Spatial transmissibility of plastic cylinder-head covers

The transmissibility technique has been traditionally used for evaluating the NVH performance of isolated, rigid structures such as the elastomer mount isolated automobile engine. The transmissibility quantity provides information on how a structure reduces vibration as subjected to dynamic loading and thereby attenuates noise. In the present study, the transmissibility is applied to a non-rigid, plastic structure - the engine cylinder-head cover module. The cover module includes primarily a thin, plate-like cover and the elastomer isolation system. At low frequencies, the cover will behave as a rigid mass and thus display a major peak at its resonant frequency. At high frequencies, the cover will vibrate as a flexible panel and thus display multiple peaks with magnitudes differing from point to point across the cover surface. As a result, the transmissibility calculated would have a spatial resolution, called the spatial transmissibility. Comparing to the traditional methods of quantification the spatial transmissibility is a more effective method in evaluating the NVH performance of the cover system, especially in the early design stage. A comprehensive study of spatial transmissibility is conducted on a production plastic cylinder-head cover module. The experiments are conducted on a full vehicle equipped with these covers. The tests are run at various driving conditions and the accelerations at the cover fastening bolts and cover surface are measured. The transmissibilities are calculated based on the acceleration data. The results show that the transmissibilities at varying driving conditions fall into the similar pattern of curves, indicating that it is an independent property of the cover system.