Octane--Attn: Mr. Bubb...

[fahrvegnugen at cox.net (David Utley)]

-----Original Message-----
From: scirocco-l-bounces+fahrvegnugen=cox.net@scirocco.org
[mailto:scirocco-l-bounces+fahrvegnugen=cox.net@scirocco.org]On Behalf Of
matt
Sent: Saturday, July 02, 2005 8:04 PM
To: scirocco-l@scirocco.org
Subject: Octane
Another interesting link...
 http://www.prime-mover.org/Engines/GArticles/octane.html

One excerpt...
There are six things to consider when comparing hydrocarbon fuels:
1. Volatility. In short, what's the fuel's propensity to vaporize. This
effects the ability to easily mix the fuel with air and the fuel's tendency
to vapor-lock. It also determines the pollution characteristics of the fuel
where evaporative pollution is a concern.
2. Pre-ignition & knock resistance. Referred to as "Octane value." How much
energy does it take to get the fuel burning - how much does it resist
auto-ignition from compressive heat? Also, what is the rate of burn of the
fuel (which affects the rate of pressure rise)?
3. Energy content. How much energy can be extracted from the fuel as a
percentage of its volume or mass.
4. Heat of evaporation.
5. Chemical stability, neutrality, and cleanliness. What additives does the
fuel contain to retard gum formation? Prevent icing? Prevent corrosion?
Reduce deposits?
6. Safety
The first three factors are often confused and interrelated when, in fact,
they measure three completely separate things. There is no natural
collelation between them.
Excerpt Two...
WHAT OTHER FACTORS AFFECT OCTANE REQUIREMENTS?
Combustion chamber design, localized hot spots, piston speed, and a host of
other factors can all contribute to a engine's propensity to ping.
Additionally, in the aviation world, altitude extremes and super/turbo
charging affect octane requirements. Increased induction pressures (such as
would be encountered in a turbo/supercharged engine) cause more rapid
flame-front propagation. Likewise, decreased exhaust pressure (as would
occur at altitude) also tends to increase flame-front propagation speed.
Both of these effects can combine to raise octane requirements - especially
at altitude.
Note that the latter effect also affects the proper fuel/air ratio for BEST
ECONOMY operation. The reason is left as an exercise for the reader.
Final Excerpt...
WHAT DOES A LOW OCTANE VALUE MEAN, TO ME?
In the absolute worst case, if the fuel is too low octane, it may
spontaneously ignite before the spark plug fires due to thermal rises from
the heat of compression or from hot spots in the cylinder itself. This kind
of ignition is called pre-ignition (as opposed to knocking) and is a
pathological case which will just turn an engine to scrap. Diesel fuel is
low enough octane that mixing it with gasoline can cause pre-ignition!
What usually happens, and what we usually call knocking or pinging is that
the fuel/air mixture does not ignite before the spark plug fires but does
ignite spontaneously after that. The sparkplug fires and this causes an
immediate, rapid, rise in combustion chamber pressure. This causes fuel on
the other side of the flame-front to ignite before the flame-front reaches
it. In turn, this causes combustion chamber pressure to rise even more
rapidly. The result is an explosion inside the combustion chamber as opposed
to the desired rapid burning.
WHAT DOES A HIGH OCTANE VALUE MEAN, TO ME?
A high octane rating ensures that it takes a REALLY hot ignition source to
ignite the fuel (such as a spark plug or the flame-front itself) and not
just the rise in pressure & temperature that's a result of normal
combustion. Note that the thermal rises in the cylinder are in direct
proportion to the compression ratio of the engine (more below). The higher
the compression ratio, the higher the octane of the fuel that's needed.
Again, if the mixture in a gasoline engine ignites before the spark plug
fires, we call that "pre-ignition." Pre-ignition can damage an engine before
you finish reading this sentence. To reiterate, what we're really concerned
with is called "knock" and that's the spontaneous ignition of the fuel-air
mixure ahead of the flame-front as a result of the rise in cylinder pressure
caused by the onset of ignition (caused by the firing of the spark plug).
So, although many articles did say you should follow your owners' manuals
recommendations, none of those articles touched on modified cars.  Granted,
there are probably WAY too many vairations to map this out in a constructive
manner, but I think you see what I am getting at.  If you have an exhaust
system, cam, Evans coolant (IE higher combustion temps), a match-poted
engine, and drive like an 18-year-old with a new toy, then you prolly could
benefit from the higher octane.  Since I fit this description, perhaps this
sheds some light on why my experiences do not duplicate other writers'
experiences...

Mr. Bubb, I would be interested in your take on this if you have any...

Cheers,
  David