Why Vapor over raw liquid gasoline?
Liquid gasoline does not burn; it is gasoline vapor in contact with air oxygen which burns or could explode. The concentration of gasoline vapor in air must be between the lower explosive limit and upper explosive limit for gasoline. The PEAC tool lists the lower explosive limit as 1.4% and the upper explosive limit as 7.6%. These numbers might vary somewhat depending how the gasoline was formulated (another reference source lists 7% as the upper explosive limit). At 68oF, (20oC) the vapor pressure for gasoline is 0.39 atm, which translates to 39% vapor concentration by volume. The Reid Vapor Pressure of gasoline [recall that this is the vapor pressure at 100oF] is about 9.5 psi, or 0.647 atm, which converts to 64.7% vapor concentration by volume. These are above the upper explosive limit, and therefore, the gasoline does not ignite. Vapor gasoline contains more potential energy then liquid gasoline.
