MODEL FREQUENCY ANALYSIS OF AUTOMOTIVE EXHAUST SYSTEM

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1 Research Paper ISSN 2278 ñ Vol. 3, No. 1, January IJMERR. All Rights Reserved MODEL FREQUENCY ANALYSIS OF AUTOMOTIVE EXHAUST SYSTEM D Jai Balaji 1*, P V Srihari 1 and Veeranna B Sheelvanth 2 *Corresponding Author: D Jai Balaji, jaibalajid@gmail.com An exhaust system has various aspects including vibrations, acoustics, thermal distribution and durability in addition to its interface with the vehicle. Presently exhaust systems are developed to minimise noise, minimise emissions, increase durability, minimise corrosion, better serviceability and make it economically viable. Modal analysis is the study of dynamic behaviour of structure at its various natural frequencies by studying its mode shapes. Marcus Myrén and Thomas Englund used beam elements, rigid elements and mass elements to represent an exhaust system in their work for simulation purpose. In this paper modal analysis of an automotive exhaust system for a passenger car was studied and a combination of shell and solid elements are used to represent the exhaust system to get better results in modal frequency domain. The exhaust system was modelled using Catia and discretized using Hypermesh, solved using ABAQUS solver. The results were viewed in HYPERVIEW and excitation points were found out in transverse direction. Keywords: Automotive exhaust system, Finite element analysis, Model analysis INTRODUCTION The automotive industry is heading in the direction of signing off the exhaust system durability based on computer simulation rather than rig simulation and physical vehicle testing. This is due to the cost, time and availability of prototype vehicles and test track. Use of Finite Element Method (FEM) enables to assure the structural integrity of the exhaust system and also contribute to better understanding of the system behaviour in the various operating conditions and evaluation of structural strength. Modal analysis is used here to find the various periods at which the exhaust system will naturally resonate. Exhaust systems are special cases in vehicles because of their geometry and the constraints placed on their design by the underside of the cars. An exhaust system is generally piping system used for muffling the noises caused by high pressure exhaust gases from the engine and avoid the hot/toxic gases from entering the passenger compartment. Exhaust systems 1 Department of Mechanical Engineering, R.V. College of Engineering, Bangalore, India 2 Asst Manager (FEA), Faurecia Emissions Control Technologies Bangalore Technical Centre, Bangalore, India 332

2 are subjected to many loads, the most important one coming from the engine and the road condition loads. The induced vibrations are spread along the exhaust system, and forces are hence transmitted to the car body through the attached points. A flex decoupler is used to reduce the vibrations transferred from the engine to the exhaust system and hanger isolators are used to reduce vibrations transferred from exhaust system to the body structure. GEOMETRICAL AND FINITE ELEMENT MODEL OF THE EXHAUST SYSTEM CAD Modeling of Exhaust System An exhaust system usually consists of pipes, acoustic and emission control components and decoupling elements. Since we are interested in only Structural analysis of the exhaust system, the internal peripherals of the muffler and the catalytic convertor are neglected. An inline exhaust system is considered for analysis in this paper which has a catalytic converter at the inlet pipe, a flexible coupling, a resonator, a mid muffler and rear muffler the exit. Figure 1 shows the CAD Model of an inline exhaust system and Figure 2 describe the parts of the same. Finite Element Modeling of Exhaust System The exhaust system cold end components such as down pipe, catalytic converter, flexible coupling, under resonator, muffler and tail pipe are made of stainless steel of various grades such as SS4501, SS308, SS4301 and FE410. CAD model was checked for geometry cleanup and mid surface extraction. FE model was developed by using preprocessing software Hypermesh. The Figure 3 shows the FE model of the exhaust system. Majority of components were modelled with shell elements (CQuad4) and flanges were modelled with solid elements (CHexa). The connections between flange and bracket bolt holes were made with rigid elements i.e. RBE2. The welded seams were modelled using solid elements (CHexa & CPenta). When modeling welded seams, it is important to ensure that at least one row of shell is lying entirely on the solids in order to depict the connection with sufficient resistance to bending. C3D6 elements form the runout of the fillet welds. The flexible coupling was represented using a Joint C spring element. To define tie contacts between surfaces, contact manager was used in Hypermesh. Perforated or punched metal pipes were also taken into account in the modelling process. This was done by inserting a separate shell component. Depending on the type of perforation, the wall thickness is to be revised lesser than the unpunched condition. The value was taken as around 80-85% of the initial value. The Table 1 shows the different materials used for different parts with their properties. Table 1: Material Properties Sl. Parts Material Name Young s modulus, Poisson s Density, No. N/mm 2 ratio T/mm 3 1 Pipes & Flanges SS E E-09 2 Weld SS E E

3 Figure 1: CAD Model of an Inline Exhaust System Figure 2: CAD Model of an Inline Exhaust System Figure 3: Finite Element Model 334

4 MODEL ANALYSIS Modal analysis is the study of properties of structures under vibrational excitation. Modal analysis uses the overall mass and stiffness of a structure to find the various periods at which it will naturally resonate during free vibration. Most of the times the only desired modes are the lowest frequencies because they can be the major modes at which the structure will vibrate, dominating all the higher modes. The objective of free-free analysis is to make sure first 6 natural frequencies equal to zero. It is also a model check for FEM. In free-free modal analysis no constraints are applied to the system. Results include natural frequency, mode shapes and displacement for each mode. Strain energy results are in various forms and are used to identify critical locations in the model for each mode. If a location appears critical for several modes, then it definitely is a potential failure location. RESULTS AND DISCUSSION Table 2 shows the modes extracted from the free-free modal analysis performed on the exhaust system. Block Lanczos method was used to extract the mode shapes of the Table 2: Modes and Frequencies Mode Number Frequency E E E E E E Figure 3: Showing 9th Mode of Exhaust System with Bending in Z-Direction 335

5 Figure 4: Showing 14th Mode of Exhaust System with Bending in Z-Direction exhaust system. The first six modes shows rigid body modes in three linear directions and rotational directions which are zero or equal to zero. This is one of the finite element validation methods for free free modal analysis. The nodal points shown in the Figure 3 and Figure 4 are at mode 9 and mode 14 due to bending in Z-direction where the displacement is zero. CONCLUSION This paper deals with the study of an exhaust system in general, using modal analysis. The mode shapes observed from Finite Element Analysis is mostly in transverse directions. So the excitation points are chosen such that it excites the system in both the transverse directions. It will give the general guidelines on how to perform modal analysis in the initial stage of the project and check for nodal and anti-nodal points from mode shape animation. The nodal points are to be considered as preliminary hanger locations for the exhaust system as this is where the displacement is minimum. SCOPE OF FUTURE WORK Further to the above analysis static and dynamic analysis, temperature distribution analysis, thermal stress analyses can be performed in order to simulate components such as catalytic converter, manifold flange, front pipes, flexible coupling etc. ACKNOWLEDGMENT The authors would like to thank everyone who was helpful in providing necessary information and for their support. REFERENCES 1. Arun Viswanathan and Dr Elaya Perumal (2009), Deciding Isolator and Mounting Points of a Truck s Exhaust System Based on Numerical and Experimental Modal Analysis, ICSV 16, Krakow. 2. Marcus Myrén and Jörgen Olsson (1999), Modal Analysis of Exhaust System, M S Thesis, University of Karlskrona/Ronneby. 3. P Verboven, R Valgaeren, M Van Overmeire and P Guillaume (1998) Some Comments on Modal Analysis 336

6 Applied to Automotive Exhaust System, 16th International Modal Analysis Conference, IMAC XVI. 4. Thomas Englund, Dynamic Characteristics of Automobile Exhaust System components, M S Thesis, University of Karlskrona/Ronneby,