E85 - Octane and Performance

Octane and Performance

Alcohol fuels reach their peak torque a bit more quickly than gasoline does, so any vehicle that receives a very basic conversion to be able to run on E85 will be very slightly faster on E85.

As more effort is put into maximizing an engine to take advantage of E85's higher "octane, engines achieve greater power advantages. One car that has higher power on ethanol is the Koenigsegg CCXR, which on ethanol is the third-most powerful production car, with 20% more hp on E85 than on gasoline. According to the manufacturer, this is due to the cooling properties of ethanol.E85 has an octane rating higher than that of regular gasoline's typical rating of 87, or premium gasoline's 91-93. This allows it to be used in higher-compression engines, which tend to produce more power per unit of displacement than their gasoline counterparts. The Renewable Fuels Foundation states in its Changes in Gasoline IV manual, "There is no requirement to post octane on an E85 dispenser. If a retailer chooses to post octane, they should be aware that the often cited 105 octane is incorrect. This number was derived by using ethanol’s blending octane value in gasoline. This is not the proper way to calculate the octane of E85. Ethanol’s true octane value should be used to calculate E85’s octane value. This results in an octane range of 94-96 (R+M)/2. These calculations have been confirmed by actual-octane engine tests."

Examples of this mis-citation can be found at the Iowa Renewable Fuels Association titled "E85 Facts" which cites a range of 100-105, and a document at the Texas State Energy Conservation Office titled "Ethanol", which cites a 113 rating.

Use of E85 in an engine designed specifically for gasoline would result in a loss of the potential efficiency that it is possible to gain with this fuel. Use of gasoline in an engine with a high enough compression ratio to use E85 efficiently would likely result in catastrophic failure due to engine detonation, as the octane rating of gasoline is not high enough to withstand the greater compression ratios in use in an engine specifically designed to run on E85. (However, Flex Fuel Vehicles are designed to run on any mixture of gasoline and ethanol, from pure gasoline to E85, and avoid this problem.) Using E85 in a gasoline engine has the drawback of achieving lower fuel economy, as more fuel is needed per unit air (stoichiometric ratio) to run the engine in comparison with gasoline. The additional ethanol required for a stoichiometric fuel ratio helps compensate for lack of energy provided by ethanol's lower heating value (LHV), which is lower than the LHV of gasoline.

Some vehicles can actually be converted to use E85 despite not being specifically built for it. Because of the lower heating value E85 has a cooler intake charge—which, coupled with its high stability level from its high octane rating—has also been used as a "power adder" in turbocharged performance vehicles. These modifications have not only resulted in lower GHG emissions, but also resulted in 10-12% power and torque increase at the wheels. Because of its low price (less than $2.00/gal in some places) and high availability in certain areas people have started to turn to using it in place of high-end racing fuels, which typically cost over $10.00/gal.

E85 consumes more fuel in flex-fuel type vehicles when the vehicle uses the same compression ratio for both E85 and gasoline, because of its lower stoichiometric fuel ratio and lower heating value. European car maker Saab, now defunct, produced a flex-fuel version of their 9-5 sedan, which consumes the same amount of fuel whether running E85 or gasoline.