01/05/2004

Ignition and Combustion


Dave likes to call combustion a square dance where oxygen and hydrogen
dow se dow, switch partners and have more fun than a gang of queers at a
hot dog factory.

It takes a certain amount of energy to break a fuel molecule down enough
to combust.  This energy takes time.  Temperature ( not pressure )
supplies the energy to break down the molecule.  The greater the energy,
the faster the molecule breaks down.

Lag time has been mentioned here.  All molecules have an auto-ignition
temperature - where in the presence of oxygen - will combust.  Once you
bring a molecule to that temperature, you must wait some time for the
reactions to occur that breakdown the molecule - lag time.  If the
temperature is greater than the auto-ignition temperature, the molecule
will burn faster than its ai lag time.

Most fuels are not pure hydrocarbon of one type - but a blend of
different hydrocarbons.  Each different hydrocarbon has a different lag
time and auto-ignition temperature.  So ( without a flame ) as the
temperature increases, each different type of fuel auto-ignites in
"layers".  

Combustion "starts" when the intake valve closes on compression.  Heat
is pumped into the fuel by compression.  Fuel starts to react with the
oxygen and combustion begins long before the sparking event.  How much
fuel in the chamber with the mass of air and its closeness (density)
determine how fast the temperature will climb, until a sustained chain
reaction(s) take place and a flame appears.  The lower the density of
the charge, the further apart the molecules are and the longer the time
it takes to heat everything up to combustion.

Now, when the plug fires, oxygen and combustion do not occur within the
plasma field.  The energy is so intense that all molecules present in
the arc are broken down to electron stripped pure elements.  When the
plasma ball collapses, the elements are shot outward with an explosive
force of several hundred tonnes.  These "radical" elements, hydrogen
penetrating 5 times further into the charge than oxygen, greatly enhance
the developing free radical pool, and combustion is now avalanched into
a flame front after a delay.

Now a low density charge like at light throttle, requires either a major
increase in heat from compression, or a fuel change to develop peak
pressure near tdc.  This is part of the reason that high compression
engines may develop more efficiency or power at throttle than a low
compression engine.

Now, if you add toluene to the fuel mix, it has both a high autoignition
temperature and a long lag time.  The fuel mix does not burn with a
single fuel characteristics, but as a strata of different fuels burning
at different times.  The toluene resists combustion till the very end
and thus enhances knock.

Key concept.  Combustion prior to flame and carbon oxidation, is
endothermic - takes energy to occur.  Only after all hydrogen is
essentially stripped from the molecule, does the remnants become
exothermic and give up massive amounts of heat from oxidation.

Fuels such as toluene, in the end gasses, take massive amounts of energy
from the growing flame to bring them to a combustion state.  They behave
as heat sinks, cooling the flame even as the mass of burning molecules
is increasing.

Thus, if the flame front can get to them before they autoignite, normal
combustion occurs.

Acetone, on the other hand, has a near methanal level octane, an
outrageous auto-ignition temperature and a very low flash point.  No
flash - Acetone is slower igniting than Toluene.  Add fire - and it
burns twice as fast as most fuels.  Thus, the use of small quantities of
acetone in Nitromethane Blends - to light the charge sometime in this
century.




> >From: "blitz"
>> >> No one has yet stated why it is a good idea to add something to cause
>> >> combustion to happen "faster". Why is faster better? Are well-tuned
>> >> engines unable to extract all the potential from the fuel, and "faster"
>> >> makes them more efficient. There seems to be an underlying assumption
>> >> that "faster is better" in the realm of combustion, but no one has said
>> >> *why*. So why?