09/15/2003

Combustion and Equilibrium

In reading non HRM or Fast And Fharting Racing mags, you will run continuously
across the term and concept of chemical equilibrium.  Lets go through that.

Combustion starts effectively on the compression stroke near bottom dead
center.  The upraising piston generates turbulence which greatly speeds the
introduction of fuel and oxygen.  The actual combustion reactions begin around
350 degrees more or less depending on the turbulence and time.

By 500 C, a cold blue flame combustion will have started with normal
hydrocarbons.  This is primarily the breaking free and oxidation of hydrogen
from the fuel.  It should be noted that combustion is insensitive to pressure,
and the peak temperature is influenced less than 5% by going from atmospheric
to 20 atmospheres at combustion.  La Chatilers Principle - for later
reference.

Compression simply adds energy to the charge to encourage combustion.  The
pressure matters not FOR COMBUSTION.  The turbulence and the thermal energy
assist greatly in breaking down LIQUID fuel and preparing it for combustion.

In a macro sense, early combustion is endothermic, with exothermicity only
after the chains begin to avalanche.  This release of energy brings us to the
peak temperature of the flame and seeds the after burning for final energy
release.

Now let us observe Dihyrogen-Monoxide in its formation and distruction.  

Assume that all molecules essentially break down to atomic level.  Free
monatomic hydrogen and free monatomic oxygen molecules have a positive heat of
formation - meaning when broken free from the baseline state, need to have
energy added to the molecule to free it.  The oxidation of free hydrogen and
free oxygen gives up energy and the resultant dihydrogen-monoxide is formed
with a negative heat of formation of a very low value.  Thus the energy
released is from the high heat to the low heat stage.

This is one side of the reaction of H and O.  This takes place at a relatively
low level of energy.  Now let us look at the other side of the equation.

Given a higher energy than realized from formation, dihydrogen-monoxide will
dissocatiate into free Hydrogen and free Oxygen.

The higher the available energy, the greater number of reactions.  As
temperature changes, one or the other side of the reaction becomes preferred.
In the case of dihydrogen-monoxide, lower temperatures favor it production,
higher temperatures favor its dissocation.  As long as the energy is high
enough, both reactions will take place, leaving much less useable energy in
the form of sensible energy.  

When the temperature ( energy ) falls below a certain point, disocation will
no longer occur and the sensible energy will rapid increase.

Thus the lower the temperature, the more energy is available for sensible
energy, and the more efficient and powerful the charge becomes.