A perspective on the origin of lubricity in petroleum distillate motor fuels
Peter Y. Hsieh, Thomas J. Bruno
Lubricity, or a substance's effect on friction and wear between two surfaces in relative motion, is affected by both chemical and physical mechanisms present at a sliding contact. The inherent lubricity of distillate motor fuels stems from surface-active compounds found in petroleum, principally heavy aromatic compounds such as polycyclic aromatic hydrocarbons (PAH) and nitrogen heterocyclic polyaromatic hydrocarbons (NPAH) containing three or more fused rings. These compounds are least abundant in motor gasoline and most abundant in diesel fuel due to differences in the final boiling temperatures of these distillate fuels. PAH and NPAH compounds can form chemical bonds with metal surfaces, and reduce the friction of metal surfaces in sliding contact. Reducing the coefficient of friction also lowers the peak stress amplitude at the sliding contact, thereby mitigating the effects of plasticity-induced wear mechanisms and delaying the transition to abrasive wear. Hydrotreatment of distillate motor fuels to remove sulfur also hydrogenates heavy aromatic compounds, leading to a reduction in fuel lubricity and increased wear of fuel injectors and pumps. The addition of linear alkyl polar compounds, such as fatty acids and their derivatives, can improve fuel lubricity in severely hydrotreated petroleum distillate motor fuels. The lubricity of a distillate motor fuel can be affected independently by the presence of both heavy polar aromatic compounds and linear alkyl polar compounds. Mechanical testing is typically employed to measure fuel lubricity due to the complex interactions between the surface-active compounds and wear mechanisms at work in a sliding contact.
and Bruno, T.
A perspective on the origin of lubricity in petroleum distillate motor fuels, Fuel Processing Technology, [online], https://doi.org/10.1016/j.fuproc.2014.08.012, https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=916044
(Accessed December 8, 2023)