A perspective on the origin of lubricity in petroleum distillate motor fuels

• Lubricity depends on fluid composition, bearing alloys, and operating conditions.
• Surface-active polycyclic aromatic compounds are naturally present in petroleum.
• Hydrotreatment removes aromatics, oxygen- and nitrogen- containing compounds.
• Removal of these compounds increase friction and wear in fuel lubricated contacts.
• PAH and NPAH concentrations in distillate fuels are a function of boiling ranges.

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 less abundant in motor gasoline and more abundant in diesel fuel due to differences in the boiling ranges 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 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 can improve fuel lubricity in severely hydrotreated petroleum distillate motor fuels. Boundary lubrication by linear alkyl polar compounds is distinct from lubrication by native heavy polar aromatic compounds found in petroleum. 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 the lack of a single SI unit like viscosity that describes the sum of interactions between the fluid, material, and mechanical forces at a sliding contact.