![]() However, rapid development in numerical methods over the last thirty years has resulted in sophisticated piston ring lubrication models that are finding application in the design process. The mathematical analysis of piston ring lubrication is complex and by necessity requires simplifying assumptions. So, even though manufacturers can produce rings that have an excellent life expectancy, these components may be far from optimum from a lubrication and friction standpoint.Īs a consequence of their importance to engine performance, the theoretical and experimental study of piston ring lubrication has received much attention in the literature, leading to major advances in our understanding of their behaviour. In terms of wear, there is insufficient understanding of the interaction with the lubrication process. The relative importance varies from engine to engine, but typically the piston assembly, comprising both the piston rings and the piston skirt, accounts for 40-50% of total engine friction. Unfortunately, the piston ring pack is one of the largest contributors to friction in the internal combustion engine over the normal range of engine speeds and loads encountered in service. All this must be achieved with minimum frictional power loss, most notably at the sliding interface with the cylinder wall, and the least possible wear in order to maximise component life. ![]() The basic roles of the piston ring pack, the collective name for the three or more rings mounted on the piston, are to form an effective gas seal between the combustion chamber and the crankcase, to limit the upward transport of oil from the crankcase to the combustion chamber and to provide a flow path for heat transfer from the piston to the engine coolant. ![]() Piston rings are critical components in the modem internal combustion engine, their optimum tribological performance having a controlling influence in minimising frictional power loss, fuel consumption, oil consumption, blow-by and harmful exhaust emissions. Priest, in Tribology Series, 2000 1 INTRODUCTION Thinning Films and Tribological Interfaces But, it is important to consider this distinction when assessing the potential use of piston rings for other duties where alternative seal types may be used, such as in hydraulic cylinders. However, as modern engines demonstrate, when correctly specified they perform a quite remarkable job in limiting oil consumption. They are a split component with a butt joint, and as such do not provide a high-integrity static seal as we may expect with something like an elastomer or a static metal compression joint. Piston rings are generally used in high-speed and very dynamic pressure situations. In a lubricated compressor, the piston rings provide a similar function to those in an internal-combustion engine, but dry compressors normally use filled plastic rings these have a somewhat different sealing action and are dealt with separately. ![]() Gas compressors can also be subdivided into two classes, lubricated and dry. The most common use is in internal-combustion engines, but they are also used in reciprocating gas compressors and in hydraulic cylinders. Robert Flitney, in Seals and Sealing Handbook (Sixth Edition), 2014 4.4.1 Introduction Details of this method has been reported in. Whereas, in the mixed lubrication mode, the friction forces were also calculated according to the dry friction law, but the friction coefficient is variable, depending on operating conditions. In the hydrodynamic regime, the viscous theory was applied to predict the friction forces. In brief, if the boundary lubrication occurs, the Coulomb (dry friction) law with a fixed friction coefficient was used to calculate the friction forces. In order to take into account the effect of the composite surface roughness of the piston-ring and liner, the friction forces were evaluated by a ‘Stribeck’ type approach. In this study, all these factors have been considered in calculating friction forces of the piston-rings. There is no doubt that the lubrication mode of the piston-ring will depend on the surface roughnesses of the ring face and liner. Further, a piston-ring can even undergo these different lubrication modes around its circumference simultaneously at a given crank angle position in situations where there is a circumferentially variable film gap between the ring and liner due to bore distortion and/or piston lateral motion. Piston-rings can experience three typical lubrication regimes on the engine stroke: namely boundary, mixed and hydrodynamic lubrication. Ming-Tang Ma, in Tribology Series, 1998 2.4 Calculation method of friction forces
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