I ran water injection in the 70's. It's big sell was that it would 
prevent/remove carbon deposits on the pistons as it would soften it, etc. 
Carburetor based engines had a lot of this problem, and there were other 
products from GM to pour in occasionally to deal with it too. They did 
advertise more power, economy, good looks ;-) etc., but that was suspect.

It did keep the pistons looking clean, but I concluded that the mileage gains 
weren't significant or were still dependent on how you drove, just in a 
different way. I finally took it out because it would cause a reduction in 
power at higher rpms as in passing. I could switch it out and the difference in 
power at 70 mph was significant.

These systems operated under acceleration and when the manifold vacuum dropped, 
not all the time. I would have a hard time thinking there would be a cooling 
problem, since the engine will run at block temperatures at start at freezing. 
  Manifold and head heat help to atomize the fuel, but I doubt a little water 
mist would make that much total temperature difference. I never saw any while 
operating, and anything beyond a fine mist at higher rpms (ie acceleration) and 
it would stumble or stall, mostly due to starving it of oxygen replaced with 
water. Of course at some point you would be 'putting the fire out'.

With fuel injection the air/fuel mixture is better regulated and I doubt there 
would be much improvement in mileage, and carbon buildup isn't much of a problem.

And there are longer term negatives... Upper cylinder and valve rust, older 
engine blow-by contaminating the oil with water, intake manifold rust, freezing 
or rupture with more possible damage, etc. I've heard of damage to O2 sensors 
with steam in the exhaust. And of course there's damage to the catalytic 
converter, exhaust rusting, and water in the exhaust mixes with the gases and 
becomes acidic, enhancing rust...

With computer controlled engines, the water would displace some air, and the 
computer would reduce fuel to compensate, leading to less power. If taken to 
far, and affecting the mixture temperature, there would be incomplete 
combustion (as in cold start) which would affect the mix of polluting gases out 
as well as making the water/exhaust mix more acidic.

This is all probably why these water systems disappeared with the carburetor.


James Newton wrote:
> On the other hand, I've often wondered if injecting a LITTLE water wouldn't
> be very good for mileage. My dad experimented with it and found that it did
> increase his MPG by a few small numbers: 2 or 3 MPG.
> 
> The (somewhat reasonable) argument is that the fire in the cylinder is
> hotter than it needs to be (obvious since the cars cooling system is pretty
> much non-optional) and that the extra heat can be used to boil off some
> water, increasing the volumn of gasses expanding and the pressure.
> 
> Too much water injected will lower the internal temperature to a point where
> combustion will be incomplete and expansion and pressure will decrease.
> 
> There may be other negative side effects of the injected water such as
> calcification and other crap building up on the cylinder crown and inside
> the head.
> 
> But the cooling is interesting to me... I wonder if you could make an air
> cooled, high power engine where the bulk of the cooling is actually done by
> injecting water...
> 
> --
> James.
> 
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