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Gasoline-derived deposits in modern automobile engines can plug fuel injectors and impair engine performance. The deposits (which originate from gasoline oxidized at moderate temperatures) contain higher sulfur concentration than the fuel. We used laboratory tests to study the effect of organic sulfur on the formation of solid deposits in an uninhibited gasoline oxidized at 100°C. Gravimetric studies showed that the effect of sulfides on deposit formation was strongly dependent on the structure of the sulfur compound. Disulfides and thiophenes showed little or no effect. Thiols enhanced the formation of insoluble deposits; but 1-dodecanethiol decreased the amount of deposits formed after long oxidation times. Analysis of thin deposit films with XPS(ESCA) showed that sulfur in the deposits was primarily bound to oxygen. When the fuel was blended with thiols, non-oxidized sulfur also appeared in the deposits.

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.