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The piston assembly is the major source of tribological inefficiencies among the engine components and is responsible for about 50% of the total engine friction losses, making such a system the main target element for developing low-friction technologies. Being a reciprocating system, the piston assembly can operate in boundary, mixed and hydrodynamic lubrication regimes. Computer simulations were used to investigate the synergistic effect between low viscosity oils and cylinder bore finishes on friction reduction of passenger car internal combustion engines. First, the Reynolds equation and the Greenwood & Tripp model were used to investigating the hydrodynamic and asperity contact pressures in the top piston ring. The classical Reynolds works well for barrel-shaped profiles and relatively thick oil film thickness but has limitations for predicting the lubrication behavior of flat parallel surfaces, such as those of Oil Control Ring (OCR) outer lands.

Five automotive oils, with different viscosity grades, were tested under different loads and speeds in a reciprocating test using piston rings and cylinder liners. Starved and fully-flooded conditions were also considered in order to analyze the influence of lubricant supplier in the lubrication regimes, especially in boundary-mixed transition. The expected Stribeck curve behavior was observed, and more interesting visualization appeared when the viscosity value was extracted from the Stribeck abscissa axis. The higher viscosity oils showed lower friction coefficient at low speed/load ratios. Such behavior is usually neglected and may be significant to understand the triblogical behaviour of engineering components. Computer simulation showed similar results, including the “cross-over” speed/load when the lower viscosity oils start to show lower friction.