[Editor's Note: We hereby welcome our latest writer, Angry Diesel Engineer. I hope you enjoy his perspective on flex-fuel vehicles - ANM]
The Big Three have announced that by 2012, half of their production will be flex-fuel vehicles. I’ll leave it to the other Angry Men to debate whether it makes sense to run your car on food at all (I’m sure Angry Biologist would have something to say about it), but ramping up on our current production FFVs is not the way to go about it.
If you’ve ever seen a FlexFuel sticker on a car in America, it means that that car contains an engine and fuel system which was designed and optimized to run on gasoline, but made to tolerate E85, a blend of 85% ethanol and 15% gasoline. Ethanol is not a bad fuel, per say. Compared to gasoline, it has 61% the energy content (Lower Heating Value), and, conveniently enough, 61% of the Air/Fuel ratio for Stoichiometric combustion. For this reason, it blends well with gasoline as a fuel – a higher volumetric consumption releases the same energy. Relative to gasoline, it actually has a much higher knock resistance (Octane rating), comparable almost to racing fuel. Coupled with the it can be made from a variety of sources, most of them renewable, and it becomes downright attractive.
But gasoline it ain’t. Gasoline has about a 90 year head start on ethanol in engine optimization, infrastructure deployment, and the like. It bears the aforementioned higher energy content, so for the same weight and/or volume (both large considerations for vehicle designers), it can be converted to produce more work. And, as Americans, this is the measure our car’s efficiency – units of work done per volume of fuel consumed, or miles per gallon.
Which brings me to the topic of this rant: chemical energy to rotational force conversion systems, or as they are commonly called, engines. The modern internal combustion engine is a marvelous piece of engineering. It has been highly refined over more than a century resulting in ridiculous increases in specific output and efficiency. Despite its advances, a modern example is rarely found operating at even 33% efficiency. That’s right, over 2/3 of the energy (gasoline) you are putting into your car is wasted.
Long ago it was discovered that raising the compression ratio (the maximum cylinder volume : minimum cylinder volume) increased the efficiency of the engine, but also uncovered the tendency of liquid fuel/air mixtures to explode spontaneously, as opposed to the desired flame-front propagation. Spark ignition engines are by design “knock limited,” meaning were it not for this spontaneous combustion, the engine output or efficiency (or both) could be improved. Historical solutions to this knock limit have been found, but in the end were deemed environmentally irresponsible.
“But wait,” you ask, “didn’t you just say that E85 has a higher octane rating, which means it should tolerate a higher compression ratio and thereby actually run more efficiently?” And you’d be right to ask that. Yes, an engine designed to run on E85 could theoretically achieve higher thermal efficiencies, achieving miles-per-gallon similar to (but probably still lacking compared to) its gasoline counterpart.
Which brings us to Detroit’s folly. In an age when fuel economy is the ultimate quest, and green house gasses are on the brink of being regulated, Detroit is saying “up yours” to mother earth. FlexFuel engines are designed to tolerate E85, not actually achieve anything from it. Actually burning the stuff results in about 2/3 the fuel economy compared to straight gasoline. All that potential for any increase in mileage is scoffed at. Detroit does with it what it does best: throws it out the tailpipe.