Study On Structural Performance Analysis And Test Method Of A Car Exhaust System

Exhaust system is an important part of the vehicle with internal-combustion engine.Under the engine moment excitation,engine vibration will be transmitted to the body through the rubber lug of the exhaust system,which will deteriorate the NVH performance of the vehicle.If the exhaust system modal frequencies couple with the engine excitation frequencies,the vibration will be amplified by the exhaust system.Under the engine torque excitation,exhaust system lug body dynamic reaction force will not only affect the ride comfort of vehicle,but also reduce the lug and the fatigue life of the system.With the car comfort requirements getting higher and higher,the vibration control of the exhaust system has important significance.In this thesis,the finite element model of a vehicle exhaust system is established using shell elements.Free modal and constrained modal tests of the exhaust system are carried out by LMS data acquisition equipment.27 measuring points are arranged on the exhaust system,the modal model of exhaust system was established in the software Test.Lab 14 A LMS,the free and constrained modal frequencies and modal properties of the exhaust system are obtained by using the hammering method.The results of finite element numerical model are compared with the modal test results,the accuracy of the finite element analysis numerical model is verified with the relative error less than 5%.Using the constraint model of the exhaust system including engine mounting system,static analysis and dynamic power frequency response of the exhaust system are obtained.Calculation results show that,the static deformation of hangers 1 and 2 is more than the engineering requirement,and the static load of the four rubber hangers is not uniform.In the start stage of the engine(excitation frequency equals 13 Hz or so),the dynamic reforce of the rubber hangers is large and the uniformity is bad,which will affect the ride comfort and fatigue life of the lugs.In the study of vibration control,the natural frequencies of the exhaust system are calculated by using the finite element method.The calculation results show that the engine idle excitation frequency couples with the2 nd order and the 3rd order natural frequencies which will cause a higher level of vibration of the car body.In order to solve the problems such as the large static deformation,the uneven pre load and the large dynamic reforce of the rubber hangers,the average drive freedom method is used to analyze the rationality of the original positions of the rubber hangers.The analysis results show that the initial hook position is reasonable.According to the structure of the vehicle chassis and the result of the average drive degree of freedom,the problem of the idling resonance of the exhaust system is solved by adding a hook,which improves the comfort of the vehicle.