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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 (C 4 −C 12 ), 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.

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Gasoline is a volatile mixture of hydrocarbons that is used in spark ignition engines. It is a complex mixture composed of inflammable olefinic, paraffinic, naphthenic, and aromatic hydrocarbons (C 4 −C 12 ). It presents low contents of oxygenates and traces of sulfur, nitrogen, and metals, which introduce instability to the mixture. In several countries, such as Brazil, ethanol is used purely as a renewable fuel or as an octane improver in blends with gasoline, especially in flex fuel engines. Nevertheless, some compounds in the fuel react slowly, at room temperature, with atmospheric oxygen, and with each other. The process is observed throughout the entire fuel production and use process and increases fuel density. These high-molar-mass insoluble oxidation products are commonly called gums and form deposits throughout the vehicle fuel system. Their accumulation has adverse effects on engine efficiency, performance, and durability, in the form of incomplete combustion, engine wear, and higher pollutant emissions. Consequently, it is necessary to prevent gum formation by improving gasoline quality and using additives. A prediction of gasoline blends behavior is also an important tool to assess critical conditions. This work studied the influence of aging period, temperature, and addition of anhydrous ethanol concentration on unwashed and washed gum content of Brazilian gasoline−ethanol blends. The effect of an additive was also evaluated, by comparing the results of regular and additivated gasolines. This study also defined predictive mathematic models for the studied properties through a Doehlert matrix with three factors, from which robustness was assessed.

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