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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1$\begingroup$ 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. $\endgroup$– dmckee --- ex-moderator kittenCommented Jul 18, 2011 at 19:55
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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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3$\begingroup$ 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) . $\endgroup$– GeorgCommented Jul 19, 2011 at 9:08
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1$\begingroup$ @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. $\endgroup$ Commented Jul 19, 2011 at 16:13