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A theoretical analysis is presented of the mixed lubrication of a piston ring pack. The analysis comprises 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 a non‐Newtonian fluid, multigrade lubricant. The friction characteristics of the piston ring pack for both monograde and multigrade lubricants are investigated. It is found that a decrease in the lubricant viscosity is effective in reducing the friction loss, although this increases the boundary friction at the beginning of the expansion stroke. The friction characteristics are markedly affected by the shear thinning effect when multigrade lubricants are used.

The performance of a reciprocating engine can be improved by reducing the friction between piston rings and cylinder liner, which significantly contributes to the mechanical friction losses of an engine.The friction force of a piston ring pack is calculated, based on hydrodynamic lubrication theory, for the piston rings. Calculations were carried out for three sets of conditions. Oil starvation is taken into consideration in the calculation of oil-film behaviour for a ring pack. The friction characteristics of piston rings are evaluated using the frictional mean effective pressure.The friction force of a piston assembly is measured experimentally by a n improved floating liner method. The effects of lubricant viscosity and engine speed on friction characteristics are investigated by both calculation and experiment. Abstract Friction Characteristics of Piston Rings in a Reciprocating Enginefriction, piston ring, lubrication, hydrodynamic, floating liner, calculation, experiment KeywordsThe tribology of sliding surfaces between piston rings and cylinder liner is one of the most complex phenomena occurring in reciprocating engines, and the tribological problem is becoming more and more severe with increases in engine power. Engine performance can be improved by reducing the friction of the piston assembly, since it significantly contributes to the mechanical power loss of the engine. Many theoretical and experimental studies on the frictional power loss of piston rings have been carried out.The oil-film behaviour of piston rings and the viscous friction of a piston-ring pack can be obtained by solving the Reynolds equation that governs the oil-film pressure. It is necessary to impose certain boundary conditions for the oil-film pressure. Many kinds of boundary condition have been pro-I NTRO DU CTlO N

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