Analytical And Experimental Investigation On Acoustic Performance Of Hybrid Muffler

The primary noise source of automotive is intake and exhaust noise. The noise level ofautomotives will be controlled significantly by taking effective noise control measures of intake andexhaust system. Muffler is one of the main devices to reduce intake and exhaust noise of engine.With the development of engine technology, the noise characteristic changes from low frequency tohigh frequency at different working conditions. Space and weight restrictions on automotive canmake it difficult to attenuate inanke and exhaust noise to a desirable level only using simple types ofmuffler. Hybrid muffler with high performances can offer the opportunity to achieve good exhaustnoise attenuation. Consequently, Mehods for calculating and designing hybrid muffler are studied inthis dissertation.Hybrid mufflers commonly have multiple chambers and different types of muffler connected toeach other by ducts. Thus, the design of hybrid muffler requires understanding of acousticcharacteristics of individual basic acoustic elements. In present study, the investigation of acousticbehavior of hybrid muffler is carried out theoretically, computationally and experimentally. First,acoustic performances of basic elements are explored in detail. Acoustic behavior of different typesof hybrid muffler is then discussed. Finnaly the common approaches for desiging hybrid muffler areconcluded for reference.The detailed work in this dissertation can be summarized as follows.The acoustic behavior of reactive mufflers is detailed discussed. The acoustic performance ofexpantion chamber is predictied by transfer matrix method, one-dimentional and two-dimentionalanalytical methods and the three-dimentional finite element method (FEM). A bus muffler ismodeled as an example and important aspects of modeling process for designer are discussed likesolid modeling, grid meshing, material properties and boundary condition setting, the methods forpredictiong transmission loss and so on. Three-dimensional analytical approaches are developed topredict the transmission loss of multi-inlet and multi-outlet muffer. The effect of the azimuthalangles, the offset distance, the quantity of inet and outlet and structural parameters of perforated tubeon acoustic performance of muffler is discussed. The improvement measures are taken to enhancethe overall performance of muffler. All disscutions above will be proved useful to the design ofhybrid muffler.The methods for prediction of acoustic characteristic of dissipative muffler are investigated.The sound absorbing material like fibrous material in muffler is treated as the fluid with complexsound speed and complex density. The relation between fiber acoustic properties and the perforationimpedance and sound proparation in muffler are developed for predictions of transmission loss ofdissipative muffler. The effect of length of muffler, thickness, hole diameter and porosity ofperforated tube and plate and acoustical properties of fibrous material on acoustic performance of dissipative muffler are then discussed. The acoustic performance of cross flow disspative mufflerand pass through flow dissipative muffler are analyzed. The sound attuention performance of threetypes of dissipative muffler is compared and the shortcomings and advantages are concluded.The acoustic behavior of Helmholtz resonator is investigated based on lumped parameter theory,one-dimentional and two-dimentional analytical method and FEM. The results obtained byapproaches mentioned above are compared with experimental result to check the validity of thesemethods. The predictions from lumped parameter theory and one-dimentional analytical methodafter end corrections are then compared with two-dimentional analytical result to check the accuracy.The impact of extended neck on resonant frequency and transmission loss of Helmholtz resonatorare discussed. Acoustic performance of Helmholtz resonator with absorbing material and is theninvestigated. The effect of filling positon and acoustic property of absorbing material ontransmission loss is examined. Since Helmholtz resonator is only suitable for narrow band noisecontrol, the serial and parallel multiple chamber Helmholtz resonators are proposed to extend itsapplication to broader band noise control. The effect of varying structural parameters and interactionbetween resonators on acoustic characteristic of Helmholtz resonators is investigated.In order to check the efficiency and validity on acoustic performance of muffler predicted byapproaches employed in this dissertation and make evaluation of overall performance of hybridmuffler, the experiment setup is built to measure the transmission loss and insertation loss of muffler.Transsmission loss of various protypes of muffler is measured on impedance tube by applingtwo-load method. Three hybrid mufflers are connected to engine experiment setup. Inesrtion lossand back pressure of mufflers are then measured to evaluate the overall performance of mufflers.The results obtained from the experiments can provide information for improvement design ofmuffler.Based on the theory and methods ultilized in presnt study, acoustic characteristic of variousmodels of hybrid muffler are investigated. The acoustic performance of mixed impedance hyrbridmuffler which consists of various perforated tubes and plates or various fibrous materials withdifferent property are studied. The acoustic attenuation characteristic of hybrid Helmholtz resonatorincorparoated serial and parallel Helmholtz resonators are explored. The analysis procedure amddesign approach for hybrid muffler is concluded. The concepts and conclusions may provideguidance for pratical hybrid muffler design.