Vibration And Modal Analysis Of Automotive Exhaust Muffler

Modal analysis is defined as a process of determining the inherent dynamic characteristics of a system in forms of natural frequencies, mode shapes and damping factors, and using them to formulate a mathematical model for its dynamic behavior. The formulated mathematical model is referred to as the modal model of the system and the information for the characteristics is known as its modal data, so modal analysis is the key and foundation of design, the basic principle is that the structural characteristics describe the independent differential equations to find the system natural frequency and modal parameters. Modal analysis has been applied in many fields such as mechanical, aeronautical engineering; in this dissertation the modal analysis is applied in exhaust system muffler, the study focus on the modal parameters of muffler which obtained by using finite element method (FEM) package,MSC.patran, after creating the geometry of muffler model by UG, then the model meshed using Altair hyper work-hyper mesh, the result of analysis is obtained for free vibration mode of muffler, Muffler is a multi-degree of freedom vibration system with multiple natural frequencies; each natural frequency of vibration has corresponding mode shape, Muffler mode is determined by the material properties (mass, damping, and stiffness), and boundary conditions of the muffler structure, the mode consider as inherent properties of a structure. Each muffler mode is defined by a muffler natural frequency, modal damping, and a mode shape, the modal assurance criterion (MAC) also has been obtained.Finite element is one of the methods used in analysis of dynamics structure, in some case the model may contain thousands of degrees of freedom, quick solutions is desirable, this leads to have a method of dynamic analysis which permits the number of degrees of freedom of the dynamic model to be reduced, the methods of reduced order model are widely applied in reduction of the large degree of freedom, there are many methods used in this field, in this dissertation model reduction used as follow, first, tow methods which are Raleigh Ritz and component mode synthesis method (CB) used for reducing the model,the methods were applied in beam consisting of two components, each component in beam was divided in substructures, the result of these methods are compare with FEM, the errors in eigenvalue and eigenvector was obtained, these methods were reapplied in the beam with attached spring at the joint point, the errors of classical and non classical were very closed, the Raleigh-Ritz method is so closed to the FEM compare with CB in eigenvalue. Second, three methods were applied, improve reduce system (IRS) method, Rayleigh Ritz vector and condensation method, (IRS) is more accurate estimated the modal model of the full system, Rayleigh Ritz vector is a most general technique for finding approximations to the lowest eigenvalue and corresponding eigenvectors, numerical example applied for three methods, IRS is strongly agreement with FEM.This dissertation reported the sensitivity and location of support, which help the designer to meets his needs, such as to maximize or minimize the frequency, stress and displacement, since the resonance in the muffler cause when its natural frequency and excitation frequency are close to each other, one of ways to control the frequency is a suspension (support points) of the exhaust system to the vehicle body, so the exhaust system vibration mode analysis is very necessary for practical value, the dynamic characteristics of structural vibration of exhaust system structural dynamics is expressed in modal parameters, which are the natural frequencies, damping and mode shapes. the merits analyze of the structure of the exhaust system modal parameters can determine to extract the modal structure of the mode shapes, to apply the suspension point the exhaust system geometric modeling is established using UG software,MSC.patran software used for both dimension, the general exhaust system available is a suspension point range, its purpose to minimize the exhaust system vibration, the theoretical basis for suspension point selection, and the choice of suspension points using average driving j-degree of freedom the displacement degrees of freedom (Average Driving DOF Displacement(ADDOFD)) is used. The sensitivity analysis is define as the changes in design variables or parameters on the objective function and constraint function in the structural design intended to meet the requirements of the various structural parameters or design variables on the structural dynamic, in this study the thickness of muffler has been change and the sensitivity of frequency, displacement and stress are obtained, the sensitivity of stress by changing the thickness has significantly effect, exactly in avoiding fatigue, the increase in thickness has relatively increase in stress, although the increase of thickness helps to be in safe for material stress but increase the weight which is not preferable, finally the designer must optimize the change in parameter to find a good performance of the system.modal test is applied to measure a structure's vibration properties of free muffler in order to compare them with corresponding data produced by theoretical method finite element (MCS patran).The modes from experiment have been obtained from a free muffler model, so choose free support, a relatively long soft two rubber ropes hanging up the muffler, to model support, experiment with single excitation point multi response point approach, using one-way accelerometer sensors along the radial ring muffler layout, the twelve sensors are located in upper and down face so the total point is 24, the pulse excitation done through the hammer impact, with LMS companies charge amplifier and finally LMS.TEST software for data processing, the optimal number of modes is determined with considerable of frequency and damping calculated from theoretical method. Physical intuition would lead to expect that estimates of frequency and damping corresponding to true structural modes should recur (in approximately the same place) as the number of modes is increased, to select the mode first the band is determined (range),then the optimal mode can chosen form stability chart, the result obtained from the experiment compared to the result obtained from finite element method, it is found there is a good agreement between two results, also the modal assurance criterion (MAC) is compared, it has a relatively good result.