Research On Driving Mode Switching Performance Of Transfer Case For Part-time 4WD Vehicle

The transfer case is the core part in the power train of the part-time 4WD vehicle.The performance of the transfer case directly affects the passing ability,fuel economy and ride comfort of the vehicle.In this thesis,a transfer case for part-time 4WD vehicle is taken as the research object,considering the specific frictional power and the performance requirements of vehicle,aiming at improving the shift quality in the driving mode switching process,the theoretical analysis and experimental research on the driving mode switching performance of the transfer case for part-time 4WD vehicle are carried out.The main work of this thesis is as follows:(1)In the driving mode switching process,the working process of the synchronizer is divided into five phases,and the working characteristics of each phase are analyzed;the power train is simplified to an equivalent system with two degrees of freedom,and the dynamic model of the power train synchronization process is established.(2)The mathematical expressions of jerk,synchronization time,specific frictional work and specific frictional power which are generated during the process of driving mode switching of transfer case are deduced respectively.Meanwhile,the effects of shifting force and synchronous speed difference on the driving mode switching performance are analyzed.The results show that the effects of the value of shifting force and synchronous speed difference on synchronization time,specific frictional work and specific frictional power are prominent and the effect of change rate of shifting force on jerk is noticeable.Considering the performance requirements of synchronizer and vehicle,the maximum allowable change rate of shifting force,the maximum allowable vehicle speed under switchable driving mode and the available range of shifting force at different speeds are determined.(3)Taking synchronization time and specific frictional power as optimization objectives,the evaluation function of driving mode switching performance that takes into account vehicle's dynamic performance and synchronizer's service life is established.The fmincon function in MATLAB Optimization Toolbox is used to solve the evaluation function,and the optimal shifting force at different speeds is obtained.The results show that under the given driving mode switching performance evaluation function the optimized shifting force can make the comprehensive performance of driving mode shifting reach the optimum.(4)The test bench is built so as to carry out the dynamic test of the driving mode switching process.Based on the test results,the characteristics of different phases and the effects of shifting force and synchronous speed difference on the driving mode switching performance are analyzed.By comparing the simulation and test curves,the correctness of the mathematical model of shift quality evaluation index is verified.