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?
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.