Transient lubrication of piston compression ring during cold start-up of SI engine

Cold start-up conditions differ from the normal operating conditions for an engine, and this discrepancy adversely affects the tribological performance of interfaces. The majority of health-hazardous engine exhaust emissions occur during the cold start-up of an engine; consequently this condition generates a major environmental concern. This study numerically investigates the transient hydrodynamic lubrication of the first compression ring during initial start-up of a cold engine by combining lubrication theory with the realistic oil rheology of multigrade oils. Specifically, in this study transient speed, variation in cylinder pressure, and high lubricant viscosity during start-up are considered to determine developing pattern of the lubricating film and of other tribological performance parameters. The cycle-by-cycle developments of lubricating film, friction force, power loss, and oil transport are analyzed as well. The start-ups of a cold and a warm engine are also compared in terms of frictional losses and oil transport. Different monograde and multigrade engine oils are used to evaluate oil-dependent performance. Results show that in all cases, performance during cold start-up is of greater significance than that of a warm engine. Low lubricant temperature and stated start-up conditions substantially increase amount of energy loss and net oil transport. Temperature-dependent variations in performance are also more significant in high viscosity-grade oils.