multi-grid method for solving,the calculation of the lubrication and friction properties under boundary,mixed and hydrodynamic lubrication modes is realized.Based on the elastic deformation theory of line contact,and taking the Reynolds equation as the control equation,the lubrication model of textured friction pair with line contact is established.By adopting Newton-Raphson iteration method for solving and setting the solving process,the calculation of lubrication and friction properties under elastohydrodynamic lubrication mode is realized.(2)Through conducting simulation of tribological performance of textured friction pairs under multiple lubrication modes,examining the influence of surface texture geometric parameters on the lubrication and friction properties of the friction pair,analyzing the law of surface texture under boundary lubrication,mixed lubrication,hydrodynamic lubrication and elastohydrodynamic lubrication modes,the optimized design method for surface texture under typical lubrication modes is formed.The simulating results show that for textured sliding friction pairs,as the relative sliding velocity increases,the lubrication mode gradually transits from boundary lubrication to mixed lubrication,then to hydrodynamic lubrication.The lubricating effect of surface texture mainly manifests in improving the oil film thickness and load carrying capacity,thus reducing the friction.To be specific,in the boundary and mixed lubrication modes,as the depth of the dimple decreases or the area occupancy ratio increases,the film pressure increases and the asperity pressure decreases,which is conducive to reducing asperity contact and enhance surface antifriction and anti-wear performance;in the hydrodynamic lubrication mode,the friction coefficient increases with the growing dimple area ratio.Also,the dimple with small depth(3 μm)and area ratio(5%)have better comprehensive anti-friction effects under various working conditions.For textured line contact friction pairs under elastohydrodynamic lubrication mode,the oil film thickness distribution increases along the elastic deformation zone,and several pressure peaks of oil film appear in the middle region of textured surface.Suitable dimple depth(4-5 μm)and diameter(30-70 μm)of the surface texture can achieve the larger oil film thickness and pressure,which can exhibit better lubrication effects under different working conditions.(3)According to the numerical simulation results,different textured surface schemes are processed by laser surface texturing technology,then a set of basic tribology experiments are conducted to analyze and verify the lubrication and anti-friction effect of surface textures.The friction test results show that compared with the untextured friction pair,the friction coefficient of the optimized textured friction pair is smaller under various bearing characteristic numbers,and the Stribeck curve of the textured friction pair move towards the lower left position(low-speed area),indicating that the existence of surface texture can improve the anti-friction performance under the multiple lubrication modes.For the surface contact oil-lubricated friction pairs,the dimple texture with small depth(or small depth-to-diameter ratio)and small area ratio can achieve optimized anti-friction effects under boundary,mixed,and hydrodynamic pressure lubrication modes.For line-contact oil-lubricated friction pair,appropriate dimple diameter can effectively connect various types of working conditions.The dimple texture with larger depth and area ratio can reduce friction under the elastohydrodynamic lubrication mode,which is consistent with the simulation results.The wear test results show that the micro dimples on textured surface can store lubricant and continuously release it to provide lubrication during the gradual wear process of friction surface.The microstructure of dimple can also capture some surface wear debris,thereby alleviating the process of abrasive wear.Therefore,surfaces with deeper and denser textures have better wear resistance property.The basic tribology experiments verified the friction reduction and wear resistance performance of the textured surface under multiple lubrication modes,and provided guidance for the design of anti-friction and anti-wear surface textures for key friction pairs of general parts in powertrain system.(4)Based on the characteristics of the Miller cycle working condition in specific combustion engine of hybrid powertrain,the coupling design of surface texture with cylinder liners nonGaussian roughness,and the lubrication adaptability of surface texture is investigated.By establishing the lubrication model of textured cylinder liner-piston ring,the influence of surface texture on cylinder liner with non-Gaussian distribution roughness was investigated,and the texture parameters are optimized.The simulation results show that for different lubrication modes of the cylinder liner-piston ring friction pair in the specific engine,the depth-to-diameter ratio of the surface dimple texture should be controlled between 0.07-0.13,and the area ratio should be between 5%-10%.At the same time,the cylinder liner surface texture can weaken the differential effect of lubricating oil dilution on tribological performance under multiple lubrication modes,narrow the fluctuation range of instantaneous friction power,and reduce asperity contact pressure and friction with the cylinder liner-piston ring friction loss reduced by about 16.2%-20.3%,which demonstrates that the ability of controlling system friction loss and the adaptability to lubricating oil dilution are enhanced.After utilizing laser surface texturing method to process micro textures on the surface of production cylinder liner,the cylinder liner-piston ring reciprocating friction and wear test is carried out to investigate the tribological performance changes of laser textured friction pairs.The test results show that surface texture can reduce the friction coefficient of this friction pair by about 9.9%-12.3%,improve the mechanical efficiency of the engine,and the micro dimples can help to store debris,thus reducing surface scratches caused by abrasive wear and improving the wear resistance of cylinder liner,which could provide technical support for the application of surface texture in high-efficiency combustion engines.(5)Based on the operating characteristics of the steel plate and friction plate pair during the engagement process of the wet clutch in transmission system,and the differential requirements for the tribological performance under various lubrication modes,an innovative design of the composite texture with micro concave and convex on the steel plate surface is proposed.The simulation results show that the composite textures on the steel plate surface can regulate the lubrication and friction properties during the clutch engagement process under boundary,mixed and hydrodynamic lubrication,respectively.At the initial stage of engagement,the sliding friction speed is high,composite textures can reduce frictional loss under the hydrodynamic lubrication mode;at the middle stage of engagement,the sliding friction speed decreases,composite textures can improve friction force to accelerate the engagement process under mixed lubrication mode;at the later stage of engagement,the sliding friction speed approaches zero,composite textures can increase the friction coefficient with the anchoring effect under boundary lubrication mode,improving the torque capacity.For the geometric feature parameters of the composite texture on the steel plate surface,the micro-concave radius of 60-80 μm and depth of 6-8 μm,the microconvex height of 6-8 μm,and the concave-convex aspect ratio of 0.6 has optimization effects in friction reducing and anchoring.After processing the composite textures on steel disc surface by laser texturing technology,the copper pin-steel disc rotating friction tests are conducted to investigate changes in lubrication and friction properties under different sliding speeds during the engagement process of the pair.The experimental results show that suitable composite texture design on the steel plate surface can reduce the friction coefficient by about 5.0%-5.5% under high-speed sliding friction at the initial stage of engagement,and increase the friction coefficient by about 37.4%-47.7% under low-speed sliding friction at the later stage of engagement.It means the composite texture with micro concave and convex could achieve the comprehensive effects of controlling drag torque loss and improving the response speed and torque capacity of power transmission,which is conducive to the surface texture differentiated design and application on key friction pair of wet clutch for specific transmissions in hybrid powertrain system. |
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