Modal Analysis of Exhaust System to Optimize Mounting Hanger Location Chetan D. Gaonkar Assistant Professor, Mechanical Engineering Dpt., DBCE, Margao, Goa Abstract— An exhaust system with a superior performance becomes unserviceable if its durability is insufficient, for example, due to excessive level vibrations. This excessive level of vibration caused by various excitation forces from engine and road surfaces are transferred on to the exhaust hangers which plays a vital role in clamping the exhaust systems in proper place, thus damaging the hangers much before its service life. Hence it becomes obligatory for NVH engineers to optimize the hanger location so that it undergoes minimum damage, thus increasing its durability. This paper presents a modal analysis approach for optimizing the hanger location using FEA and comparing the results with experimental modal analysis. For FEA technique, Hypermesh was used as a pre and post processor whereas Nastran was used as a solver. The methodology adopted here was to determine node and antinode points on the exhaust system so that the mounting hangers can be shifted to node points. Hence after the identification of critical frequencies, its mode shapes were analyzed to identify optimum hanger location. Further the results were compared by performing experimental modal analysis using LMS Data Acquisition System. Keywords— NVH; Exhaust system; Modal Analysis; Natural Frequency; Mode Shape; MAC Diagram I. INTRODUCTION Exhaust systems are subjected to many dynamic input loads, the most important one coming from the engine and road surface. The induced vibrations are spread along the exhaust system, and forces are transmitted to the car body through the attached points, mainly holding hangers. Due to this, these holding hangers undergoes maximum damage. To tackle this problem, a NVH engineer can use modal analysis technique. Modal analysis has turned into a real option to give an accommodating commitment in understanding control of numerous vibration phenomena which are experienced in everyday practice. Deciding the nature and degree of vibration response levels and confirming theoretical models and prediction are both significant targets that can be accomplished with experimental modal testing. II. FEA TECHNIQUE First, a FEA technique was used to perform modal analysis. In this approach, starting from the structural geometry, the boundary conditions and material characteristics i.e. mass, stiffness and damping distribution of the structure is expressed in terms of respective matrices. Theses contain sufficient information to determine the system modal parameters. Nowadays advanced software packages such as Hypermesh and Nastran are available. Hypermesh is a general purpose finite element modelling package whereas NASTRAN for numerically solving a wide variety of mechanical problems. These problems include: Static/Dynamic Structural Analysis (both linear and nonlinear), Heat Transfer and Fluid Problems, as well as Acoustic and Electro-Magnetic problems. The combination of these two software packages is effectively used for solving modal analysis problem in this work. A. Description of the elements used Shells are essentially 2-D elements that represent 3-D space, thus the term 2.5-D is also used. Shells are excellent elements for thin 3-D structures, such as body panels, sheet metal, injection moulded plastic or any part that can be described as having a thickness that is small relative to its global dimensions. Deflections are given at the nodes, but stresses can be found at the upper and lower surfaces as well as at the midplane. This gives the analyst the ability to extract membrane effects versus bending effects in the results. The welds and bolted connections are simulated using rigid and Beam elements respectively. Outer surface of the muffler was meshed with shell elements of appropriate thickness. Suitable material properties were assigned to the shell elements of outer surface of muffler to match the mass of the muffler. Flex pipe is modelled with CBUSH element and its mass is represented by point mass. Contact between straps and muffler is modelled with CBUSH elements. In order to account for the weight of glass wool and other substrate materials, NSM (Non Structural Mass) element was used. B. Boundary Conditions Much attention is required in specifying boundary conditions. Improper specification of the boundary conditions may cause various problems. In this case to extract all natural frequency at the system level, free-free boundary condition is used. Fig. 1 shows boundary conditions for a conventional exhaust. C. Material Properties The material properties such as Young’s Modulus, poisons ratio and density are required as an input values for performing modal analysis. For the flex pipe present in the system, the stiffness values in X, Y and Z directions are also required. The mechanical properties of the constituents of International Journal of Engineering Research & Technology (IJERT) ISSN: 2278-0181 www.ijert.org IJERTV4IS030050 Vol. 4 Issue 03, March-2015 24