Powertrain Friction Reduction by Synergistic Optimization of the Cylinder Bore Surface and Lubricant Part 1: Basic Modelling

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. In these cases, a deterministic-based model was used to evaluate the role of surface roughness on the hydrodynamic pressure build-up and its impact on the lubrication performance of OCRs. Samples’ characteristics and results of piston ring-cylinder bore tribological tests were used as input data for the mathematical models and support the discussion of the simulated results. This paper also provides an overview and a conceptual examination of mixed lubrication models commonly used to simulate piston ring conjunctions. A companion paper, namely “Powertrain Friction Reduction by Synergistic Optimization of Cylinder Bore Surface and Lubricant - Part 2: Engine Tribology Simulations and Tests”, complements this work with additional simulation results and empirical data that advances the understanding of the interplay between lubricant viscosity and surface topography and its contribution on the performance of reciprocating engines.