Flex-fuel vehicles are a compromise between straight gasoline powered engines and straight alcohol powered engines. They are designed as gasoline burners (since that is still the most abundantly available fuel) that can also accept high alcohol content fuel E85). The problem is that while they can operate on high concentrations of alcohol, they can't do so efficiently, and are unable to take advantage of some of alcohol's superior properties.
Good Octane Going to Waste
Ethanol has an octane rating of about 113 (R+M/2 method) while gasoline ranges between 87 and 93 (R+M/2 method). A fuel's octane rating is its ability to control detonation (also commonly called pre-ignition or pinging) in an engine's combustion chamber--and the higher the octane number, the better the detonation control. To a large extent, detonation is a function of cylinder pressure (compression ratio) since the higher the pressure, the more heat is produced, and the more prone to pre-ignition a particular fuel is.
Conversely, the higher the cylinder pressure, the more forceful and efficient the power stroke of the piston becomes (and this is what ultimately moves the vehicle). A flex-fuel engine has to have a lower compression ratio to prevent detonation when it's fueled with regular pump gas. Thus, it can't take advantage of alcohol's efficient high pressure, high octane power when it's fueled with ethanol. In essence, ethanol's superior octane rating is "wasted" in a flex-fuel engine.
The Atomization Penalty
Ethanol requires more heat to atomize (reduce to a fine mist) than does gasoline. In a straight alcohol engine, the fuel delivery system would be designed in such a way that the fuel would be preheated to some degree to assist atomization before delivery to the combustion chamber. Again a compromise must be reached to burn both fuels in a flex engine. This is the main reason for the existence of the E85 blend. The addition of 15 percent gasoline to the 85 percent alcohol overcomes the cold evaporation problems of pure ethanol.
Is it Really Just the BTUs?
Ethanol's lower energy content (76,100 BTUs/Gal.) is a big contributor to its reduced fuel economy compared to gasoline (114,100 BTUs/Gal). Even though FFVs burn E85 (81,800 BTUs/Gal) and the energy penalty isn't as severe, it still requires more fuel to do the same work. On paper, according to our GGE chart, it takes 1.39 gallons of E85 to move a flex-fuel vehicle the same distance as 1 gallon of gasoline. Again, that's theoretically. A whole host of contributing factors come into play regarding any particular flex-fuel vehicle's fuel economy at any given time:
- Engine characteristics, from cylinder compression ratio to valvetrain geometry and drivetrain variations including overdrive and final gear ratio
- Engine state-of-tune
- Percentage of ethanol in the fuel
- Vehicle weight
- Road conditions
- Ambient temperature
- Driving habits, including heavy acceleration and braking.
And while the list of variables goes well beyond any static limitations that can be determined by fuel characteristics alone, it's obvious that alcohol is more than just a niche or blending fuel. E85 is fine as a transitional fuel, and alcohol holds a lot of promise as a renewable motor fuel, but it must be used in dedicated engines that are engineered to take advantage of its pros while mitigating its cons.