Study Of Vibro-acoustics Of The Automotive Exhaust System

Energy short and environment pollution are the two serious problems which faced by the automobile industries. With the development of automobile industry, automobile noise pollution has been severer than before. One important component of automobile noise control is the exhaust noise of engine. At present, the main method to reduce exhaust noise is by setting muffler in automobile exhaust system.In the study of exhaust system, most of research concentrated on acoustic characteristic of muffler, there is few research into vibration, but vibration and acoustic are not separate. Therefore, the thesis presents the deep study of exhaust system which integrate multiple methods, such as theory ,simulation and experiment from two aspects of acoustic and vibration. The vibratory characteristic of exhaust system was deeply studied. The simulation and amelioration of the muffler which is assembled in a 480 diesel engine is presented. Applied with FEM and EMA methods, the modal parameters can be obtained and the reason that the exhaust system vibrates heavily is that natural frequency is equal to excited frequency of engine. The problem of vibration was solved by transferring the position of the suspensory. This provided a method on vibration control of other exhaust system. These results are also the basis of noise radiation prediction.In the aspect of simulation of muffler performance, used ANSYS and SYSNOISE , analysis software ,the several kinds of muffler performance were simulated and the results were verified by test. Some conclusions that different structures affect muffler performance were summed up. Compared one dimensional theory, three dimension numerical method can more accurately calculate the acoustic characteristic of complicated model, especially in the range of middle and high frequencies. Laid sound-absorbing material in muffler, it can reduce noise in high frequency. Simulation of muffler was carried out by multi-field method.The improvement of reducing noise was carried out by affording two schemes: both is based on the original muffler, the first is appending short tube and the tube from which gas spills was instead of perforate tube with sound-absorbing material; the second is that clapboards were transformed into perforate board and the tube from which gas spills was instead of perforate tube with sound-absorbing material. Made use of result of simulation and SYSNOISE FEM fluid modal, the transmission loss of muffler was predicted and the results were verified by test. The result indicates that both schemes are correct and the first is better than the second. This study shows that the trend of simulation and test is consistent despite difference of the result. This means provides an effective measure to design of muffler, more resources are saved and the design is accomplished in short time.Based on boundary element method(BEM),a prediction modal of noise radiation for exhaust shell was set up. Applied the result of dynamic response by FEM to calculation of noise radiation, the results of sound pressure level could be obtained and this method provides a basis on noise evaluation during design period.