Trace heteroatom speciation governs boundary lubricity in paraffinic aviation fuels: Implications for synthetic fuel design

Highly upgraded synthetic paraffinic fuels can exhibit marginal boundary lubricity because severe hydrotreating removes naturally occurring polar species that form protective surface films. Understanding which trace chemical classes drive this lubricity loss is essential for specification development for current blended and future 100 % synthetic fuels. In this work, a HEFA-SPK matrix was systematically doped with representative sulfur-, oxygen-, and nitrogen-containing model compounds, and lubricity was measured by Ball-on-Cylinder Lubricity Evaluator. In parallel, neat hydrocarbons and commercial synthetic blending components (SBCs) in which heteroatoms are largely absent were screened to isolate the role of bulk viscosity. Carboxylic acids were the most potent lubricity-restoring species, reducing wear scar diameter (WSD) below the 0.85 mm specification limit at oxygen-equivalent concentrations of only 1–2 ppm. The corresponding methyl ester produced only modest improvement. Heteroaromatic nitrogen compounds (indole, quinoline) required approximately 10 ppm fuel-bound nitrogen to reach the specification limit. Among sulfur species, a long-chain aliphatic thiol improved lubricity only at concentrations far exceeding jet-fuel sulfur limits, while aromatic sulfur compounds provided little benefit and could degrade lubricity at high concentrations. Viscosity showed only a weak correlation with WSD for neat hydrocarbons (R 2 = 0.48) but none for real SBC fuels (R 2 = 0.04). An inverse association between total acidity and WSD further supports the dominant role of trace acidic oxygenates. These results provide a chemistry-based framework for managing lubricity in highly paraffinic fuels: assurance should target trace heteroatom speciation or approved additives rather than bulk viscosity constraints.