It's also an oxygenation agent, and replaces MTBE (methyl tert-butyl ether) which ... oh, also substituted for tetraethyl lead. (I thought MTBE was an anti-smog treatment.)
MTBE turned out to leach into groundwater quite readily and there was a pretty widespread outcry about it in the early aughts. Again, ethanol is a replacement (mentioned in the Wikipedia article above).
And yes, since anti-knock agents effectively slow combustion (and ethanol has lower energy-density than petroleum), net fuel efficiency is slightly reduced on a volumetric basis. Net effect remains better overall engine performance.
https://en.wikipedia.org/wiki/MTBE_controversy
MTBE turned out to leach into groundwater quite readily and there was a pretty widespread outcry about it in the early aughts. Again, ethanol is a replacement (mentioned in the Wikipedia article above).
And yes, since anti-knock agents effectively slow combustion (and ethanol has lower energy-density than petroleum), net fuel efficiency is slightly reduced on a volumetric basis. Net effect remains better overall engine performance.