Investigation To Vibration Modeling And Dynamic Characteristics Of Vehicle Exhaust System

Exhaust system is an important part of vehicle with internal combustion engine propulsion system for discharging exhaust gas from engine. Though performances of acoustics, exhaust purification and pressure loss of the exhaust system have been widely investigated, its dynamic behaviors still haven't been attracted enough research attention. One of traditional exhaust systems weights about 15kg, connecting between the vehicle body and the engine. With increase of its weight, the exhaust system not only affects the vehicle NVH performance but also disturbs the matching design of the powertrain mounting system, therefore, the dynamic behaviors of the exhaust system are required a comprehensive study.Due to its complexity in structure, the exhaust system can't be directly modeled by the classical vibration theory. Based on the beam element, one dimension numerical model of the exhaust system is established. The simplification of the flange joint is verified by the test and a simple and reasonable approach to one dimension modeling for the muffler with complex topology structure is provided. According to the one dimension modeling, numerical and experimental modal analyses are carried out, and the engine's effect is considered in the constrained modal analysis. By comparison between the simulation and the experiment results, it is shown that the numerical predictions are in fairly good agreement with the real test results in the vibration characteristics of the exhaust system, and maximum deviation in frequency is less than 10Hz.Further by the one dimension modeling of the exhaust system, the effect of the engine mounting system on the exhaust modeling is evaluated. It is shown that by the simulation results the engine mounting system has an important effect on the exhaust modeling and can not be negligible. Because there are many physical and geometric parameters in the design of exhaust units, it is quiet difficult to optimize accurately the dynamic properties of the exhaust system. Thus, the sensitivity analysis of the natural frequencies of the exhaust system to the unit parameters is performed and the sensitive parameters are provided thereafter.Although the exhaust system is connected with the engine, the weight of the traditional exhaust system is much lighter in comparison with that of the engine system. The effect of the exhaust system on the dynamics of the engine mounting system may be ignored. With increase of weight of exhaust system and its stiffnesses of hangers, much attention must be paid to that unnegligible effect. According to the exhaust modal analysis, an 18 DOF dynamic model of the engine mounting system considering the effect of the exhaust system is proposed, and the exhaust system is simplified to be a vibration system described by the lumped mass, linear stiffness and damping. Through the studies it is indicated that the exhaust system has the largest effect to the engine-mount system in X andθx directions, the larger in Z andθz directions and small in Y andθy directions, the maximum deviation in frequency is 1.6Hz appearing in theθx direction, meanwhile, except in the Y andθy directions, the rates of decoupling in the other directions are all decreased 10% as well.Road and engine excitations are the main vibration sources of the exhaust system, the vibration transfer characteristics of the exhaust system from the road and engine excitations should be investigated. In the study of road excitation, the exhaust theoretical modeling method is introduced and an 8 DOF vehicle model in the Z direction in association with the exhaust model is established. On the other side, in the study of the engine excitation, the numerical model of the exhaust system counting for effect of the engine system plays a leading part and a numerical vehicle model including body, chassis, powertrain and exhaust system is developed, in which energy dissipative components such as suspensions, bushes and exhaust hangers are considered in the modeling and excitations of the engine in X, Y and Z directions are analyzed, respectively. In order to verify the rationality of two types of the models, proving ground experiments are made. By the test results it is indicated that the predictions of the both models to the dynamic responses of the exhaust system are in good agreement with experiments, and at the same time, due to the model simplification and the complexity of the excitations, there appear still some discrepancies of the simulation results from the experimental observations.Finally, methods of the vibration control for optimization of the exhaust system are summarized in some details.