I am not a chemist but I am looking for a satisfactory explanation of
the chemstry of adding water to the air/fuel mixture in a petrol
internal combustion engine.

From researching on the internet I understand there are two effects:

1. Cooling to reduce the volume of incoming air. (increasing the air mass flow)
2. Knock reduction (in the same way as a lead additive) promotes an
even combustion allowing for timing advance and more power output.

My interest is that I am considering the possibilty of adding a system
to my car wherby water held at a fixed pressure is sprayed into the
inlet manifold in the correct proportion (i.e by having the control
unit intercepting signals from engine rpm / air mass meter signal /
fuel injector duty cycle ).

If you want some parameters the details of my car are:

2 Litre naturally aspirated engine 4 cylinders/4 injectors does 43mpg
cruising at 65mph so will flow approx 7 litres of petrol an hour which
is about 1.75L per injector of fuel. I dont know how much air will be
flowed in this time.



Any supplementary pointers to text books etc that might have covered
this welcome. I have installed one such system already that runs off
manifold vacuum and found no change in hydrocarbon emissions or power
output on the rolling road, although it consumes approximatly a litre
of water every 50 miles irrespective of engine speed/load conditions.

I have heard that a trace of water is essentual for most chemical
reactions including combustion. Since air contains some water vapor;
my guess is a tiny milage and/or performance improvement with most
engines, with more water added. If you supply the same ammout of water
while idling as other conditions; improvement is likely to be
negligible. A freind is of the opinion that drops of water will knock
the carbon deposits off the inside of the engine. My guess is there is
some risk of damage if large drops of water hit very hot engine parts.
  Neil

I don't have the slightest idea if your proposal is feasable, but in
theory, H2O will supply additional fuel, the hydrogen and oxygen
combusting.
Years ago, I saw a design for an oil heater that had water dripping
into the flaming area with this effect.

I wonder if there are any studies about variances in power with gas or
diesel engines due to the humidity of the air?

I am a professional chemist, and the idea of adding water to a
gasoline/air mixture to improve combustion doesn't make any sense.
Perhaps, you need to ask someone with technical expertise in internal
combustion engines, and not chemisty.

I would recommend checking out MIT's free online class IC engines:

http://ocw.mit.edu/OcwWeb/Mechanical-Engineering/2-61Spring2004/CourseHome/index.htm

Buy the book:  Heywood, John B. Internal Combustion Engine
Fundamentals. New York: McGraw-Hill, 1988. ISBN: 007028637X.  We used
the same book at U Wisc-Madison where I received my BS in Mech Eng in
2001.

Water is incompressible and you risk serious damage to your engine if
you add it to your fuel/air mixture.  That is why you should never
drive your car through deep water and why Hummvees have snorkels on
them.

The minute amount of water present in air as vapor will not damage
your engine, but you will not gain anything from increasing the
content.

Heywood's book is great and can explain my answer using hard numbers. 
Hope this helps!

Wizzlon --

The technique has been used in high-compression engines.  It
originated with  engines in turbocharged and supercharged aircraft
during World War II, primarily to reduce detonation (engine knock)
that is catastrophic in high-compression engines --
http://www.rallycars.com/Cars/WaterInjection.html

Suggested Google search strategy:
water "high-compression" engine

Best regards,

Omnivorous-GA

As arch2's comment implies, there is no direct "chemical" effect of
adding water to the fuel-air mixture in an internal combustion engine.
 Contrary to myoarin's comment, water will not "supply additional
fuel" to the system.  Water is, in fact, one of the *products* of
combustion of hydrocarbon fuels, and the combination of hydrogen and
oxygen in a water molecule is the lowest energy state for these
elements under ambient conditions.  Any water that's added to the
system before combustion simply exits the system unreacted as part of
the exhaust.

The addition of water *does* have a physical effect, namely the
reduction of the temperature (due to the heat capacity and latent heat
of vaporization of water).  This, in turn, reduces the likelihood of
detonation or premature ignition in the engine.  See
http://www.rallycars.com/Cars/WaterInjection.html

I am a Mechanical Engineer working as the Technical Manager of a
Petroleum Marketing company. Adding water or water vapour drastically
reduces the Octane Number rating of the charge sent into the Engine
Cylinder. The engine will develop cracks due to 'Knocking'. Conversion
of Water into Hydrogen and then using that Hydrogen for combustion (it
needs oxygen to burn) is a very long process and is not applicable in
this case. Please do not try it. It is a wasteful experiment.