E85 is a fuel mixture containing 85 percent ethanol alcohol and 15 percent gasoline and is intended for use in flex-fuel vehicles (FFVs). And what the heck is an FFV? An FFV is a vehicle that is designed to operate on straight gasoline, E85 or any combination of the two. We like to think of FFVs as transitional vehicles and E85 as a transitional fuel. They are engineered to bridge the gap between our current predominately petroleum based fuel supply (gasoline), and an evolving and steadily growing bio (plant-based alcohol) fuel supply.
But there's the rub. Any device, process or idea that must go through a transition is, by default, a compromise. It must be able to straddle the difference between--and function in--the conditions that exist now, while still keeping emphasis on its true intended purpose at some point in the future.
Challenges in Transitioning to Ethanol Fuel
The whole issue with E85 and flex-fuel vehicles is a little bit of a chicken/egg thing mixed in with a vast amount of change that must be managed effectively--and in a fairly linear fashion. Here's why:
- The ultimate goal is to be able to fuel alcohol powered vehicles with 100 percent (neat) ethanol; however, ethanol has different burn characteristics than gasoline.
- Engines that burn pure alcohol must be designed differently than engines that burn pure gasoline.
- The fuel distribution infrastructure for ethanol is still in its infancy, and it will take many years and many millions of dollars to fully develop and have true intra-continental availability.
- During the transition from petrofuel to biofuel, vehicles will need to be able to run on both types of fuel as availability of one or the other may be spotty. E85 fills that immediate need by blending the two fuels together.
FFV Engine Design Limitations
Flex-fuel engines can burn both fuels; however, they make efficiency compromises that neither straight gasoline nor straight alcohol engines need to consider, for example:
- Flex-fuel engines have low compression ratio/cylinder pressure parameters to be able to burn regular pump gasoline without the worry of detonation.
- Compared to gasoline, alcohol has a higher heat threshold for proper atomization (reduction to a fine mist). Flex engines can't burn straight alcohol; they need to have the 15 percent gasoline that is in E85 for cold start situations.
- Per gallon, alcohol has a lower energy content than gasoline. Because flex fuelers have a low compression ratio, they can't exploit alcohol's superior octane rating to make up the difference.
As a result of the sundry compromises flex-fuel engines must make to burn either type of fuel, both efficiency and economy suffer when the engine is burning E85. Let's take a deeper look a how/why fuel mileage drops in an E85 powered FFV.