> The The use of water injection with petrol engines seems
> to mainly be useful with turbo- or super-charging, and allows higher boost
> ratios to be used without causing detonation. So it's a means of getting
> more power, but it wouldn't help a normally-aspirated engine much, I suspect,
> and I can't see it would do anything for fuel economy.

The basic principle of water injection is to cool the air going into
the cylinder, making it denser and therefore having more oxygen to
support more fuel to make more power. When a mechanical supercharger
is used, it heats up the air in the act of compressing it, so water
injection is one method to get that loss back again.

With any engine (naturally aspirated or forced induction), maximum
power is made when the (dynamic) compression ratio, spark advance,
etc is just before the onset of detonation. One way to get that is to
have the engine right on the edge and then add a little water to cool
it back into the "keep engine intact" zone. (kinda like torqueing a
bolt by doing it up till it strips and then backing off half a turn).
In a new fangled car the same thing is done with a knock sensor and
ignition retard. These things are all to make more (or rather get the
most) power from the engine and to make power you are running it rich
anyway.

There is an alchemist form of water injection that claims to do
one of the things that exhaust gas recirculation does, which is to
take up some of the space in the cylinder with an inert gas (exhaust)
and effectively make the engine a bit smaller.

If you really want to cool an intake charge, Nitrous Oxide is a far
more fun way. :-)

Steve.