An Experimental and Modeling Investigation into the Comparative Knock and Performance Characteristics of E85, Gasohol [E10] and Regular Unleaded Gasoline [87 (R+M)/2]

In the near future increasing use of ethanol in motor fuels will occur due to legislative mandates. E10 (Gasohol) and E85 will see more widespread use in spark ignition engines. This study looks at the performance and knock characteristics of E10 and E85 in comparison to regular gasoline. Detailed experimental engine data and analysis as a function of compression ratio, ignition timing and fueling are presented with associated physical explanations. Comparative results are presented. Increasing ethanol content provides for greater engine torque, efficiency and knock tolerance, yet fuel consumption worsens. Knock limited trends and sensitivities are presented, for example, 5 degrees of spark retard are required with E10 and gasoline for each compression ratio increase, while the much less sensitive E85 requires only 2 degrees of retard for each compression ratio increase. Trends with efficiency and torque are described amongst the fuels tested. Companion modeling was performed to isolate the chemical kinetic differences between E85 and isooctane fuels. An adiabatic-core model revealed significant differences in reaction rates with an E85 fuel versus a pure isooctane fuel. These differences can be traced back to reactions that produce HO₂ radicals which ultimately lead to H₂O₂ buildup as a precursor to autoignition.