Research On Acoustic Performance Of Automotive Exhaust Muffler Based On Virtual.Lab Acoustics

Exhaust noise is the main noise source of car engine, using exhaust muffler is the mainway to control automotive engine exhaust noise. When the muffler reduces noise, its shell maybe motivated to generate new noise, acoustic analysis of muffler should include calculate pipenoise and shell radiation noise. In the design of muffler if considered only one aspect, it islikely to worsen another performance. In the past, when calculating the muffler acousticperformance，only paid attention to its pipe noise, without considering the influence of surfacevibration on muffler performance and the influence of outer field radiation noise. The studyshows that in some cases, the structural radiation noise of muffler’s shell can be equal to theexhaust pipe noise. Therefore, these two factors should also be considered, so that the totalacoustic performance of muffler can reach optimization. According to the above problem, bothpipe noise and shell noise were considered, the acoustic performance of muffler was studied Inthe LMS Virtual.Lab.First of all, acoustic FEM of the exhaust muffler was established in the VL.This paperpresents two different methods for the study of transmission loss of muffle: The first methodused defining velocity of particle vibration on the inlet; another method used technology ofAML to calculate sound power. The results show that the calculation results of the two methodsare highly consistent. The second method has simple calculation steps and faster calculationspeed as well as is suitable for the large cross-section duct muffler, shows its superiority, andprovides a new method for the calculation methods of transmission loss of large cross-sectionduct muffler. Preliminarily studied the influence of flow velocity on transmission losscharacteristics of muffler, Results show that the inlet velocity of60m/s have little effect onthe transmission loss of muffler in the low-frequency range, effect is more obvious in highfrequency range. The radiation noise of muffler outlet was studied with AML technology basedon the effect of flow velocity, the finite element calculation model of insertion loss was alsoestablished.Secondly,according to the problems of the acoustic performance of original muffler,according to the exhaust muffler design theory, transmission loss is as the evaluation index ofacoustic performance, without changing its shape and size, by changing the muffler internalstructure, increase the perforation structure and porous absorption material and other measures,two kinds of optimization improved scheme was proposed. In order to solve the perforationplate hole mesh which leads to computational difficulties, Choose to define the transfer admittance on perforated plate on both sides to simulate these holes.Through analysis andevaluation of these two schemes，the results show that the two schemes are feasible, and thesecond kinds of optimization scheme is the best. Finally selected scheme-2as the target of theoptimization design,improved-2muffler as research object for the muffler coupledvibro-acoustic and radiation noise below.Then, taking improved-2muffler as the object of research,carried on the vibration analysis,analysis and determine the vibration sources of muffler. Study of acoustic modal in the mufflerand shell structure modal, coupled modal, find the three modal natural frequencies and modeshapes. By contrast to the dry mode and coupled modes, find that both the natural frequency isnot the same, have certain differences, indicates that fluid has a certain influence on structuremode of muffler. Considering interaction of fluid and structure acoustic, coupled vibro-acousticanalysis was carried out on the muffler, Considering acoustic performance of muffler in theinfluence of coupled vibro-acoustic.Finally, vibration response of the muffler shell after coupled vibro-acoustic was mapped tothe acoustic mesh as boundary conditions,using FEM-AML method muffler shell radiationnoise analysis model was established. Radiation noise prediction was studied for the mufflershell, then got the distribution of sound pressure level on the surface of the muffler and shellradiation noise sound pressure level distribution on field mesh. Through the analysis ofdistribution of sound pressure level on field mesh and sound pressure（dB）frequency responsefunction of IO points on field mesh, the results show that the second order modal of the mufflerhave larger influence on radiation noise. In view of the prediction results for radiation noise ofmuffler shell, some measures that control exhaust muffler shell radiation noise were given.