In article , "Capt. Geoffry
Thorpe" The Sea Hawk at wow way d0t com wrote:

Water can reduce the charge termperature as it evaporates in the cylinder
which can increase the charge mass a little, but mostly it improves knock
resistance by cooling the charge and increasing the specific heat of the
charge mass. But to really take advantage you have to either increase the
compression ratio or boost.
NOx is recduced somewhat since the peak temperatures are reduced.

If you are trying to change the fuel chemestry to get more energy out - you
have to put energy in. Lose - lose.

If you are trying to make the fuel burn "better" to make it "all" burn -
gasoline burns just fine. Unburned fuel in the exhaust comes from three
primary sources - mixture that is pushed into the crevices around the piston
/ sparkplug / etc. where it can't burn (it comes out of the crevices during
the exhaust stroke), mixture that is quenched at the cylinder walls due to
heat loss. And excess fuel from running rich. The bulk of the charge mass
burns just fine. Cow magnets, magic additives, whatever aren't going to
change any of that.

Actually, ( I was trying to make it simple ) either compression or boost can
be expressed as as BMEP, which takes both into account.
As BMEP rises you get better combustion of any unburned fuel from being
too rich (not a factor in today's autos) , but the higher pressure also
suppresses
the boundary layer at the cylinder wall, giving better combustion there also
and lower unburned hydrocarbons as a result.
Using water injection, as you say, allows you to increase BMEP without
risking knock because it cools the intake charge, acting (as I said) like
an octane improver.

--
Harry Andreas
Engineering raconteur