Dynamic characteristics of automobile exhaust system components

Sammanfattning: Demands on emission control, and low vibration and noise levels have made the design of automobile exhaust systems a much more complex task over the last few decades. This, combined with increasing competition in the automobile industry, has rendered physical prototype testing impractical as the main support for design decisions. The aim of this thesis is to provide a deeper understanding of the dynamic characteristics of automobile exhaust system components to form a basis for improved design and the development of computationally inexpensive theoretical component models. Modelling, simulation and experimental investigation of a typical exhaust system are performed to gain such an understanding and evaluate ideas of component modelling. Modern cars often have a gas-tight bellows-type flexible joint between the manifold and the catalytic converter. This joint is given special attention since it is the most complex component from a dynamics point of view and because it is important for reducing transmission of engine movements to the exhaust system. The joint is non-linear if the bellows consists of multiple plies or if it includes an inside liner. The first non-linearity is shown to be weak and may therefore be neglected. The non-linearity due to friction in the liner is, however, highly significant and gives the joint complex dynamic characteristics. This is important to know of and consider in exhaust system design and proves the necessity of including a model of the liner in the theoretical joint model when this type of liner is present in the real joint to be simulated. It is known from practice and introductory investigations that also the whole system sometimes shows complex dynamic behaviour. This can be understood from the non-linear characteristics of the flexible joint shown in this work. An approach to the modelling of the combined bellows and liner joint is suggested and experimentally verified. It is shown that the exhaust system is essentially linear downstream of this joint. Highly simplified finite element models of the components within this part are suggested. These models incorporate adjustable flexibility in their connection to the exhaust pipes and a procedure is developed for automatic updating of these parameters to obtain better correlation with experimental results. The agreement between the simulation results of the updated models 5 and the experimental results is very good, which verifies the usability of these component models. A major conclusion is that in coming studies of how engine vibrations affect the exhaust system it may be considered as a linear system if the flexible joint consists of a bellows. If the joint also includes a liner, the system may be considered as a linear sub-system that is excited via a non-linear joint.