On the Effectiveness of the Spatial Transmissibility to Drive the NVH Design of Cylinder Head Covers

Many suppliers and OEMs adopted the concept of transmissibility ratio <TR> as a method of choice to evaluate the NVH performance of cylinder head cam covers. The <TR> was defined as the transfer function between the cam cover and the cylinder head average vibrations. The <TR> was shown to be independent of the engine speed and declared to be a characteristic intrinsic to the cam cover system.

This paper examines the correlation between the <TR> predictions and the measured cam covers sound power. For this purpose, a comprehensive study was conducted using several cam covers with different materials, designs and isolation systems. The results indicated a moderate correlation between the <TR> and the sound power for the isolated covers only. Analysis of the measured cam cover and cylinder head vibrations shows the potential cause for this weak correlation and demonstrates the need for improving the <TR> definition in order to accurately guide the cam cover NVH design.