The influence of contaminants and their interactions of diesel engine oil tribology using electrostatic condition monitoring

Abstract

In modern automotive engines, especially in diesel engines, consumer demand for ever increasing service intervals for vehicles has led to longer oil drain periods. Consequently this has increased contamination levels in lubricating oils that will in turn reduce engine efficiency and increase the possibility of system failure due to increases in viscosity and the potential of oil starvation leading to scuffing and catastrophic failure of the engine. Therefore it is necessary to investigate the effects of diesel engine oil contaminants on the tribological performance of tribocontacts and also the possible interaction between the contaminants. The thesis aims to investigate the influence of contaminants and their interactions on diesel engine oil using Electro sensing (ES) monitoring. ES monitoring suggest sensitivity to tribocharging, triboemission, surface charging, and wear debris generated in the tribocontact. However, ES monitoring is still in its initial stages of development but the relationships between the ES charge signal and corresponding wear mechanisms are established. Initial experiments showed the sensitivities between charge and wear to changes in additive chemistry, as well as contamination (specifically a soot stimulant). The results indicated some interesting findings which were difficult to elucidate but paved the way for a parametric study. The parametric matrix examined the effect of contaminants and it was observed that wear rate was mainly influenced by acid and soot additions. The best correlation of steady state charge with the other measured tribological parameters of wear rate, friction and temperature is seen for the series of oxidised oils. The multi-contaminated oil (L4×4) shows remarkably little degradation in tribological performance with a reduced friction level compared with the uncontaminated oil and only a modest increase in wear rate, slight increase in charge levels. This clearly showed that there were interactions between the contaminants. The factorial matrix studied interactions between the diesel engine oil contaminants on various tribo-couple materials, including silicon nitride, using ES monitoring. For charge, pin material – sulphuric acid and oxidation –sulphuric acid, interactions were statistically identified as having significant influence on performance. For friction, both pin and disc materials were identified as main influences along with pin material – sulphuric acid interactions. For wear, the presence of soot and the influence of pin material were identified statistically as the main effects. To conclude, the ES monitoring was sensitive to the presence and levels of contaminants in diesel lubricating oil, particularly diesel soot. The change in charge levels indicated the concentration of soot level present in the contact, which was directly related to wear. ES monitoring also detected interactions between the contaminants through statistical analysis. ES monitoring has shown that monitoring lubricant performance and the effects of contamination are feasible under laboratory conditions.

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ORCID for Robert Wood: orcid.org/0000-0003-0681-9239

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