IHL,
the water injection does lean out the mixture, but that is not why it adds power (in fact, that effect robs a little of the power gain it creates). This is true in either case, piston or turbine. For turbocharged piston engines, the cooling prevents detonation, primarily, allowing the high boost pressures, but it is the cooling that makes the injection work.
The water injection works because it cools off the inlet air to the pistons or compressor, mostly due to the evaporation of the water.
The effect of cooling off the air is strong, an engine gains power by about 1% for each degree C that we cool the inlet air.
Alcohol is added to the water so that it does not freeze, and has little effect on the general purpose of the spray (it does burn, so it has a bit of fuel effect, and on pistons it does increase the octane of the mix, and so helps prevent detonation in addition to the cooling effect).
Last edited by NickLappos; 16th March 2005 at 10:26.