The compatibility of semi-synthetic engine oil with conventional diesel and biodiesel fuels

Recent trends to downsize diesel engines have increased the stress on lubricants. Oils naturally degrade during operation, undergoing continual reactions, changing chemically and physically, detracting in performance from initial specifications. This thesis investigates the role of fuel in the ageing of diesel engine oils, specifically Ultra Low Sulphur Diesel (ULSD) and Rapeseed Methyl Ester (RME – a common European biodiesel). Oil ageing is assessed distinctly with fuel dilution, the entrainment of exhaust gases; and the effects of soot loading. Results show fuel dilution has the greatest influence on oil performance. Effects are seen with an instant ‘dilution’ of properties, with the resultant blend performing as an amalgam of the fluids. This can be both positive and negative, depending on the property being measured, with the entrainment of biodiesel generally beneficial. The entrainment of exhaust gases in the oil leads to increased unburnt hydrocarbons and fuel content, with similar dilution effects. Soot loaded oil performance is heavily dependent on the respective fuel content. RME contamination has a positive influence which far outweighs its negligible soot production, whereas ULSD detracts from oil performance, also producing more soot. During an equivalent timeframe, the influence of RME is less detrimental than ULSD on overall performance