Water Injection Combustion Chemistry
As has been discussed before, the use of any type of compressor, whether turbocharger, belt driven centrifugal supercharger, roots compressor or lysholm, heats the air during compression. The amount of heat is a function of the pressure ratio of the output to input of the compressor and the efficiency of the compressor. Heating the air has side effects: reducing the density of the air for a given manifold pressure, causing less power and resulting in the need for reduced spark timing or an over rich air/fuel mixtures (which cost power) to control detonation. Detonaton can result in blown head gaskets, destroyed pistons, rods, bearings, etc.. By cooling the compressor outlet air, we regain the lost power due to all of these factors and reduce the risk of detonation. There are a number of ways to cool the air: 1) one of the simplest is just to inject water into the hot air stream and use the heat of vaporization to cool the air; 2) use alcohol injection in the same manner as 1) but which has more benefits in that it raises the average octane level of the total fuel charge and it will take you right to the fuel class (unless you are already there!); 3) air to air intercooling which has plumbing and air flow issues, and 4) water to air intercooling which can lower the charge air temepratures even below ambient but with the penalty of adding weight to the car, as well as the plumbing issues and failure modes of the plumbing and pumps.
Did you ever think about what happens in the combustion chamber when you inject water into the charge air? In addition to cooling the air, they are many other things going on. The presentation below, taken from a web site with the authors permission, I might add, describes in understandable terms what happens in the combustion chamber. Wish I was smart enough to have written it!
I wish to thank the author for letting me use his words. Every little bit of knowledge helps us to run faster without blowig any thing up and scattering car parts all over the track.
"[Reformatted slightly for readability, but otherwise as posted.]
From: Robert Harris
Subject: Water and its effect on combustion.
Date: Mon, 10 Jul 2000 10:24:08 -0700
Let us take a quick look at ignition. Those who have a Heywood can look it up
- mines on loan so going by memory. The first thing that happens is a plasma
cloud is formed by the arc consisting of super heated electron stripped atoms.
When this cloud "explodes" a ball of high energy particles is shot outward.
The highest energy particles are the hydrogen atoms - and they penetrate the
charge about 5 times as far as the rest of the particles. As they lose energy
and return to normal temps - about 5000 k - they begin to react chemically
with any surrounding fuel and oxygen particles. The effectiveness of spark
ignition is directly related to the availability of free hydrogen. Molecules
containing tightly bound hydrogen such as methanol, nitromethane, and methane
are far more difficult to ignite than those with less bonds.
During combustion - water - H2O ( present and formed ) is extremely active in
the oxidation of the hydrocarbon. The predominate reaction is the following:
OH + H ==> H2O
H2O + O ==> H2O2
H2O2 ==> OH + OH
Loop to top and repeat.
The OH radical is the most effective at stripping hydrogen from the HC
molecule in most ranges of combustion temperature.
Another predominate process is the HOO radical. It is more active at lower
temperatures and is competitive with the H2O2 at higher temps.
OO + H ==> HOO
HOO + H ==> H2O2
H2O2 ==> OH + OH
This mechanism is very active at both stripping hydrogen from the HC and for
getting O2 into usable combustion reactions.
Next consider the combustion of CO. Virtually no C ==> CO2. Its a two step
process. C+O ==> CO. CO virtually drops out of early mid combustion as the O
H reactions are significantly faster and effectively compete for the available
Then consider that pure CO and pure O2 burns very slowly if at all. Virtually
the only mechanism to complete the oxidization ( Glassman - Combustion Third
Edition ) of CO ==> CO2 is the "water method".
CO + OH ==> CO2 + H
H + OH ==> H20
H2O + O ==> H2O2
H2O2 ==> OH + OH
goto to top and repeat.
This simple reaction accounts for 99% + of the conversion of CO to CO2. It is
important in that fully two thirds of the energy of carbon combustion is
released in the CO ==> CO2 process and that this process occurs slow and late
in the combustion of the fuel. Excess water can and does speed this
conversion - by actively entering into the conversion process thru the above
The peak flame temperature is determined by three factors alone - the energy
present and released, the total atomic mass, and the atomic ratio - commonly
called CHON for Carbon, Hydrogen, Oxygen, and Nitrogen. The chemical
reactions in combustion leading to peak temperature are supremely indifferent
to pressure. The temperatures and rates of normal IC combustion are
sufficient to cause most of the fuel and water present to be dissociated and
enter into the flame.
As can be seen above, water is most definitily not only not inert but is a
very active and important player in the combustion of hydrocarbon fuel.
Ricardo and others have documented that under certain conditions ( normally
supercharged ) water can replace fuel up to about 50% and develop the same
power output, or that the power output can be increased by up to 50% addition
of water. This conditions were investigated by NACA and others for piston
aircraft engines. It is important to note that these improvements came at the
upper end of the power range where sufficient fuel and air was available to
have an excess of energy that could not be converted to usable pressure in a
As a side note - Volvo recently released some SAE papers documenting the use
of cooled EGR to both reduce detonation and return to a stoic mixture under
boost in the 15 psi range - while maintaining approximately the same power
output. Notice - they reduced fuel and still get the same power output.
When you consider that EGR consists primarily of nitrogen, CO2, and water ( to
the tune of about two gallons formed from each gallon of water burned ), you
might draw the conclusion that it also was not "inert". They peaked their
tests at about 18% cooled EGR - which would work out to about 36% water
injection and got about the same results under similar conditions that the
early NACA research got."