Lubrication properties of fatty acid methyl esters as low-sulfur diesel enhancers

Increasingly rigorous restriction on the sulfur content in diesel leads to a decrease in fuel lubricity. This reduced lubricity can cause damage to the fuel injection system of an engine. Biodiesel, which is derived from animal fats or vegetable oils by transesterification under alkali or acid catalysts, has been prevailed as an alternative fuel. It has been observed that the fatty acid esters in biodiesel play an active role in enhancing lubricity. Therefore, now biodiesel can serve as an additive to low-sulfur diesel fuel. To correlate the carbon chain length, unsaturation and hydroxylation of the fatty acid methyl ester to its lubricity, methyl myristic (C14:0), methyl palmitic (C16:0), methyl stearate (C18:0), methyl oleate (C18:1), methyl linoleate (C18:2), methyl linolenate (C18:3), methyl ricinoleate (C18:1 OH), castor oil methyl ester and cooking waste oil methyl ester were added to low-sulfur diesel fuel by 0.5%, 1.0%, 1.5% and 3.0%, respectively. Two types of biodiesel from castor oil and cooking waste oil, representing the mixtures of multiple fatty acid methyl esters, were also added to diesel fuel for wear test. The specified components of the two mixture solutions were determined by GC-MS. The low-sulfur diesel used in the test was manufactured by a hydrogenation process. The lubricity of various samples was studied using a high frequency reciprocating rig (HFRR) analysis method. Each wear test was repeated three times to minimize the error. As for methyl myristic (C14:0), methyl palmitic (C16:0) and methyl stearate (C18:0), we found that the fatty acid methyl ester with longer carbon chain had a better lubricity. For the C18 series, such as methyl stearate (C18:0), methyl oleate (C18:1), methyl linoleate (C18:2) and methyl linolenate (C18:3), an enhancement in lubricity was observed with the increase in the unsaturation degree of esters. Although with the same carbon chain length and unsaturation, methyl ricinoleate (C18:1 OH), as a hydroxylated ester, had better lubricity than methyl oleate (C18:1) compared with other treatments. Meanwhile, the individual fatty acid methyl ester did not show remarkable lubricating performance as biodiesel which was composed of several esters as a mixture. Biodiesel can perform better with more hydroxylated esters. From the wear test results, the lubricity of low-sulfur diesel can meet the requirement of the national standard with the addition ratio of saturated fatty acid methyl esters, unsaturated fatty acid methyl esters and biodiesel reaching 3.0%, 1.5% and 1.0%, respectively. In all, we concluded that there was a high correlation between lubricating properties with the unsaturation degree and hydroxyl groups in ester molecules. Studying the effects of molecule structure of fatty acid esters on the lubricating properties and its operation mechanism will be greatly beneficial for choosing the suitable biodiesel components as the lubricity enhancers in low-sulfur diesel. © 2016, Chinese Society of Agricultural Engineering. All right reserved.