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Bob, Jerry B's explanation is a good one. I'll add a little "LIGHTER SIDE" to the light end debate---This section is copied from a fuel handling report (not the whole report)----- --- 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.
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