Numerical Simulation And Design Investigation Of Automotive Exhaust Muffling Systems

Exhaust muffling system is an important part of automobile. A good exhaust system requires low back pressure and high noise attenuation performance. In this dissertation, the GT-POWER software is used to build the simulative models of gasoline engine and exhaust muffling system, and then to investigate the acoustic attenuation characteristics of mufflers as well as the effect of exhaust system on the engine performance.The GT-POWER software is used to calculate the acoustic attenuation performance of two reactive mufflers with stationary medium and high temperature gas flow. Based on the comparisons of transmission loss predictions with experimental measurements, the accuracy and applicability of GT-POWER for calculation of the acoustic attenuation performance of mufflers are discussed. In the planar wave range, this software could predict the transmission loss of mufflers accurately. However, the transmission loss prediction is not accurate when the frequency of analysis is very low due to the software drawback in its computational approach. At the higher frequency near the cut-off frequency, the software may predict the trend of transmission loss of mufflers only. The temperature and velocity of the engine exhaust gas have significant influences on the acoustic attenuation performance of mufflers. The transmission loss curve is moved to the lower frequency as the flow velocity increases, and the transmission loss curve is moved to the higher frequency as the temperature increases. The GT-POWER software is also used to calculate the transmission loss of a three-way catalytic converter, and the predictions are compared with the results by the frequency-domain method and experiment. The comparisons showed that the predictions by GT-POWER have errors, and the reasons may attribute that the shape of capillary tubes in a catalytic monolith is different from the default shape in the software, and the limitation in the expression of the capillary tube acoustic impedance in GT-POWER.The resistance loss of above reactive mufflers is calculated by GT-POWER at room and high temperature, and the predictions are compared with the results calculated by FLUENT software. The reasons for calculation errors by GT-POWER are analyzed in detail and the corrective method is offered. The temperature and velocity of the exhaust gas have significant influences on the pressure drop of mufflers. The resistance loss increases rapidly as the flow velocity increases, and it is directly proportional to the square of velocity. The resistance loss decreases rapidly as the temperature increases, and the resistance loss curve is a conic type. The resistance loss of the above three-way catalytic converter is predicted by GT-POWER at room and high temperature. The result shows that the resistance loss of the three-way catalytic converter is made up of pressure loss of substrate and local pressure loss under the room temperature condition. At high temperature, the resistance loss of the three-way catalytic converter is mainly caused by the pressure loss of substrate.Based on the study above, a three-way catalytic converter and two reactive mufflers are used to build the exhaust system. The exhaust system model coupled with engine model to calculate the insertion loss of muffler and composite structure, and to investigate the effect of muffler and composite structure on engine performance.Finally, based on the original exhaust system model, the GT-POWER is used to conduct the optimum design of exhaust system and muffler configuration in order to meet the requirements of the exhaust noise and back pressure.