The interaction between the piston and the liner in a reciprocating engine is of much interest because it affects reliability, noise, and efficiency. This study evaluated various changes to the piston skirt with the specific goal of minimizing friction. An analytical model of the piston, previously developed at MIT, was used to perform parametric studies of various designs in order to predict the effect of each on engine efficiency. The model incorporated hydrodynamic, boundary, and mixed lubrication modes, and it allowed for either fully-flooded or partially-flooded skirts. It also considered the effects of skirt deformation in response to applied loads. A dominant factor influencing net friction between the skirt and liner was the distribution between hydrodynamic lubrication (support by the oil film) and boundary lubrication (direct metal-to-metal contact). Design changes that shifted support from the high-friction boundary lubrication regime toward the hydrodynamic regime generally reduced net friction. For example, the model predicted that if a piston is originally supported largely by boundary contact, increasing the viscosity of the oil can reduce friction by enabling the oil film to sustain a greater load. If, however, the load is already supported primarily hydrodynamically, decreasing the viscosity reduces hydrodynamic drag and may reduce net friction.
展开▼
