Tribological Behavior of Low Viscosity Lubricants in the Piston to Bore Zone of a Modern Spark Ignition Engine 2014-01-2859

Most major regional automotive markets have stringent legislative targets for vehicle greenhouse gas emissions or fuel economy enforced by fiscal penalties. Large improvements in vehicle efficiency on mandated test cycles have already taken place in some markets through the widespread adoption of technologies such as downsizing or dieselization. There is now increased focus on approaches which give smaller but significant incremental efficiency benefits such as reducing parasitic losses due to engine friction. Fuel economy improvements which achieve this through the development of advanced engine lubricants are very attractive to vehicle manufacturers due to their favorable cost-benefit ratio.

For an engine with components which operate predominantly in the hydrodynamic lubrication regime, the most significant lubricant parameter which can be changed to improve the tribological performance of the system is the lubricant viscosity. Low viscosity lubricants are increasingly being specified by vehicle manufacturers who are now more frequently working directly with the lubricant supplier to design fluids specific to their requirements. As lubricant viscosity grades far below those currently in the market are investigated it is vital that the detailed operating environment of the oil within specific engine components is properly understood. Losses in the piston / cylinder bore zone account for the largest portion of the overall engine friction. This paper investigates the tribological contact in this zone using advanced analytical techniques to predict the performance of two low viscosity lubricants. The relative contributions of hydrodynamic and boundary friction are resolved and the accuracy of the simulation is compared against motored engine test results.