06/03/03

long rant - save till bored.

Ricardo defined effective combustion as the time when the pressure diverged
from the (Ideal Gas) PVT curve.

Thus it would begin sometime before TDC and extend well past TDC terminated by
time or the opening of the exhaust valve.

Combustion actually starts much earlier - given time - when the fuel meets the
oxidizer.  Oily rage syndrome.

Lets look at some of the chemistry involved.  

As the charge absorbs heat, oxidation of the charge increases.  Oxidation is
always an exothermic ( heat releasing ) operation.  But - early combustion is
always an endothermic ( heat absorbing ) operation.  The charge will absorb
heat from combustion as fast as it is released to create new chemical events.
This leads to cool or "blue flame" combustion at around 500c.

After a period of time, the amount of energy released by oxidation will be
sufficient and the charge will begin to raise in temperature terminating in a
full flame.

Spark ignition always occurs during the cool flame/early combustion/induction
period.  The spark does not start the reaction - that is already underway.
What it does is avalanche the reaction by releasing free radicals and creating
a hot spot.

All of the heat needed for combustion prior to the spark comes from charge
heating - particularly compression.  The warmer the coolant, the less time it
takes.  Ditto for compression.  Compression is the major heat source for
combustion.  The higher the compression, the greater the heat, the faster the
charge starts to burn.  High enough compression will result in the charge
going to full flame without a spark.

Not Diesel - but Homogeneous Charge Compression Ignition.  If the compression
ratio is high enough - enough energy will be put into the charge for
combustion to start thru out the charge.

Diesel works by delaying the fuel introduction until cylinder heat is high
enough to ignite it.

More heat = shorter induction.  The more the external heat in the charge, the
less lead angle ( spark advance ) needed for ignition.  ( Always assuming that
you wish the peak pressure to occur at the optimimum point ).

The cylinders and chambers initially heat the charge but somewhere around
midstroke, the charge temperature rises above the surface temperatures and
they start to cool the charge.  Remember the truism that iron head make better
milage - but aluminum heads make more power?

Iron conducts much less heat away from combustion than aluminum.  If you are
detonation limited, the aluminum would carry more heat away during this stage,
delaying HCCI until past spark release, thus allowing a higher compression or
more intense charge.  The Iron, conducting less heat - would require less fuel
in throttled conditions.

Same for air cooled engines - the hotter heads allow better economy - but make
less power than for a liquid cooled head.

Of course, all of these considerations are about as meaningful as a fueling
merit discussion on diy-efi.  The advent of ceramic coating brought into full
frutation by the Army's tank corp in the eighties and heavily advocated by
Dave Vizard since has totally changed the picture.

A properly coated chamber of any size or design ( valves, chamber, piston )
has incredibly small heat losses during early combustion.  Whether the head is
air, liquid or uncooled, aluminum or iron or brass - means virtually nothing.

Properly coated, aluminum heads advantage is strictly weight.

Consider that 50% of all compression - regardless of the geometric ratio or
the intake supercharge - occurs in the final clearance volume - from 2
clearance volumes to one clearance volume at TDC.  ( Because of previous
charge heating - this can actually be significantly more than 50% ).

Virtually all of the work is done very close to tdc.  Contemplate that
carefully.  

Now lets look at the chemistry taken place during the induction period.
Hydrogen is being oxidized.  Very little carbon.  Hydrogen oxidation is
significantly faster that carbon oxidations.  Carbon is buried in the
hydrocarbon molecule.  All the hydrogen must be stripped from the particle
before the carbon can be oxidized.  

Free hydrogen is important.  Most mixtures do not include free hydrogen being
inducted - so to burn, we must have free-able hydrogen.  Methods to break
hydrogen free are heat on weak molecules and spark ionization.  We use both in
otto combustion.  First we build enough heat to cause some molecules to break
down and release hydrogen.  Then, sometime later, we avalanche the breakdown
by an ionizing spark.

Oxygen is critical for breakdown.  When heated, O2 will pick up a charge and
will extract a hydrogen from the molecular mix - forming OOH - then H2O
somewhat later.  Plus around a zillion lesser reactions.  We need available
oxygen to oxidize the fuel.  But its not real useful without freely available
hydrogen.

It is virtually impossible to combust CO in pure oxygen - until a small amount
of water vapor is introduced.  Hydrogen source.

The speed of the induction period is proportional to the amount of free
hydrogen in the charge.  More free hydrogen - the shorter the induction
period.

There is a time relationship - bounding about the availability of oxygen to
free hydrogen.  The shorter to time for a hydrogen to make a date with oxygen,
the shorter the induction time.  To a point.

Oxidation is exothermic - but combustion is endothermic until we reach a
runaway point.  Speed increases with available oxygen until about 10% oxygen
and then the excess oxygen becomes a dilutation.  This is not flame speed -
this is mass consumption we are talking about.  Even tho the flame speed is
lower because of less hydrogen, the rate the charge is consumed is increased
peaking about 10% or less excess air.