1993-05-01

Optimization of Engine Mounting Systems to Minimize Vehicle Vibration 931322

A new simulation-based method for design of powerplant mounting systems is presented. Unlike traditional methods, in which the objective is to obtain a set of powerplant rigid body modes found from experience to be favorable, this new method directly seeks minimal response in the vehicle passenger compartment, regardless of what powerplant modes are obtained. Therefore, the simulation objective exactly matches the design objective: minimal response.
The new method, which uses optimization based on response sensitivities, has been implemented into software. Results show that the final response levels are significantly reduced from the baseline, and that typically the final mounting configuration is much different, and better, than the mounting system that would have resulted from application of the traditional method.
POWERPLANT MOUNTING is one of the fundamental design characteristics of a motor vehicle. A vehicle's powerplant, in particular the way it is mounted to the vehicle body, plays a major role in determining the vehicle's vibration characteristics. A well-designed mounting system needs to isolate the engine inputs from the vehicle body, and use the engine mass to minimize, not amplify, the effects of road/wheel inputs.
Through the years, vehicle designers have developed certain “rules-of -thumb” that generally lead to good mounting systems. These are rules that are applied to the powerplant's six rigid body modes: frequency targets, coupled vs. decoupled modes, etc. Although this traditional method for powerplant mounting has proven useful, it is very indirect and often difficult to apply. While the real goal is to minimize vibration felt by the vehicle occupants, the rules-of-thumb apply to powerplant modes.
A new simulation-based method for powerplant mounting is presented here. The objective of this new method is more direct than traditional methods. The goal is to find a mounting system that results in minimal response in the vehicle passenger compartment, regardless of what powerplant rigid body modes result. Thus, the simulation method has the same objective as the design itself: to minimize vibration perceived by the vehicle occupants.
The new method uses computed response sensitivities to determine changes to the mounting system that will lead to minimum response at user-defined locations in the vehicle. Design variables are mount locations, mount stiffnesses, and mount damping values. Special purpose software automates the method. The software changes the design variables until the lowest level of response is found. Typically, the final mounting configuration that results from the application of this method is quite different from, and better than, the mounting system that would have resulted from application of traditional methods. The reason is that, while traditional methods seek favorable powerplant rigid body mode characteristics, hoping for minimal vehicle response, the method presented here directly seeks minimal vehicle response.
Obviously, the rules-of-thumb that are used to apply traditional methods have been developed through experience with past designs. While it is certainly beneficial to take advantage of past experience whenever possible, good design methods should not keep the designer from considering alternative, and perhaps radical, new designs. Perhaps the biggest advantage of this response-based method is that it allows the investigation of alternative designs, designs for which the traditional rules-of-thumb may not apply.

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