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.

在以生物柴油为燃料的压缩点火（CI）发动机中，未燃烧生物柴油的稀释已发现对完全配制的发动机润滑剂中添加剂的边界润滑性能有不利影响。对于在较高混合生物柴油浓度下运行的CI发动机，发动机润滑剂的这种稀释甚至会更加明显。考虑到相互作用的纳米性质，本研究旨在确定用脂肪酸甲酯（例如油酸甲酯）稀释后的发动机润滑油添加剂（例如二烷基二硫代磷酸锌（ZDDP））的纳米摩擦学性质。使用侧向力显微镜（LFM）结合流体成像技术，当用70％（体积）的油酸甲酯稀释时，ZDDP的最低纳米摩擦力和摩擦系数值（0.068-0.085）被测量。还观察到这些值低于在相似条件下分别对纯ZDDP和纯油酸甲酯测得的值。随后，用Eyring热活化能方法解释数据，然后可以阐明，用这种体积混合物润滑的接触所观察到的较低的摩擦损失是较低的势能垒和引发滑动所需的活化能的结果。这些能量值分别比纯ZDDP润滑的触点的能量值分别低2.6％和28.9％。还发现，在此体积浓度下，混合物具有尽可能高的压力激活能（承载能力）以及最低可能的剪切激活能（剪切），可能表明边界润滑的最佳摩擦学条件。因此，这项研究的结果表明存在一个最佳的浓度阈值，在该阈值中可以实现添加剂与脂肪酸甲酯之间的协同纳米摩擦学相互作用，从而有可能减少润滑接头的边界摩擦损失。这些发现可能证明对于有效配制生物柴油燃料CI发动机所用发动机润滑剂的协同添加剂浓度至关重要。