Measurement of Vapor Pressures and Enthalpies of Vaporization of Gasoline and Ethanol Blends and Their Effects on Mixture Preparation in an SI Engine

Global ethanol trade is forecast to increase 25-fold by 2020. Most of it will be blended with gasoline to make biofuel. However, blending ethanol with gasoline has a profound effect on the evaporation characteristics of the mixture. In particular, the thermodynamic properties of the blends can be significantly different than the constituents. A clear understanding of the blend's properties is essential for optimizing engine design, e.g. utilizing charge cooling effect. Data available in the literature is very limited, considering ethanol-gasoline blends will be used as a fuel in large scale worldwide. In this work, comprehensive measurements of vapor pressures were carried out. The enthalpies of vaporization were derived from vapor pressure data using the Clausius-Clapeyron equation. Maximum vapor pressure occurs with 20% ethanol-gasoline blend at which a positive azeotrope is formed. The trend is different in enthalpy of vaporization. The results presented here contradict previous claims that the enthalpy of vaporization is a linear function of ethanol content. Such a trend is true up to 20%; the value then decreases a little and appears to flatten out between E30 and E60. After E60, it begins to increase again. The ethanol-gasoline blends were tested in a port injected SI engine. The evaporation characteristic of the fuel was assessed by measuring the cycle-resolved temperature in the cylinder. The results have shown that low ethanol blends (0-30%) tend to evaporate readily, but the evaporative cooling is limited by the relatively low fuel flow rate (at fixed stoichiometry) and enthalpy of vaporization. High ethanol blends (>50%) with reduced vapor pressure cannot evaporate readily, but if they do vaporize, then they absorb large amounts of energy per unit mass. The implication of these competing effects is that maximum cooling is achieved using a blend with about 50% ethanol. A parameter "potential evaporative power" is introduced to estimate these effects.