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Why is straight gasoline (or whetever the mixture was before the introduction of ethanol) more efficient (ie, more miles/gallon) than E85? I've known since it's introduction that E85 was less efficient, but why is it?

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Rather than "less efficient" (which is not terribly well defined) you probably mean "lower in energy density", and while I'm happy to leave this open it might be better on the Chemistry site if it ever goes live. – dmckee Jul 18 '11 at 19:55
up vote 2 down vote accepted

There is less chemical energy per unit mass in ethanol than there is in the major chemical components of gasoline.

Standard enthalpies of formation for:

Ethanol: -277.0 kJ/mol

n-Hexane: -40.0 kJ/mol (other hexanes have similar (40-60 kJ/mol) standard enthalpies)

Carbon Dioxide: -393.5 kJ/mol

Water: -285.83 kJ/mol

Combustion of ethanol: 1365 kJ/mol

Combustion of n-Hexane: 4381 kJ/mol

Even taking into account that n-Hexane has a molecular mass ~2 times greater than ethanol, you can see that burning hexane releases a lot more energy.

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Because motor fuel is sold by volume (in most countries at least) one should compare heat of combustion per volume. Another problem is, that if You would construct motors dedicated to Ethanol, the efficency might be better ( eg. by higher compression ratio) . – Georg Jul 19 '11 at 9:08
@Georg. With the higher compression ratio possible with ethanol you can make up a lot of ground and get similar mpg (which is remarakable considering the lower chemical energy density). But that means the engine can no longer run on gasoline. – Omega Centauri Jul 19 '11 at 16:13

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