Octane Numbers of Ethanol- and Methanol-Gasoline Blends Estimated from Molar Concentrations

When expressed using volumetric concentrations (as is industry practice), the addition of relatively small amounts of ethanol or methanol (e.g., 10% by volume) to gasoline appears to result in disproportionately large, nonlinear increases in research octane number (RON) and motor octane number (MON). As a result, volumetric "blending octane numbers" are of limited value for estimating the octane number of alcohol-gasoline blends because they vary with alcohol content and base gasoline composition. We show that RON and MON increases with alcohol content are approximately linear when expressed using molar concentrations. Moreover, molar-based blending octane numbers are effectively equal to the octane numbers of the pure alcohols for most base gasolines. A limited dependence on gasoline composition was observed, namely, greater-than-predicted octane numbers for ethanol-gasoline blends with unusually high isoparaffin content. We suggest that octane numbers of methanol-gasoline and ethanol-gasoline blends can be estimated conveniently and more accurately from their molar composition by linear interpolation between the octane numbers of the base gasoline and the pure alcohol.