DESIGN & OPTIMIZATION OF EXHAUST MUFFLER & DESIGN VALIDATION 1 RAHUL D. NAZIRKAR, 2 S.R.MESHRAM, 3 AMOL D. NAMDAS, 4 SURAJ U. NAVAGIRE, 5 SUMIT S. DEVARSHI 1,2,3,4,5 Department of Mechanical Engineering, Sinhgad Institute of Technology, Lonavala-410401, India Abstract- For an automotive exhaust system the noise level, transmission loss & back pressure are the most important parameters for the driver & engine performance. In order to improve the design efficiency of muffler, resonating of the exhaust muffler should be avoided by its natural frequency. Mufflers are most important part of the engine system and it is commonly used in the exhaust system to minimize the sound transmission level which is caused by exhaust gases. The design of muffler becomes more and more important for noise reduction. The solid modeling of exhaust muffler is created by CATIA-V5 and modal analysis is carried out by ANSYS to study the vibration and natural frequency of muffler. So as to differentiate between the working frequency from natural frequency and avoid resonating.. Keywords- Exhaust muffler; Creating 3-D model in CATIA-V5; Modal analysis; Mode shape I. INTRODUCTION Expansion chambers are effective tools for reducing noise in several applications. The most familiar example is probably the automotive muffler, where a single tuned expansion chamber is utilized. Therefore, automotive exhaust system is a significant part of exhaust system. There are several parameters that describe the acoustic performance of a muffler and/or its associated piping. These include the noise reduction (NR), the insertion loss(il), and the transmission loss (TL). The NR is the sound pressure level difference across the muffler. Though the NR can be easily measured, it is not particularly helpful for muffler design. The IL is the sound pressure level difference at a point, usually outside the system, without and with the muffler present. Though the IL is very useful to industry, it is not so easy to calculate since it depends not only on the muffler geometry itself but also on the source impedance and the radiation impedance. The TL is the difference in the sound power level between the incident wave entering and the transmitted wave exiting the muffler when the muffler termination is anechoic; the TL is a property of the muffler only. The muffler TL may be calculated from models but is difficult to measure. This paper will focus on measuring the muffler TL. [1] II. BASIC THEORY Since the invention of the internal combustion engine in the latter part of the nineteenth century, the noise created by it has been a constant source of trouble to the environment. Significantly, the exhaust noise in terms of pressure is about 10 times all the other noises (structural noise) combined. So the problems of reducing engine noise consist, mainly in attenuating exhaust noise. The design of mufflers has been a topic of great interest for many years and hence a great deal of understanding has been gained. Most of the advances in the theory of acoustic filters and exhaust mufflers have come about in the last four decades. Hence good design of the muffler should give the best noise reduction and offer optimum backpressure for the engine. Moreover, for a given internal configuration mufflers have to work for a broad range of engine speed. III. DESIGN There are numerous variations of the two main types of muffler designs commonly used, namely absorptive and reactive. Generally automotive mufflers will have both reactive and absorptive properties. [3] A. Benchmarking The first step in any design is to set a target by doing the benchmarking. The same will be applicable for the silencer here, to set a target in terms of transmission loss. B. Target Frequencies After doing benchmarking exercise, there is needs to calculate the target frequencies to give more concentration of higher transmission loss. For calculating the target frequencies engine max power rpm is required and calculation follows, 31
Theoretical Computation: The exhaust tones are calculated using the following Formulae: CFR = Engine Speed in RPM/60. For a two stroke engine (1) = Engine Speed in RPM/120.For a four-stroke engine (2) EFR = n X (CFR) (3) C. Muffler Volume Calculation Volume Of the muffler (Vm): Design & Optimization of Exhaust Muffler & Design Validation D. Internal Configuration And Concept Design Based on the benchmarking transmission loss and target frequencies the designer draws the few concept of the internal configuration. The diameter of the hole to be drilled on the pipe is calculated as; In matlab simulation the value of transmission loss becomes 6.078 db and by using mathematical equation transmission loss becomes 6.04 db. So, the MATLAB simulation model is validated and percentage error in between this is 0.625 %. G. Single Expansion Chamber shape 1 is 562.9 Hz. 32
Design & Optimization of Exhaust Muffler & Design Validation shape 5 is 1529.2 Hz. H. Double Expansion Chamber shape 2 is 765.9 Hz. Modal Analysis Fig.7 shape 3 is 765.21 Hz. shape 1 is 583.62 Hz. shape 4 is 1029.7 Hz. shape 2 is 637.7 Hz. 33
shape 3 is 650.72 Hz. Design & Optimization of Exhaust Muffler & Design Validation In this model the first protrusion i.e., inlet pipe is included 100mm inside the muffler and then analysis is carried out. shape 4 is 650.72 Hz. The TL result for given model is 19.62 db. shape 5 is 1001.1 Hz. IV. OPTIMIZATION (A) Single Expansion Chamber The different transmission loss of single expansion chamber is at different protrusion. These different models are as below; I. Single Expansion Chamber with First Protrusion Transmission loss should be more than 30 db. In this muffler, transmission loss becomes 19.62 db. So it is not satisfying condition. II. Single Expansion Chamber with Front And Back Protrusion Muffler 34
Design & Optimization of Exhaust Muffler & Design Validation In this double expansion chamber the inlet protrusion is increased by 100mm and outlet protrusion is increased by 50mm and then analysis is done. Transmission loss should be more than 30 db. In this muffler, transmission loss becomes 21.24 db. So it is not satisfying condition. The transmission loss of single expansion chamber is 6.04 db which is less than requirement i.e., 30 db and it is not satisfactory. So, it is not satisfying condition. Therefore, for the better sound quality and transmission loss we are doing optimization in the double expansion chamber. (B) Double expansion chamber 35 Transmission loss should be more than 30 db. In this muffler, transmission loss becomes 42.48 db. So, it is satisfying condition. So, it is optimized condition in the consideration of transmission loss. V. RESULTS The results of modal analysis for the six natural frequencies of single expansion chamber of muffler is in Table I
TABLE I. COMPARISON OF MEASURED AND EXPERIMENTAL NATURAL FREQUENCY Design & Optimization of Exhaust Muffler & Design Validation The results of modal analysis for double expansion chamber of inlet and outlet at different fixing positions are shown in tabulated form; TABLE II. FIXED AT FRONT END TABLE III. FIXED AT FIRST AND LAST END TABLE IV. FIXED AT MIDDLE PIPE 36
Design & Optimization of Exhaust Muffler & Design Validation CONCLUSION Double expansion chamber gives better results as compared to single expansion chamber. Transmission loss of double expansion chamber is 42.48 which is more than requirement and satisfactory. Also Natural frequency of double expansion chamber is within range of 583.62 to 1001.1 Hz resulting in no resonance. By fixing the muffler at first and double expansion chamber we can increase the frequency and avoid the resonance. Transmission loss of the muffler can be increased by adding protrusion pipe at inlet and outlet. It can be seen that the finite element modal analysis has certain significance in the study of vibration characteristics of the muffler. The time required for optimization of muffler using ANSYS and MATLAB is very short and can be repeated simply after changing the input parameters which provides an easy way to find an optimum solution for muffler design. VII. FUTURE WORK The muffler which we are going to create that is little big in size. So, in future the size of muffler can be minimize to a proper size which can be suitable for the motorcycle. Also there is a scope to calculate back pressure. Also because of reduction in size of muffler the manufacturing cost of muffler can also be reduce. Due to reduction in the muffler the requirement of space is also less. REFERENCES [1] Z. Tao & A.F.Seybert, A Review Of Current Techniques For Measuring Muffler Transmission Loss, Society Of Automotive Engineers (SAE), 2003 [2] Shital Shah, Saisankaranarayana K, Kalyankumar S. Hatti, & Prof. D.G.Thombare, A Practical Approach Towards Muffler Design, Development and Prototype Validation, 2010 [3] Potente, Daniel, General Design Principles For an Automotive Muffler, Proceedings of Acoustics, 2005 [4] Tetsuo Kaneda, Mitsuaki Oda, Prediction of Transmission Loss for Motorcycle Muffler, Society Of Automotive Engineers (SAE), 1999 [5] Sachin Wagh, Development of Exhaust Silencer for Improved Sound Quality, Society Of Automotive Engineers (SAE), 2010 [6] I. J. Lee and A. Selamet, Acoustic Characteristics of Coupled Dissipative and Reactive Silencers, Society Of Automotive Engineers (SAE), 2003. 37