Techniques for Engine Mount Modeling and Optimization

This dissertation presents techniques for the design of engine mounting system to address the issue of vibration isolation. While the techniques presented herein are general, the application of proposed techniques is demonstrated primarily through applications in motorcycles. The dynamic loads that are generated due to the shaking forces within the engine and the road loads that are transmitted to the engine through the tire patch are discussed. The geometrical shape of the engine mount is also considered in this work. All models discussed herein deal with solving the optimization problem for the engine mount system such that the transmitted forces to and from the engine are minimized in which the mount parameters are used as design variables. While work has been done in the past in the area of engine mount design, this dissertation tries to fill in the gap when it comes to designing a comprehensive mounting system that takes into account modeling of the mount characteristics, the excitation load present in the system, and a determination of the final geometrical shape of the engine mount. The work presented in this dissertation discusses three major problems. The first problem addresses comprehensive mount modeling wherein mathematical mount models range from a simple Voigt model to a complex Voigt model that captures hysteresis and nonlinear behavior are presented. The issue of mechanical snubbing is also considered in these models. An optimization problem is formulated to determine the mount parameters by minimizing the difference between the transmitted loads predicted by the theoretical model and experimentally measured values. The second problem addressed in this dissertation deals with mounting system optimization. The optimization is carried out such that the loads transmitted through the mount system from/to the frame are minimized. The road loads that are generated due to the irregularities in the road profile and the shaking loads that are generated due to the engine imbalance are discussed in detail. The mount parameters are considered as design variables. Displacement constraints, both static and dynamic are considered to account for packaging requirements and to prevent mechanical snubbing of the engine mount. Numerical examples dealing with mount system optimization are presented first for a six degree of freedom model that deals only with the powertrain assembly. This is followed by twelve degree of freedom model that builds on the previous model by considering the swing-arm assembly dynamics in addition to the powertrain assembly. The third problem presented in this dissertation deals with finding the optimum geometrical shape of the mount itself. The shape optimization of the mount is done using a nonlinear finite element model of the mount developed in ANSYS®. An optimization problem is formulated to minimize the difference between the target stiffness obtained from the dynamic analysis and stiffness values obtained from the mount geometry. The mount geometrical parameters such as the mount diameter and the thickness are used as design variables. Numerical examples are provided quantifying how mount geometrical parameters vary for different operating engine speeds. All the models and techniques developed in this work will help designers comprehensively design a mounting system that achieves an effective vibration isolation of the powertrain assembly