High octane number ethanol–gasoline blends: Quantifying the potential benefits in the United States

Abstract

Ethanol provides a significant contribution to road transportation fuel in the US, Brazil, and elsewhere. Renewable fuels regulations in the US and EU imply that ethanol use will continue to increase in the near future. The high octane rating of ethanol could be used in a mid-level ethanol blend to increase the minimum octane number (Research Octane Number, RON) of regular-grade gasoline. Higher RON would enable greater thermal efficiency in future engines through higher compression ratio (CR) and/or more aggressive turbocharging and downsizing, and in current engines on the road today through more aggressive spark timing under some driving conditions. Such an approach would differ from the current practice of blending ethanol into a gasoline blendstock formulated with lower octane rating such that the net octane rating of the resulting final blend is unchanged from historical levels.Developing scenarios of future ethanol availability, we estimate that large increases (4–7points) in the RON of US gasoline are possible by blending in an additional 10–20%v ethanol above the 10% already present. Keeping the blendstock RON at 88 (which provides E10 with ∼92.5RON), we estimate RON would be increased to 94.3 for E15 to as much as 98.6 for E30. Even further RON increases may be achievable assuming changes to the blendstock RON and/or hydrocarbon composition. For example, an increase in blendstock RON from 88 to 92 would increase the RON of E10 from 92.5 to 95.6, and would provide higher RON with additional ethanol content (e.g., RON of 97.1 for E15 to 100.6 for E30). Potential CR increases are estimated for the different estimates of future octane number, including the effect of increased evaporative cooling from ethanol in direct injection engines. For the ethanol and blendstock RON scenarios considered, CR increases were estimated to be on the order of 1–3CR-units for port fuel injection engines as well as for direct injection engines in which the greater evaporative cooling of ethanol can be fully utilized. Impacts to the fuel refining and blending sector and transition considerations are discussed. While additional work is needed to quantify and optimize the costs and benefits for both the automotive and refining sectors and for consumers, it appears that substantial societal benefits may be associated with capitalizing on the inherent high octane rating of ethanol in future higher octane number ethanol–gasoline blends.