Here’s an interesting commentary from the Providence (RI) Journal written by a “former rocket scientist and combustion specialist.”
Maurice Webb is his name and he explains the gasoline vs. ethanol mileage question as follows:
The fact is that higher fuel-heating values do not necessarily provide better engine performance (mileage). Engines equipped with carburetors do not perform well with ethanol because it has a higher density (more pounds per gallon) than gasoline, which means that the air-to-fuel ratio is no longer optimum.
If one makes the comparison with an older vehicle or lawn mowers that are equipped with carburetors, then it is probably true. However, almost all new vehicles have electronically controlled fuel injection and the performance should be the same for both fuels.
This was confirmed to me while traveling in Brazil three years ago, when fuel at gas stations was available with ethanol content varying from 10 percent to 90 percent. The drivers I spoke with preferred the higher ethanol content because it was less expensive and gave better mileage.
The reason for this equality in performance has to do with the composition of the two fuels. Gasoline has a much higher proportion of carbon to hydrogen than ethanol. Following combustion of the fuel in the engine, the exhaust gas contains 24 percent of carbon dioxide for gasoline and 16 percent for ethanol. The carbon dioxide requires more heat (energy) to raise its temperature than for other gases present.
Also, gasoline requires more air for combustion, and that means that there is more gas to be heated. The air-to-fuel ratio for complete combustion is 12 percent greater for gasoline. Complete combustion gives the maximum temperature, which also provides for maximum engine efficiency. The two fuels have almost identical maximum temperatures. So what happens to the extra heat for gasoline? It goes out the exhaust!
Webb also goes on to point out the other advantages of ethanol over gasoline, including less pollution, higher octane, and renewability. I just thought it was interesting.