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The Impact of Low Octane Hydrocarbon Blending Streams on the Knock Limit of E85

Ethanol is a very attractive fuel from an end-use perspective because it has a high chemical octane number and a high
latent heat of vaporization. When an engine is optimized to take advantage of these fuel properties, both efficiency and
power can be increased through higher compression ratio, direct fuel injection, higher levels of boost, and a reduced need
for enrichment to mitigate knock or protect the engine and aftertreatment system from overheating.
The ASTM D5798 specification for high level ethanol blends, commonly called “E85,” underwent a major revision in
2011. The minimum ethanol content was revised downward from 68 vol% to 51 vol%, which combined with the use of
low octane blending streams such as natural gasoline introduces the possibility of a lower octane “E85” fuel. While this
fuel is suitable for current “ethanol tolerant” flex fuel vehicles, this study experimentally examines whether engines can
still be aggressively optimized for the resultant fuel from the revised ASTM D5798 specification.
The performance of six ethanol fuel blends, ranging from 51-85% ethanol, is compared to a premium-grade
certification gasoline (UTG-96) in a single-cylinder direct-injection (DI) engine with a compression ratio of 12.87:1 at
knock-prone engine conditions. UTG-96 (RON = 96.1), light straight run gasoline (LSRG, RON = 63.6), and n-heptane
(RON = 0) are used as the hydrocarbon blending streams for the ethanol-containing fuels in an effort to establish a broad
range of knock resistance for high ethanol fuels.
Results show that nearly all ethanol-containing fuels are more resistant to engine knock than UTG-96 (the only
exception being the ethanol blend with 49% n-heptane). This allows ethanol blends made with low octane number
hydrocarbons to be operated at significantly more advanced combustion phasing for higher efficiency, as well as at higher
engine loads.
While experimental results show that the octane number of the hydrocarbon blend stock does impact engine
performance, there remains a significant opportunity for engine optimization when considering even the lowest octane
fuels that are in compliance with the current revision of ASTM D5798 compared to premium-grade gasoline.

James Szybist
Contact Person
Tim Theiss
Contact Organization
Oak Ridge National Laboratory
Contact Email
Bioenergy Category
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Funded from the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Bioenergy Technologies Office.