Vapor pressure is an important quality specification for gasoline. Specifically, vapor pressure is a measure of the volatility of a fuel, or the degree to which it vaporizes at a given temperature.

For gasoline, vapor pressure is important for both performance and environmental reasons. First, because gasoline engines require the fuel to be vaporized in order to burn, gasoline must meet a minimum vapor pressure to ensure that it is volatile enough to vaporize under cold start conditions. Engines also have a maximum limit for vapor pressure set by concerns over vaporization in the fuel line that can result in vapor lock, or a blocking of the fuel line. However, the most critical limit to vapor pressure in most markets now is environmental concern about evaporative emissions outside of the vehicle, which contribute to pollution. Typically, it is this concern that sets the critical maximum vapor pressure specification for most grades of gasoline.

Measurement of vapor pressure

The most common measure of vapor pressure for gasoline is Reid vapor pressure. This is the pressure, in psi (pounds per square inch) or kPa (kiloPascals), necessary to keep a liquid from vaporizing when at 100 degrees F (37.8 degrees C).

Vapor pressure limits vary seasonally, with higher limits in the cold months and lower limits in the warm months. Typical ranges are 7-15 psi, or 48-103 kPa.

Vapor pressure does not blend linearly with volume. To estimate the vapor pressure of a blend, a vapor pressure index is used. This is simply:

Vapor pressure index = (Vapor pressure)^(1.25)

High and low vapor pressure blend stocks

Most gasoline blend stocks have vapor pressures below the maximum specification for finished gasoline. So achieving the vapor pressure spec is not difficult. However, one of the highest vapor pressure blend stocks, butane, also happens to be very high in octane and very cheap relative to other blend stocks. Consequently, there is typically a strong incentive for refiners to blend as much butane into gasoline as possible, up to the point where it meets the maximum vapor pressure limit.