Lubricity improvement in bio-hydrogenated diesel by organic friction modifiers

Bio-hydrogenated diesel (BHD) is an alternative fuel that can help address the problems of dwindling fossil fuel supplies and pollution. BHD has a chemical composition similar to conventional diesel, with excellent clean combustion properties and reduced greenhouse gas emissions. However, BHD exhibits poor lubricity, which must be improved to prevent engine wear. This study investigates the use of organic friction modifiers (OFMs) as additives to address the poor lubricating properties of BHD. OFMs have a molecular structure featuring a long hydrocarbon chain and a polar end group, which allows them to form a lubricating film on the contacting surfaces within the engine. The effect of the functional group and degree of unsaturation of the hydrocarbon chain on the lubricity of BHD was investigated using a high-frequency reciprocating rig under the ISO 12156-1:2018 standard test. This study found that stearic acid, with its carboxyl functional group, exhibited the most significant enhancement of lubricity. When OFMs were added at concentrations below 1% by weight, stearic acid reduced the friction coefficient by 57% and the wear scar diameter by 70%. Interestingly, oleic acid, containing a double bond, exhibited lubricity enhancements comparable to stearic acid, while other OFMs with different functional group like stearamide, stearyl alcohol, and methyl stearate offered moderate lubricity improvements. Therefore, the functional groups of the OFMs played a more significant role in determining the lubrication properties than the degree of unsaturation. Following the lubricity tests, the worn surface of the disc specimens was analyzed using an OLS5100 3D laser measuring microscope. The microscopic examination of the worn surfaces for the blends with stearic acid and oleic acid revealed slender wear with thinner deposits around the worn surface. This suggests the formation of a robust lubricating film that effectively separates the contacting surfaces. Meanwhile, methyl stearate, stearamide, and stearyl alcohol showed acceptable improvements in lubricity. This study demonstrates the potential of OFMs, particularly stearic acid, as a solution to the limited lubricity of BHD, potentially reducing engine wear.