Modification of gasoline blendstock composition in preparing ethanol-gasoline blends has a significant impact on vehicle exhaust emissions. In “splash” blending the blendstock is fixed, ethanol-gasoline blend compositions are clearly defined, and effects on emissions are relatively straightforward to interpret. In “match” blending the blendstock composition is modified for each ethanol-gasoline blend to match one or more fuel properties. The effects on emissions depend on which fuel properties are matched and what modifications are made, making trends difficult to interpret. The purpose of this paper is to illustrate that exclusive use of a match blending approach has fundamental flaws.
For typical gasolines without ethanol, the distillation profile is a smooth, roughly linear relationship of temperature vs. percent fuel distilled. Hence the use of three points on the curve (T10, T50, and T90, defined as the 10%v, 50%v, and 90%v evaporated temperatures) has been sufficient to define their volatility-related behavior in engines. These parameters are commonly “matched” in studies intended to evaluate fuel composition effects on emissions. For ethanol-gasoline blends, higher boiling-point hydrocarbons must be added to match T50 and T90 with fuels having less ethanol. The degradation of emissions which can result is primarily due to the added hydrocarbons, but has often been incorrectly attributed to the ethanol. Studies to evaluate the effects of ethanol should be conducted by adjusting the blendstock only as necessary to satisfy ASTM D4814 requirements. Blending ethanol at up to 30%v with an E10 blendstock should generally require only minimal changes in composition to meet ASTM D4814.
