Monday, June 30, 2008

More Ethanol thought experiments

As I learned here, the octane rating is a comparison of the actual fuel being tested to a theoretical fuel. Gasoline is a mixture of various hydrocarbon molecules, and different hydrocarbon molecules have different burn characteristics. Refiners put the raw petroleum stock in one end of the process, and gasoline comes out the other end. The theoretical gasoline used in the comparison is a fuel containing only octane and heptane. (It is theoretical only because no real world gasoline can be that pure, not because it is impossible to create such a fuel.) The octane rating of that fuel is the ratio of octane to heptane in it. If it has 87% octane, that's what the octane rating is.

To test actual fuels, they put the fuel into an engine and compare its characteristics to the known characteristics of the octane/heptane control fuel.

Also note that it is seemingly cheaper to produce fuels with lower octane ratings.

On to ethanol, which has an octane rating of 130. It is, after all, racing gas. The point of all of this testing and whatnot is that the higher a fuel's octane rating, the more it can be compressed in the cylinder before it behaves badly.

So, it seems to me that to get normal E10 fuel at an 87 octane rating, you need to mix gasoline of a lower octane rating with that ethanol of a higher rating to get 87.

.9(x) + .1(130) = 87, right? So .9(x) + 13 = 87 and .9(x) = 74 Meaning the X ends up being 82.

My point is, I wonder what effect blending this shitty gasoline with the less-energy-containing ethanol has on an engine? That ethanol blends aren't worse because of ethanol, but because of the worse gasoline being used?

More importantly, when are we going to start building cars that can actually take advantage of ethanol, instead of hacking on engines tuned for gasoline to make them tolerate ethanol? As it is, the engines now are basically de-tuned to tolerate the cheaper 87 octane, and now we're detuning them even more to tolerate ethanol. No wonder ethanol seems to be a lie.

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