Piston-Pin Rotation and Lubrication

Hannes Allmaier,David E Sander

doi:10.3390/lubricants8030030

Abstract

The rotational dynamics and lubrication of the piston pin of a Gasoline engine are investigated in this work. The clearance plays an essential role for the lubrication and dynamics of the piston pin. To obtain a realistic clearance, as a first step, a thermoelastic simulation is conducted for the aluminum piston for the full-load firing operation by considering the heat flow from combustion into the piston top and suitable thermal boundary conditions for the piston rings, piston skirt, and piston void. The result from this thermoelastic simulation is a noncircular and strongly enlarged clearance. In the second step, the calculated temperature field of the piston and the piston-pin clearance are used in the simulation of the piston-pin journal bearings. For this journal bearing simulation, a highly advanced and extensively validated method is used that also realistically describes mixed lubrication. By using this approach, the piston-pin rotation and lubrication are investigated for several different operating conditions from part load to full load for different engine speeds. It is found that the piston pin rotates mostly at very slow rotational speeds and even changes its rotational direction between different operating conditions. Several influencing effects on this dynamic behaviour (e.g., clearance and pin surface roughness) are investigated to see how the lubrication of this crucial part can be improved.

Highlights

From all journal bearings that are employed in an internal combustion engine, the piston pin has to endure the most extreme working conditions, as it suffers from insufficient, random lubrication, and has to withstand extreme forces
The range of average rotation speed and oscillation speed is diverse, the piston pin changes its direction of rotation which indicates that there exists numerous operating conditions for which the piston pin has no average rotation speed at all
By analyzing the asperity friction power loss to quantify mixed lubrication, it was found that the full-load operating points show the largest amount of mixed lubrication

Summary

Introduction

From all journal bearings that are employed in an internal combustion engine, the piston pin has to endure the most extreme working conditions, as it suffers from insufficient, random lubrication, and has to withstand extreme forces. In the development of new, highly efficient engine generations with high power to volume displacement ratios, the occurrence of failures of the piston pin increases In these cases, there generally exists little understanding as to what the causes of these failures are and how they can be prevented; a costly trial-and-error approach is taken to fix these issues. There exist different forms of friction that are usually represented as the Stribeck curve: purely hydrodynamic lubrication, boundary friction, and the mixed lubrication regime, where both of these different forms of friction coexist Lubricated contacts—as they are present in internal combustion engines—might be assumed to operate in purely hydrodynamic lubrication For this form of lubrication, a sufficiently thick oil film separates the two gliding surfaces from each other and the friction in this oil film generates the observed losses. The reality is more complex as already the transient nature of operation for internal combustion engines prevents pure hydrodynamic lubrication (e.g., during starting)

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Conclusion