Technical Papers

　

Studies on the Mixed Lubrication of Piston Rings^*

By Toshiro Hamatake^**, Yutaro Wakuri^***, Mitsuhiro Soejima^**** and Tatsumi Kitahara^***

　

A great deal of attention has been focused on decreasing the piston assembly friction in piston engines, since this friction accounts for the majority of total mechanical power loss in these type of engines. The tribology of the sliding surfaces between piston rings and cylinder liner includes extremely complicated phenomena which will become even more severe with an increase in the engine power. In the present study, a theoretical analysis on the mixed lubrication of piston rings is made. The analysis is comprised of Patir and Cheng's average flow model and Greenwood and Tripp's asperity interaction model, and is developed to consider the shear thinning effect of, non-Newtonian fluid, multigrade oil. The friction characteristics of piston rings for both monograde and multigrade oils are investigated and compared with experimental results obtained by the floating liner method, which can measure the instantaneous friction force of a piston assembly under firing conditions.

　

1. Introduction

　

Recently, the engine engineers have been strongly required to improve the thermal efficiency and control the exhaust emissions in reciprocating engines, since measures have to be taken for wide area and global environment problems and exhaustion of petroleum resources. One of the ways to improve the thermal efficiency of the engine is to decrease the mechanical power loss in the engine. It is important to decrease the piston assembly friction, since this friction accounts for about 40% of total power loss^l). The current trend is that viscous friction losses of sliding parts in engine components may be reduced by using the multigrade lubricant with low viscosity. At both ends of a stroke, the piston ring may operate in a regime of mixed lubrication and/or boundary lubrication, since the sliding speed of the piston ring is low. The use of low viscous lubricant results in the increase in the mixed and boundary frictions, and enhances the wear and the occurrence of scuffing.

The lubrication phenomena of the sliding surfaces between piston rings and the cylinder liner is complicated by many factors such as sliding speed, film temperature, oil starvation and surface roughness. It is assumed in the hydrodynamic lubrication theory that the surface roughness amplitude is small compared to the film thickness and the lubricant flow between two surfaces is almost parallel. The Reynolds equation is applicable to the analysis of hydrodynamic lubrication. Patir and Cheng proposed an average flow model based on the stochastic process theory for the thin film lubrication that cannot neglect the surface roughness^{2) 3)}, and they introduced the average Reynolds equation. Rohde⁴⁾ made the mixed lubrication analysis of piston rings by using the average flow model and the asperity contact model via Greenwood and Tripp⁵⁾. Rohde investigated in detail the effects of the surface topography on the film thickness and friction of the piston ring. Since then, a number of mixed lubrication analyses accounting for the oil starvation inlet condition have been reported^6)-11) although Rohde did not consider the oil starvation.

The property of lubricant viscosity at high temperatures and high shear rates has been given attention in connection with the reliability on the anti-wear characteristics of engine components. Taylor et al.^{12) 13)} evaluated the change of lubricant viscosity with the temperature and the shear rate by means of the Gross equation¹⁴⁾, and examined the influences of lubricant rheology on the friction of a piston ring pack. Unfortunately the surface roughnesses were not taken into consideration in analyses. Tian¹⁵⁾ and Arcoumains¹⁶⁾ studied the mixed lubrication characteristics of both Newtonian and non-Newtonian fluids by using the average Reynolds equation.

The authors also performed theoretical analyses on the mixed lubrication of piston rings by using the average flow model, and studied the effects of surface roughness on the mixed lubrication characteristics of piston rings^{1) 10) 11)}.

　

* Translated from Journal of MESJ Vol.34. No.12 (Manuscript received Apr.30, 1999)

** Oita University (Oita City)

*** Kyushu University (Fukuoka City)

**** Kyushu Sangyo University (Fukuoka City)

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