Study On Structural Performance And Optimization Of Automobile Exhaust System

As an important part of vehicles with internal-combustion engines, the performance ofexhaust system directly influences a vehicle’s vibration, harshness and the amount ofcontaminants, which are important indexes to evaluate an automobile’s quality. Therefore, theNVH performance of an exhaust system should be considered and reasonably matched withthat of the engine and the whole vehicle when it is designed. Additionally, with the intenselystrict rules, the interior structure of an exhaust system is more complicated. Also, as thesylphon bellows and rubber hangers are nonlinear isolators, the exhaust system becomes acomplicated, with multi-degree of freedom, and nonlinear system, and is hard to be modeleddirectly by classical vibration theory.In this thesis, the finite element model of a vehilce exhaust system is established usingshell element. The FE model is verified by comparing the calculated free modal frequencyand the experimental data, it is shown that the numerical predictions are fairly goodagreement with the real test results in vibration characteristics of the exhaust system, andmaximum deviation in frequency is less than10Hz. Then, the static(hanger deformation andpreload) and dynamic frequency responses(peak dynamic force transferred to vehicle body)are conducted in which the powertrain mounting system is coupled with the exhaust system，the whole system is constrained the same as real boundary. Simulation results of the initialscheme show that the static deformation of the rubber hangers is higher than the specifiedvalue, dynamic forces transferred to the vehicle body is too big and uneven during the enginestarting phase, the isolation system meet requirement while fatigue life of the exhaust systemis too low.To minimize the forces transferred to the vehicle body by the exhaust system and assurethe fatigue life of the exhaust components, a multi-objective optimization model is proposedand solved using the experimental design method and the optimization algorithm, with thestiffness of sylphon bellows and four rubber hangers as design variables. Optimization resultsof the calculated structural performance show that by assigning different stiffness to sylphonbellows and four rubber hangers, better structural performance is received. The largest force among the four dynamic force peaks transmitted to the vehicle body is effectively decreasedby7.308N and standard deviation decreased from15.72to5.02, with the static deformation ofthe rubber hangers is less than3mm, and lowest fatigue life increased by80%. Also, anotheroptimization scheme based on ADDOFD algorithm is given, a hanger is added to the exhaustsystem and its static and dynamic performances are calculated and optimized. Comparingwith the optimization results when the extra hanger is added before, adding a hanger to theexhaust system can get more ideal dynamic performance and especially the biggest staticforces decreased by10.7N, also, lowest fatigue life after optimization meet the requirement.The simulation results demonstrate that the proposed optimization method of the exhaustsystem is convenient and effective, and is valuable to engineers in optimizing structuralperformance of the exhaust system. Also, the best result is found easily using optimizingsoftware.