Abstract
In biodiesel-fueled compression-ignition (CI) engines, dilution by unburned biodiesel has been found to have adverse effects on the boundary lubrication properties of additives in fully formulated engine lubricants. Such dilution of engine lubricants could be even more pronounced for CI engines running on higher blend concentrations of biodiesel. Given the nanoscopic nature of the interaction, this study seeks to determine the nano-tribological properties of an engine lubricant additive (e.g., zinc dialkyldithiophosphate (ZDDP)) when diluted with a fatty acid methyl ester (e.g., methyl oleate). Using lateral force microscopy (LFM) together with a fluid imaging technique, the lowest nanoscopic friction forces and coefficient of friction values (0.068–0.085) were measured for ZDDP when diluted with 70 vol% of methyl oleate. These values are also observed to be lower than those measured for neat ZDDP and neat methyl oleate, respectively, under similar conditions. Subsequently, interpreting the data with the Eyring thermal activation energy approach, it could then be elucidated that the lower frictional losses observed for the contact lubricated with this volumetric mixture are a result of the lower potential energy barrier and activation energy required to initiate sliding. These energy values are approximated to be 2.6% and 28.9% (respectively) lower than that of the contact lubricated with neat ZDDP. It was also found that the mixture, at this volumetric concentration, possesses the highest possible pressure activation energy (load-carrying capacity) along with the lowest possible shear activation energy (shearing), potentially indicating optimum tribological conditions for boundary lubrication. Thus, the findings of this study suggest that an optimum concentration threshold exists in which a synergistic nano-tribological interaction between additives and fatty acid methyl esters can be attained, potentially reducing boundary frictional losses of lubricated conjunctions. Such findings could prove to be essential in effectively formulating synergistic additive concentrations for engine lubricants used in biodiesel-fueled CI engines.
Highlights
The International Energy Agency (IEA) outlined that the transportation sector consumes up to 65.1% of the total consumption of oil and oil products globally [1]
Aside from Langmuir–Blodgett films studied by Briscoe and Evans [42], it should be noted that interpretation of lateral force microscopy (LFM)-measured nano-frictional properties of fluids using this thermal activation energy assumption has been reported in literature for methyl esters
The present study sought to determine the nano-tribological properties of zinc dialkyldithiophosphate (ZDDP) when diluted with methyl oleate at different volumetric proportions, through the use of an LFM coupled with a fluid imaging technique, which simulates a surface asperity-pair contact along a boundarylubricated rough surface conjunction
Summary
The International Energy Agency (IEA) outlined that the transportation sector consumes up to 65.1% of the total consumption of oil and oil products globally [1]. Most studies on biodiesel diluting of engine lubricants reported in the literature mainly involve tribological characterization approaches at the micro-to-macro length scale This lacks the capacity to fundamentally elucidate the underlying mechanisms affecting the formation of boundary films by engine lubricants when diluted with biodiesel, which is nanoscopic in nature. The present study extends the work of Hamdan et al [39], where the impact of palm oil-derived biodiesel diluting various types of fully formulated SAE grade engine lubricants was investigated using a pin-on-disk tribometer In their study, they found that an acceptable tribological threshold of biodiesel dilution exists for different types of engine lubricants, in which any dilution levels beyond this threshold are observed to increase. Such fundamental knowledge could prove to be significant in theoretically aiding the additive package selection for lubricants designed for use in biodiesel-fueled engines, rather than resorting to the trial-and-error approach commonly adopted in the lubricant industry
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