Piston Design for Optimizing Trade-off of Friction and NVH

Requirements for reducing powertrain NVH drives the selection of low piston skirt to liner clearances contradicting the requirement to maintain larger skirt clearances for minimizing engine friction. Whilst this clearance trade-off between low friction and low NVH is fundamental, piston design features have a significant effect on where the trade-off curve sits on the friction/NVH map. Design features can therefore be viewed not by either friction or NVH improvement measures but a shift in the friction-NVH trade off curve. Specifically, some piston design features which may be targeted at reducing friction can be viewed as either a friction benefit for similar NVH or an NVH improvement for similar friction levels. The ability to realistically quantify the effect of the design changes on NVH is therefore critical to determining what design changes to recommend, the direction of the piston design being highly sensitive to the process by which the impact on NVH is assessed. Assessment of liner accelerations have frequently been used to quantify the effects of piston design features on NVH. However, this is not necessarily a good indicator of the perceivable NVH within the vehicle cabin, due to the variation in transfer path sensitivity and potentially dominating influence of other vibration sources across the speed and frequency range. This paper demonstrates an analytical process for more realistically assessing the perceivable NVH due to the piston secondary dynamics whilst maintaining sensible analysis run-times. This process is applied in order to direct design improvements of a 2.5L gasoline engine piston to target reduced friction and NVH.