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There are two analytical methods for optimizing automotive structural dynamic characteristics to improve vehicle ride quality and minimize structural mass for improved fuel economy. The first method, the traditional approach, is to move the undesired structural resonant frequencies out of the range of the forcing functions by modifying the mass and stiffness parameters appropriately. However, in some cases the resonant frequencies are insensitive to parameters; these cases normally are difficult to improve. Fortunately, there is a second method, based on the natural phenomena that an anti-resonance exists for each resonant frequency. Furthermore, the sensitivity of these anti-resonance nodes to the structural parameters of mass, stiffness and damping are uniquely different. It is this difference in sensitivity that permits cases to be solved, which resist solution by the traditional first method.

This paper describes a new method for engine design using dynamic optimization and substructure synthesis method. A very important theme in engine design is how to shift the peak of the natural frequency of the vibration mode that causes some noise and vibration problems. This must be resolved by effective modification of structural design. In order to carry out effectively vibration analysis of a large scaled structure like engine assembly and conduct dynamic optimization with many iterative calculations, we have used substructure synthesis method that devides a whole structure into a number of substructures and solves each substructure. Vibration analysis of engine assembly (cylinder block, crank shaft, bearing caps and flywheel systems) was carried out by using this substructure synthesis method.

In this study, the genetic algorithm so called GA is newly applied for the optimization of many engine mounting parameters, calculations of stiffness matrix and inverse matrix to obtain 6 degrees of freedoms displacements at mounting points and a center of gravity. As a result, the optimized result could be shortly obtained in a minute, and an inexperienced engineer could easily make the optimum engine mounting layout, which can satisfy the vibration isolation and the non-interference in an engine compartment.

This paper describes a method for effectively reducing a level of idling vibration in heavy-duty trucks, which has been the point at issue lately. In this method, the vibration level is significantly reduced by using a full vehicle model, which is made by finite elements, and varying parameters to study effects. In order to achieve high accuracy, engine excitation forces calculated from the measured fluctuation in the flywheel angular velocity are input to the model. An effective use of this method in an early development stage has enabled us to reduce development cost and the lead-time.

In this study, the air suspension is newly applied to the engine mounting layout for getting the significant vibration isolation effect. In this case, the genetic algorithm so called GA is also applied for the optimization of many parameters, calculations of stiffness matrix and inverse stiffness matrix to prevent the coupled vibration of lateral and rolling modes and to obtain the displacement of each mounting point. As a result, inexperienced engineers can easily obtain the optimum engine mounting layout in a minute. By the confirmation test of FEM, the engine lateral vibration level at 25Hz dropped below 1/10 and its effect was significant.