Volatility
Volatility describes a gasoline's tendency to form vapors. Liquid gasoline does not burn; only gasoline vapor burns. To start a cold engine, enough low boiling components ("light ends") must vaporize at the engine temperature to form a combustible vapor-air mixture.
This is one reason that the volatility of gasoline is tailored for the range of temperatures expected in the locality where is it sold. "Winter gasoline" has a higher volatility for easy starting in cold weather. "Summer gasoline" has a lower volatility because hydrocarbon vapor in the atmosphere contributes to smog formation.
Evaporation
The gasoline light ends needed for easy starting have the same tendency to vaporize in storage as they do in an engine. If the storage container is not tightly sealed, some of the light ends gradually will be lost. Too great a loss decreases the gasoline's ability to start an engine.
Evaporation of gasoline from a vented fuel tank or a can with a loose cap would be minimal if the temperature of the container were constant. But daily temperature changes cause the temperature of the container to cycle. The heating portion of the cycle raises the pressure of the gas (gasoline vapor and air) above the liquid gasoline which, in turn, drives some of the vapor-air mixture out of the container. The succeeding cooling cycle lowers the pressure of the gas, drawing fresh air into the container. Light ends evaporate from the liquid gasoline to saturate the new air. The daily repetition of this cycle gradually pumps light ends out of the container.
The cycle also brings air and water vapor into the container, especially during periods of high humidity. The oxygen in the air contributes to gum formation. (See Oxidation section.) And the water vapor, if it condenses during the cooling cycle, contaminates the gasoline with liquid water.
A larger volume of gas will be pumped in and out of the container when the air space above the liquid fuel is larger and when the daily temperature change is larger. Consequently, keeping the container almost full of gasoline and controlling the temperature fluctuations will minimize the loss of light ends, the exposure of the gasoline to air, and the contamination of the gasoline with water.
Oxidation
Except for any added oxygenates, gasoline is made up almost entirely of hydrocarbons–molecules constructed from the building blocks of elemental carbon and hydrogen. Hydrocarbons, as a class, are chemically stable molecules.
However, there are types of hydrocarbons (olefins and diolefins) that can combine slowly with the oxygen in the air ("oxidize") at ambient temperatures. The products of the reaction are larger molecules, collectively called "gum."
The gum-forming reactions become faster as the temperature of the gasoline increases. This is why this bulletin recommends controlling the temperature of stored gasoline.
Most gasolines contain negligible amounts of gum when they are manufactured, and most contain special chemicals ("stabilizers") to retard gum formation. It is the stabilizers that make it possible to store Chevron gasolines for a year when the conditions are good.
Soluble Gum
The gum formed by oxidation is usually soluble in gasoline. However, it remains behind as a sticky residue when the gasoline evaporates. Since gasoline begins to evaporate in the carburetor of a carbureted engine or in the injector of a fuel-injected engine, a gasoline containing soluble gum may leave a deposit on these parts and on the intake valves. These deposits will be in addition to the deposits normally formed by a gum-free gasoline — a formation triggered by the elevated engine temperatures.
Modern engines are designed to run best when vital engine parts are clean. Carburetor and fuel injector deposits can cause hesitation and stumbling on acceleration, lower fuel economy, lower power output, and higher emissions of hydrocarbons and carbon monoxide. Excessive intake valve deposits can cause many of the same performance problems, plus higher emissions of hydrocarbons, carbon monoxide, and nitrogen oxides.
Because the Federal Environmental Protection Agency recognizes that fuel system deposits increase emissions, they require all gasolines to contain a deposit-control additive. All deposit-control additives keep deposits from forming; the best ones clean up deposits formed by lower-quality gasolines.
If the gasoline contains a lot of soluble gum, the normal level of deposit-control additive may not be sufficient. This is why Chevron recommends treating a tank of gasoline with an extra dose of deposit-control additive if a vehicle displays driveability problems after being stored. (Note: Chevron does not recommend adding a deposit-control additive to the fuel for a two-stroke-cycle engine.) The gum-forming reactions become faster as the temperature of the gasoline increases.
Insoluble Gum
Severe oxidation of gasoline may produce insoluble, as well as soluble, gum. The insoluble gum will take the form of brown or black particles which float in the gasoline or settle to the bottom of the container.
When an engine is fueled with gasoline containing insoluble gum, the fuel filter will remove the gum. If the engine has an in-tank fuel pump, the screen on the pump's feed also may capture some of the gum. However, these devices can become plugged if the gasoline contains too much insoluble gum. This will cause the engine to lose power or stall because it is starved for fuel. Adding a deposit-control additive will not keep insoluble gum from plugging filters and screens.
Other Issues
Storage, including storage involving gum formation, does not change the bulk properties and most of the performance characteristics of gasoline (excluding the characteristics affected by the gum itself). For example, storage does not change a gasoline's anti-knock index (octane) or energy content. However, these properties will change if the storage is accompanied by evaporative loss. The evaporation of light ends decreases a gasoline's antiknock index and increases its energy content.
Federal and California reformulated gasolines will survive storage as well or better than conventional gasolines. The regulations require reformulated gasolines to have less light ends and less olefins (federal, later; California, now) than conventional gasolines. As explained above, it is the oxidation of olefins that leads to gum formation. Reformulated gasolines also contain oxygenates. The common oxygenates are stable molecules that do not form gums.
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