Thin Deposit Films from Oxidized Gasoline on Steel Surfaces as Determined by ESCA

Fuel-derived deposits on injectors and elsewhere in engines can severely impair engine performance. A laboratory test procedure was developed to produce thin deposit films from oxidized fuel on steel. The deposit films were analyzed using ESCA (XPS) and depth profiling with Ar i-ons. The deposits were carbonaceous in nature with lesser amounts of oxygen, and small amounts of sulfur and nitrogen. The total sulfur concentration in the deposits was approximately five-ten times higher than the concentration of sulfur in the original gasoline. Ion bombardment preferentially removed oxygen from the deposit layer, revealing that sulfur in the deposits was in the form of oxygenated compounds (RSO₂ R, RSO₂OR, RSO₂OR, RSO₂OSO₂ R) and removal of oxygen converted them to lesser or non-oxygen-containing compounds (RSR, RSOR, RSSR, RSSO₂ R). Fuel samples were spiked with two sulfur-containing chemicals, thioanisole and thianaphthene. Neither compound affected the relative concentration of sulfur in the deposits, indicating that they do not participate directly in the deposit formation process. Thinner deposit films were formed on steel surfaces which were polished and oxidized prior to being used in the deposit formation test than on surfaces which were only polished before the test.