Study of Break-In Process and its Effects on Piston Skirt Lubrication in Internal Combustion Engines

Zhen Meng,Tian Tian,Linfeng Zhang

doi:10.3390/lubricants7110098

Zhen Meng, Tian Tian + Show 1 more

Open Access

https://doi.org/10.3390/lubricants7110098

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Abstract

The piston skirt is one of the main contributors to the total mechanical loss in internal combustion engines. Usually, the skirt friction experiences a rapid change during the break-in period largely due to the wear of the machine marks or roughness against soft coatings. It is thus important to consider the effect of the change of the roughness for a realistic prediction of the piston skirt friction and system optimization. In this work, an existing model of piston skirt lubrication was improved with the consideration of a breaking in process for the most commonly used triangle machine marks. A new set of flow factors in the averaged Reynolds equation were analytically derived for the trapezoid shape formed after wear of the original triangle shape. A new asperity contact model was developed for the trapezoid shape. The calculation results reflect the trend of friction mean effective pressure (FMEP) during break-in in an engine test and showed quantitative agreement under the same amount of wear.

Highlights

The piston skirt can be a major contributor to the total engine friction loss in internal combustion engines
The skirt friction can be maintained to a negligible level if solid–solid contact does not hydrodynamic pressure
The skirt friction can be maintained to a negligible level if solid–solid contact hydrodynamic pressure

Summary

Introduction

The piston skirt can be a major contributor to the total engine friction loss in internal combustion engines. Piston skirt friction is a consequence of complex interactions at different scales. At the largest scale is the piston secondary motion and the deformation of the piston skirt and the cylinder liner due to the impact of the piston skirt. The profile of the piston skirt and bore distortion determines the exact nominal clearance distribution for the lubrication of the skirt and the liner. At the lowest level is the roughness or waviness of designed patterns on the skirt whose peak-valley height is in the order of sub-microns to 10 microns, which will be referred to as the Level III feature hereafter. A realistic prediction of skirt friction needs an adequate understanding of and models for these interactions

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