Optimum Design of Engine and Body Mounts at Total System Level Using Unconstrained Minimization

The paper presents the mathematical formulation and procedure of an ongoing developmental work on the optimum design of engine and body mounts at a total system level. The objective of this study is to develop engine and body mount characteristics to minimize the engine induced and the road load induced vibrations over a wide frequency range. The computer software LOHITSA/SYS_OPT, a total system level non-linear unconstrained optimizer with built-in component mode synthesis solution technique, is used to develop the optimum engine and body mount characteristics. A typical total system level model consists of the body and/or frame, powerplant, suspension, body mount, engine mount etc. Complex structural systems such as body, frame and powertrain are represented as typical modal models (assumed), along with typical physical models of engine mount, body mount and suspension. The main goal of this study is to minimize the forces transmitted into the body structure in order to reduce the tactile and acoustic response with constraints on geometry and material properties, (physical locations, orientations and stiffness values) and manufacturability (shear stiffness to compression stiffness ratios). A forced frequency response optimization with component mode synthesis technique is then used to develop various sets of mount stiffnesses that satisfy all constraints and results in minimum transmissibility corresponding to each excitation frequency. A selection scheme is then used to identify a set of optimized values of the constraints which will produce minimum transmissibility over a wide frequency range of interest.

Engine excitations and the road load induced excitations are simulated using unit harmonic excitations over a wide frequency range. However the software is capable of handling actual forcing functions with real and imaginary or magnitude and phase components. Engine excitations are simulated by simultaneous application of pitch, roll and yaw couples at the engine center of gravity. The road induced excitations are simulated by applying three translational forces at the spindle.