From: Robert Harris <bob@bobthecomputerguy.com>
To: DIY_EFI@lists.diy-efi.org
Subject:
Water and its effect on combustion.
Date: Mon, 10 Jul 2000 10:24:08 -0700
Message-ID: <9ptjms0uu4oe292mpk6a6vhm2hn8bu9h1j@4ax.com>
Let
us take a quick look at ignition. Those who have a Heywood can look it up
mine is on loan so I am 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
oxygen.
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
mechanism.
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 [by] up to about 50%
and develop the same power output, or that the power output can be
increased by up to 50% [with the] addition of water.
These 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 timely manner.
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 [the amount of] 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.
