Optimization Of Mechanical Fatigue Life And Acoustics Performances Of A Vehicle Exhaust System

With the transformation of automobile market and the change of consumer’s consumption concept,whether the Chinese independent brands have enough product strength determines whether its automobile products can maintain a foothold in the increasingly fierce market competition.As one of the critical subsystems of gasoline cars and hybrid electric vehicles,the acoustic performance and fatigue performance of exhaust system have been paid more and more attention.This paper takes a vehicle exhaust system as the research object.Focusing on the acoustic performance and fatigue performance,an optimization method of structural mechanical fatigue life is studied on the basis of improving the noise reduction ability of mufflers.The bench test simulation model is established and calibrated in GT-Power software.Comparing and analyzing the acoustic performance of two initial scheme of exhaust systems,it is found that the tailpipe noise in scheme 1 is higher than that in scheme 2 under common rotational speed,and the pressure loss in scheme 2 does not meet the requirements of enterprises.By analyzing and comparing the transmission loss of the two systems,it is found that the main reason for the poor performance of acoustic performance is the insufficient transmission loss of the mufflers at middle and low frequencies.According to the results of acoustic performance analysis,with a guidance of muffler design theory,the acoustic cavity structure is optimized.The acoustic performance simulation of the optimized scheme in GT-Power software shows that the total transmission loss of the exhaust system is effectively improved at 20-200 Hz and 600-1000 Hz,the tailpipe noise of second-order and fourth-order at low-speed is significantly reduced,and the pressure loss and power loss meet the requirements.The real vehicle test is carried out on with optimized exhaust system,and the results verify the effectiveness of the optimization measures.The finite element model of the acoustic optimization scheme of the exhaust system is established in HyperMesh software.The basic vibration performance of exhasut system and the accuracy of the finite element model are verified by modal analysis.Considering the influence of excitation frequency on the system response,a method of the whole-life-span analysis based on dynamic response is proposed,and the simulation results show that the mechanical fatigue life of exhaust system is lower than the requirements.Based on response surface methodology,a multi-objective optimization function is proposed to minimize the extreme values of the vertical dynamic load transmitted to chassis including its standard deviation and maximize the mechanical fatigue life.The deterministic optimization results show that the amplitudes and standard deviation of the vertical dynamic load significantly reduces while the durability of the structure as well as the ride comfort of the vehicle is improved.Considering that the real stiffness of elastic elements will fluctuates randomly due to random factors,the 6σ rubustness design method is applied to the optimization problem.On the basis of deterministic optimization,the probabilistic characteristic parameters of target variables and constraint variables are introduced into the 6σ rubustness optimization problem of design variables.After optimization,the standard deviation of vertical dynamic load and mechanical fatigue life are reduced compared with the deterministic optimization results,which indicates that the vibration isolation performance and fatigue performance are more robust.The quality level of constrained variables of robustness optimization design scheme is higher than the 6σ level,which indicates that the optimization results are more reliable.The constrained modal analysis of the exhaust system proves that the rubustness optimization design schemes is feasible in engineering.