Transient effects in engines operating at steady speeds & loads

Abstract

For an engine operating at a steady speed & load, the many individual lubricatec contacts in the various engine components can behave in a highly transient manner. These effects can manifest themselves in various ways. For a new engine, the piston rings “run-in” in the first few hundred hours, changing their shape causing a reduction in ring friction, an increase in minimum oil film thickness at dead centre positions, and a change in the proportion of hydrodynamic/boundary friction. Piston secondary motion causes the piston to move rapidly from one side to the other close to top dead centre. This can affect piston ring motion in the grooves, which can in turn alter the gas flows in the piston ring pack. In addition, the piston rings can rotate. These effects can lead to changes in oil consumption, for the same engine speed and load, measured on a scale of minutes. Transient effects also occur in the bearings and the valve train, and of course, the lubricant composition is continually changing with time, due to lubricant oxidation, and examples from each of these areas are discussed.