Vapor Pressures of Mixtures of Gasolines and Gasoline-Alcohol Blends

An investigation was conducted to determine the change in Reid vapor pressure (RVP) which results when gasoline and various gasoline-alcohol blends are mixed. Such mixing occurs in vehicle fuel tanks when a motorist buys gasolines and blends alternately.

When mixing a gasoline with a gasoline-alcohol blend of the same RVP, the resulting mixture always had a higher RVP, due to the non-linear effect of alcohol concentration in gasoline on RVP. Even when a blend had a much lower RVP than gasoline, some mixtures of the two still had higher RVP's than the gasoline. When two common commercial blends, 10 percent ethanol and 10 percent Oxinol™ 50, both having the same RVP, were mixed in various proportions there was essentially no change in RVP.

The results of this study suggest that the presence of both gasolines and blends in the marketplace can lead to higher in-use evaporative emissions from vehicles, even if the blends meet the same volatility standards as gasoline.