The goal of this paper is to quantitatively assess the implications of congressionally mandated biofuel targets on requirements for ethanol blending, distribution, and usage in spark ignition engines in the U.S. light-duty vehicle fleet. The “blend wall” is a term that refers to the maximum amount of ethanol that can be blended into the gasoline pool without exceeding the legal volumetric blend limit of 10%. Beyond the blend wall, the additional ethanol fuel must be used in higher blends of ethanol like E85. Once the blend wall is reached, the existing fleet of flex fuel vehicles (FFVs) will be required to use E85 for some percentage of vehicle miles traveled (VMT) in order to achieve the Renewable Fuel Standard (RFS) targets. The feasibility of this requirement will depend on several major variables including (i) the total amount of ethanol to be used; (ii) the volumetric blend limit of ethanol in gasoline; (iii) the annual sales and fleet accumulation of FFVs; (iv) the availability of retail stations offering E85; (v) a measure of the attractiveness of E85 relative to gasoline relative to the price; and (iv) whether engine technology can be deployed to take advantage of the antiknock advantages of ethanol.
A fleet model has been developed by several researchers in the Sloan Automotive Laboratory at MIT that can simulate the interaction of these variables in the US light-duty vehicle (LDV) fleet. Different scenarios can be adjusted individually or as a set of coordinated policies to observe the effect on the total energy equivalent fuel demand. Results from the model show that in order to meet current biofuel targets the retail availability of E85 will need to increase drastically, and at a faster rate than the current trends indicate. The certification of E15 can delay the blend wall by a few years, but results still show likely shortfalls in reaching current biofuel targets even with aggressive deployment of FFVs.