An Improved Friction Model for Spark-Ignition Engines

A spark-ignition engine friction model developed by Patton et al. in the late 1980s was evaluated against current engine friction data, and improved. The model, which was based on a combination of fundamental scaling laws and empirical results, includes predictions of rubbing losses from the crankshaft, reciprocating, and valvetrain components, auxiliary losses from engine accessories, and pumping losses from the intake and exhaust systems. These predictions were based on engine friction data collected between 1980 and 1988. Some of the terms are derived from lubrication theory. Other terms were derived empirically from measurements of individual friction components from engine teardown experiments. Recent engine developments (e.g., improved oils, surface finish on piston liners, valve train mechanisms) suggested that the model needed updating. So modifications were made to the piston ring tension and gas pressure loading contributions to piston assembly friction, the impact of liner roughness, and to the valvetrain mechanism friction. Lubricant viscosity scaling was added to the hydrodynamic terms in the rubbing friction component models. The improved model now gives reasonable estimates of individual friction component and total spark-ignition engine friction mean effective pressure. The inclusion of oil viscosity scaling with temperature results in cold engine friction predictions about twice the value for warmed up engines. This agrees with the limited cold engine friction data available in the literature.