The Role of Piston Distortion on Lubrication in a Reciprocating Engine

The microgeometry of the piston, rings, and skirt relative to the liner strongly influences lubrication in a reciprocating engine. This study develops an approximation technique that decouples the thermomechanical piston-skirt distortions from the complex lubricant support in a large diesel engine. The model considers the limiting case of starved skirt lubrication with large clearance. It permits efficient design of machined three-dimensional piston-skirt contours for piston support. In the calculations, a three-dimensional finite-element model is coupled with a postprocessing algorithm to predict skirt distortions, piston tilt, operating clearance, and oil-film contact area as a function of machined profile, thermal expansion, cylinder pressure, piston inertia, and transient side loads. A piston dynamics model is developed that defines the transient piston side force based on engine geometry, cylinder pressure, inertial loads, and wrist-pin offset. The results of this study indicate that (1) the transient skirt distortions due to cylinder pressure on the compression and power strokes result in a significant increase in oil-film contact area; (2) the piston skirt operating shape depends on the location and area of oil-film contact; (3) the contact area and location during intake and exhaust strokes vary substantially from that during the compression and power strokes; (4) the wrist-pin offset reduces the maximum side load and piston slap intensity occurring in the region of maximum cylinder pressure; (5) effective three-dimensional skirt profile design may result in significant changes in oil-film contact area and location on the skirt throughout the cycle.