Light-duty diesel vehicles that make use of a diesel particulate filter (DPF) generally require periodic active regenerations. This is achieved by late, in-cylinder post-injections designed to add unburned fuel into the exhaust system to raise the DPF temperature and burn off the soot. These late injections do not atomise and evaporate as readily as during normal combustion causing a portion of this fuel to impinge on the cylinder wall and wash down, with the engine oil, into the sump. This can result in degradation of the engine oil which could lead to increased engine wear and/or engine failure.
When investigating which fuel properties have an effect on oil dilution, most literature suggests high final boiling point fuels increase fuel addition rates to engine oil. Through the use of accelerated oil dilution testing on an engine test bench, the findings of this paper suggest that a fuel with low viscosity, density and surface tension, and high front-end volatility (which are linked to fuel spray droplet size and associated in-cylinder droplet evaporation rates) is better correlated with low fuel addition rates to engine oil.
Fuel subtraction is where fuel is evaporated out of the engine oil during elevated oil temperatures which is a different phenomenon to fuel addition. Fuel subtraction rates were shown to follow the derivative of the back-end of a fuel's distillation curve (percentage volume distilled with respect to temperature).
Blends and neat forms of diesel fuels derived from crude oil, gas-to-liquids (GTL), coal-to-liquids (CTL), rapeseed methyl ester (RME) and soya bean methyl ester (SME) were used. GTL and CTL diesel resulted in significantly reduced oil dilution when compared to the other fuels used in this study.
