An alternative diesel fuel that is steadily gaining attention and significance is biodiesel, which
is defined as the monoalkyl esters of vegetable oils and animal fats. Previous literature states
that low blend levels of biodiesel can restore lubricity to (ultra-)low-sulfur petroleum-derived
diesel (petrodiesel) fuels, which have poor lubricity. This feature has been discussed as a major
technical advantage of biodiesel. In this work, the lubricity of numerous fatty compounds was
studied and compared to that of hydrocarbon compounds found in petrodiesel. The effects of
blending compounds found in biodiesel on petrodiesel lubricity were also studied. Lubricity was
determined using the high-frequency reciprocating rig (HFRR) test. Dibenzothiophene, which is
contained in nondesulfurized petrodiesel, does not enhance petrodiesel lubricity. Fatty compounds
possess better lubricity than hydrocarbons, because of their polarity-imparting O atoms. Neat
free fatty acids, monoacylglycerols, and glycerol possess better lubricity than neat esters, because
of their free OH groups. Lubricity improves somewhat with the chain length and the presence of
double bonds. An order of oxygenated moieties enhancing lubricity (COOH > CHO > OH >
COOCH3 > CO > C−O−C) was obtained from studying various oxygenated C10 compounds.
Results on neat C3 compounds with OH, NH2, and SH groups show that oxygen enhances lubricity
more than nitrogen and sulfur. Adding commercial biodiesel improves lubricity of low-sulfur
petrodiesel more than neat fatty esters, indicating that other biodiesel components cause lubricity
enhancement at low biodiesel blend levels. Adding glycerol to a neat ester and then adding this
mixture at low blend levels to low-lubricity petrodiesel did not improve petrodiesel lubricity.
However, adding polar compounds such as free fatty acids or monoacylglycerols improves the
lubricity of low-level blends of esters in low-lubricity petrodiesel. Thus, some species (free fatty
acids, monoacylglycerols) considered contaminants resulting from biodiesel production are
responsible for the lubricity of low-level blends of biodiesel in (ultra-)low-sulfur petrodiesel.
Commercial biodiesel is required at a level of 1%−2% in low-lubricity petrodiesel, which exceeds
the typical additive level, to attain the lubricity-imparting additive level of biodiesel contaminants
in petrodiesel.
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