On Fuel Efficiency, Or Lack Thereof

Car and Driver’s staff engineering wonk Patrick Bedard has written a doozy of an article this month on why the so-called “Ethanol Solution” to high gas prices is little more than a thinly veiled (or not-at-all veiled) farm subsidy. Bedard’s article, as is usual for his writing, takes a popular idea — in this case the vague idea of a solution to our endless appetite for dead dino juice put forth by the Bush administration — and utterly destroys it with hard numbers.

Using ethanol to power internal-combustion engines isn’t a bad idea, but it’s not going to wean us of our dependency on foreign oil any time soon, either. (As an aside, did you know only 19 percent of our petroleum imports in 2004 came from the Middle East?) The ethanol production process simply isn’t efficient enough right now to replace a significant fraction of petroleum imports with ethanol. A process that allows for more diverse feedstock, like thermal depolymerization, might be a viable alternative. Bedard, unfortunately, does not address TDP at all in his article (though, in his defence, the article is focused on ethanol and is not a general treatise on “green energy”).

What caught my eye as I read the article, though, was Bedard’s mention of the relatively poor fuel efficiency realised by vehicles powered by E85, an 85-15 mixture of ethanol and traditional gasoline. The reason for this is the lower energy density of ethanol: a gallon of ethanol contains only about 75% of the stored chemical energy of a gallon of gasoline. In C&D’s testing, a 2007 Chevy Tahoe (which has an engine capable of running on up to 85 percent ethanol) showed a 30 percent drop in fuel economy running on E85 relative to 87-octane pump gas. That’s directly proportional to the lower energy density of E85.

Diesel fuel throws yet another wrench in the works; the heavier and less-refined fuel oil sold as fuel for diesel engines is slightly more energy-dense than regular gasoline, and contains a lot more chemical energy than E85. Let’s step into the world of hypotheticals for a moment and imagine that a piston engine exists that will run equally well on all three fuels — diesel (or a biofuel equivalent, like WVO or the output from a TDP plant), gasoline, or ethanol. Let’s say we’ve installed this engine into a Honda Accord, a fairly “typical” family car in the United States. Assume our hypothetical flex-engined Accord gets 30 MPG on gasoline. That means, assuming the engine is equally as efficient at converting all fuels to forward motion, this car gets 35 MPG on diesel, and a dismal 22.5 MPG on pure ethanol (about 23.6 MPG on E85).

Thinking this through very quickly in my head while reading the article, I came to a realisation: what we really need is a new measure of efficiency. I alluded to it above. The important number is not how many gallons of fuel we’ve added to our vehicle. What matters is how well said vehicle turns that fuel into forward motion. Without getting into a 40 rods to the hogshead sort of measurement, I think it would far more useful if we were to say that a vehicle got X number of miles per megajoule (or some other usefully large unit of energy). In our above example, using gasoline as the fuel, 30 MPG is equivalent to .227 miles per megajoule. (Gasoline contains approximately 132 megajoules of energy per gallon.)

“But that doesn’t make any sense,” you say. Just like the metric system, it doesn’t make any sense because you haven’t bothered to use it regularly. If window stickers gave EPA ratings in miles per megajoule instead of MPG, and provided conversion factors for two or three common fuels, people would get used to it soon enough. The other obvious solution — putting MPG figures for different fuels on the window sticker — would work, too. The desired outcome is for people to become more aware of how good a given vehicle is at converting energy into forward motion, in the face of a confusing array of alternative fuel choices.

Getting 35 MPG on diesel isn’t nearly as impressive as getting 35 MPG on E85. Conversely, paying $2.50/gallon for diesel is a far better deal than paying $2.00 a gallon for E85, since our hypothetical engine will actually go 14 miles per dollar on diesel and only 11.8 miles per dollar on E85.

Me? I’m glad I ride a motorcycle.

posted by Chris on 10 June 2006 at 2035 in car


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