Lubricant transport by misting and the link to degradation in the piston assembly of fired engines

The presence of lubricant mist in the piston assembly of an automotive engine has been known of for many years, but has not been specifically investigated. This study, therefore, focuses on understanding the lubricant misting process and its effect on the engine in terms of lubricant degradation. Informed by existing literature, it was hypothesised that mist is formed in the piston ring gaps. A laboratory rig was
designed and built to simulate this environment, producing and measuring representative mist flows. Using this rig, the contribution of the components of a commercial lubricant to its misting properties was studied. Base oil viscosity and molecular weight, and the presence of polymeric viscosity modifiers were identified as being the key parameters controlling the tendency of an oil to form mist. Further investigation using viscosity modifiers of varying size and molecular structure was performed, indicating differences between the behaviour of linear and star polymers.
Measurements of the droplet flows in the crankcase of an engine were made using a particle sizer. These confirmed the dependency of misting properties on viscosity, and
validated the hypothesis that lubricant mist in the crankcase is formed in the piston assembly by a similar mechanism to that produced in the laboratory rig. Longer engine tests were used to investigate the effect of lubricants with varying misting tendency on lubricant degradation. A range of chemical and rheological analyses were performed to compare fresh and used lubricant samples in different flows and positions in the engine. These tests were not able to differentiate between the degradation of the different lubricants tested, though the degradation of lubricant in different positions in the engine varied significantly. It was shown that lubricant mist is formed in the piston assembly and that lubricant misting is a significant lubricant transport mechanism.