Friction Reduction via Piston and Ring Design for an Advanced Natural-Gas Reciprocating Engine

Friction from the power cylinder represents a significant contribution to the total mechanical losses in internal combustion engines. A reduction in piston ring friction would therefore result in higher efficiency, lower fuel consumption, and reduced emissions. In this study, models incorporating piston ring dynamics and piston secondary motion with elastic skirt deformation were applied to a Waukesha natural gas power generation engine to identify the main contributors to friction within the piston and ring pack system. Based on model predictions, specific areas for friction reduction were targeted and low-friction design strategies were devised. The most significant contributors to friction were identified as the top ring, the oil control ring, and the piston skirt. Model predictions indicated that the top ring friction could be reduced by implementing a skewed barrel profile design or an upward piston groove tilt design, and oil control ring friction could be reduced by decreasing ring tension. Piston design parameters such as skirt profile, piston-to-liner clearance, and piston surface characteristics were found to have significant potential for the reduction of piston skirt friction. Designs were also developed to mitigate any adverse effects that were predicted to occur as a result of implementation of the low-friction design strategies. Specifically, an increase in wear was predicted to occur with the upward piston groove tilt design, which was eliminated by the introduction of a positive static twist on the top ring. The increase in oil consumption resulting form the reduction in the oil control ring tension was mitigated by the introduction of a negative static twist on the second ring. Overall, the low-friction design strategies were predicted to have potential to reduce piston ring friction by 35% and piston friction by up to 50%. This would translate to an improvement in brake thermal efficiency of up to 2%, which would result in a significant improvement in fuel economy and a substantial reduction in emissions over the life of the engine.