Acoustic Performance Optimization Of Exhaust Muffler For A Passenger Vehicle Considering Pressure Loss

The exhaust noise is a main source of the vehicle noise.Reducing the exhaust noise is significant for reducing the vehicle noise.In practice,installing an exhaust muffler with excellent noise reduction performance and enough aerodynamic performance is an effective way to reduce the exhaust noise.The aim of this thesis is to improving the acoustic performance of a car-use muffler by optimization.The stationary noise tests and the passing noise tests are conducted to evaluate the current noise reduction ability of the muffler,and the objective function of the optimization is determined.In particular,a parametric study to analyze the varying tendencies of the pressure loss with the design variables-these key structural parameters of the muffler-is done before the optimization.In order not to deteriorate the aerodynamic performance of the muffler,the values of the design variables are limited to the ranges that ensure a lower pressure loss.Under this constraint,the optimization is carried out by using a 2D acoustic finite element method and its result is validated by a re-simulation with 3D finite element models.The details of this thesis are as follows:Firstly,under different engine speeds,the stationary noise and the passing noise of the car,with and without the exhaust muffler,are collected in a noise test.A time-frequency analysis to the passing interior and exterior noise is done,which shows that interior noise and the exterior noise are much smaller when the muffler is on.Consequently,the mean noise reduction ability of the muffler is confirmed.However,by comparing the interior and exterior noise spectrums in the stationary case with and without the muffler,it is found that the mid-frequency noise reduction ability of the muffler is weak.This conclusion is also proved by a correlation analysis to the exhaust noise and the interior/exterior noise,which demonstrates that the exhaust noise has a high contribution to the interior/exterior noise in the middle frequency range.Secondly,the flow pressure loss and the noise transmission loss of the muffler are calculated by finite element simulations.Specifically,the varying tendencies of the pressure loss with the main structural parameters are analyzed and the ranges of these parameters that lead to a lower pressure loss are summarized.Finally,an optimization towards improving the mid-frequency transmission loss of the muffle is conducted by using a 2D acoustic finite element method code developed for the muffler.Considering the improvement of the acoustic performance always results in the deterioration of the aerodynamic performance,the lower and upper boundaries of the design variables are set so as to guarantee a lower pressure loss.Based on the optimization result,a new topology of the muffler is designed and its noise transmission loss and flow pressure loss are checked by re-simulations.The result of the simulations shows that both the acoustic performance and the aerodynamic performance of the muffler are increased.Particularly,its mid-frequency transmission loss is improved by 17.5dB while its pressure loss is reduced by 15.3%.Therefore,the scheme of the performance improvement for car-use mufflers proposed in this thesis:(1)transmitting the restriction of the pressure loss to the limitation of the corresponding design variables;(2)acoustic optimization through 2D finite element method and result check-up with 3D finite element models is validated and proves to be an effective way for the car-use muffler design in engineering.