pre-charger water injection

[Brutus]

No, it can?t be used up. A little warmed up, yes. But I?ve been told that chilling the intake charge is just a side effect of water injection. The main cooling comes from the latent heat, when the water evaporates in the combusting chamber. That?s clear if you consider that the specific heat is 4.18 KJ/kg/K against the 2256KJ/kg latent heat.
What?s not exactly clear to me is why you inject proportional so much more water pre-charged.
And, at what point becomes oxygen displacement an issue?