Vapor pressure—i.e., the pressure of the vapor resulting from evaporation of a liquid (or solid) above a sample of the liquid (or solid) in a closed container—is used as a measure of volatility and is an important property of automotive gasoline fuels. (Higher vapor pressure indicating higher volatility.) Vapor pressure can affect proper cold starting of the engine; vapor lock tendency in older engines without fuel injection; and quality of starting in engines with fuel injection. It is also a critical factor in meeting evaporative emission requirements.

Evaporative emissions from gasoline—volatile organic compounds (VOCs)—are precursors to the formation of tropospheric ozone and contribute to ground-level ozone. Gasoline standards in the US and globally specify allowable vapor pressures depending upon the type of fuel and ethanol content, geographic location, and season.

While the vapor pressure is independent of the vapor/ liquid ratio for a pure compound, this is not true for mixtures, such as gasoline. Even highly volatile compounds that are present at small concentrations can contribute greatly to the vapor pressure, but their impact on the measured vapor pressure is progressively reduced with an increasing vapor/ liquid ratio.

...When blending alcohols with gasoline, especially the shorter chain alcohols, methanol and ethanol, the blend exhibits reductions in distillation temperatures and does not behave like an ideal mixture, because of the formation of a near azeotropic mixture. These non-ideal mixtures also have higher vapor pressures than would be predicted by Raoult’s law. This effect is particularly noticeable when a small amount of alcohol (a few volume percent) is blended into gasoline, because the blend has a vapor pressure higher than either the gasoline or the alcohol alone.

...With the expected increasing use of bioalcohols in gasoline blends and the possibility that alcohols other than ethanol and methanol will be used, it is important to have an accurate understanding of the vapor pressures of such blends.

—Andersen et al.

The study determined the Reid vapor pressures (RVPs) for alcohol-gasoline blends containing 5-85% by volume of methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, i-butanol (2-methyl-1-propanol), and t-butanol (2-methyl-2-propanol). The team then compared the results to literature data.

Among the findings of the study:

• The RVP of the base gasoline was higher than that of the C₁-C₄ alcohols. The alcohols and hydrocarbons in the gasoline formed non-ideal mixtures with RVPs that were higher than would be expected for ideal mixtures.

• Blends of methanol and ethanol have RVPs higher than the base gasoline over a wide concentration range. The highest RVPs were observed with relatively low concentrations (5-10%, v/v) of these alcohols.

• The propanols and t-butanol had RVPs higher than the base gasoline only at 5-10% (v/v) concentration.

• The other three butanol isomers decreased the RVP when added to the base gasoline at any concentration.

• The propanols and butanols may be more attractive blending components than ethanol at low to medium blending concentrations (10-30%, v/v) because of their lesser impact on RVP. At high blending concentrations (70-85%), however, the propanols and butanols may produce problematically low RVPs.

• When more than one alcohol is added to gasoline, it is possible to offset the RVP changes and obtain a blend with a RVP equal to that of the base gasoline.

Resources

• V. F. Andersen, J. E. Anderson, T. J. Wallington, S. A. Mueller and O. J. Nielsen (2010) Vapor Pressures of Alcohol-Gasoline Blends. Energy Fuels, Article ASAP doi: 10.1021/ef100254w

• Tables of the vapor pressure (VP₄) data

• Guide to Federal and State Summer RVP Standards for Conventional Gas Only (US EPA)