Gum Formation in Gasoline and Its Blends: A Review

Gasoline is a volatile mixture of hydrocarbons that is used in spark-ignition (SI) engines. It is a complex mixture composed of olefinic, paraffinic, naphthenic, and aromatic hydrocarbons (C4–C12), among other substances in a smaller concentration. In several countries, such as Brazil, ethanol is used pure as a renewable fuel for SI engines, especially in flex fuel engines, and/or an additive to improve the octane number of gasoline. During storage, some classes of hydrocarbons in gasoline blends, particularly olefins and diolefins, are able to slowly react, at ambient temperatures, with the oxygen in the air. The formed oxidation products are responsible for the formation of an insoluble solid, commonly called deposits or gums, which sticks to the metal surfaces along the vehicle-fuel system, from the tank to the combustion chamber. Accumulation of these products can cause engine wear and can have adverse effects on engine efficiency, performance, emission, and durability. Consequently, it is necessary to predict gasoline blend behavior and prevent gum formation, improving gasoline quality and using additives. Even if the number of publications dedicated to gum formation in gasoline blends is reduced, results available in the literature for other fuels can be applied to the gasoline issue. This review intends to define more precise fuel stability concepts and what is considered a gum. It also aims to present the oxidation mechanism involved in gum formation, determine the main parameters influencing gum deposition in gasoline blends, and describe the experimental tools available to measure gum content.