On Sun, 13 Apr 2008 19:04:55 -0400, "Ed Huntress"
wrote:



More to do with the actual fuel composition than the actual octane
rating, wouldn't you agree?

No, they're talking about octane ratings, and you'll find the same pattern
for gasoline and other high-octane fuels versus diesel fuel, throughout the
literature. The issue here is the differences in performance relative to
different octane rating systems.


You will admit, will you not, that propane, ethanol, and leaded racing
fuel will have 3 very widely varying burn speeds, very different
flamability ranges, extremely differing autoignition specs, widely
disparate energy densities,just as widely disparate specific gravities
and reid vapour pressures, yet very close to the same octane?

SO.
Octane has NOTHING to do with flame front speed, nothing to do with
autoignition temperatures, nothing to do with vapour pressure, power
density,specific gravity, or ANY other property except the resistance
to detonation in a spark ignition internal combustion engine.

It MAY have an effect on the operation of a compression ignition
engine as well - I would say there is little chance that any fuel with
high octane would run successfully in a standard CI engine - but
octane, by it's definition has little if any to do with compression
ignition engine operation.

The big MYTHS out there are several, including but not limited to the
following:

ALL high octane fuels MUST burn slower than low octane fuels.
ALL high octane fuels must produce less power in a low
compression engine than low octane fuels.
Using ANY fuel with higher than recommended octane rating MUST
produce poorer fuel economy
SG of a fuel indicates octane
High octane fuel can be differentiated from low octane fuel by
RVP.

The truth is:
FASTER burning fuels are less prone to detonation, all else
being equal.
SOME high octane fuels will produce just as much power (or
even more power) than some low octane fuels in low compression engines
SOME high octane fuels will provide as good or better fuel
economy in low compression engines than low octane fuels.
RVP in itself has no bearing on Octane

There is so much TOTAL MISUNDERSTANDING of fuel octane and detonation
issues out there in both automotive and aviation worlds.

Your explanations (or the explanations which may or may not exist in
the citations given) may go a lot farther in the explanation of how it
actually works, but without paying for and reading the complete
documentation (which in most cases would go right over the heads of
most on this list) it is impossible to say for certain. They may or
may not disprove (or support) the theory I have been taught and that
has been accepted for years. ANd just because some scientist or
scholar has presented a thesis, and supported it with scientific
method, does not mean that thesis will not be proven totally false (or
substantially in error) by some other scientist or scholar in the
short or long term.

It looks to me like someone gave you some misinformation regarding flame
speeds, but that the rest of what you're saying looks reasonably right. As I
was reading all of this yesterday (I have a respiratory infection, or you
wouldn't catch me dead inside, and not fishing, on a Saturday in April g),
some of what I was taught 40 years ago came back to me. I remembered that
gasoline was harder to ignite by compression than diesel. I had forgotten
that flame speed was slower for gasoline, which was counterintuitive and
struck me funny 40 years ago. But that's the same thing they're saying in
all of this current research. Abstracts or full text, those conclusive
statements are the same either way.

Flame speeds vary with fuel mixture and fuel composition, as well as
with compression pressure. Lean mixtures burn faster than rich. High
compression causes faster fuel burn ( at least in most cases, as
chemical activity goes up with temperature, and compression raises
temperature)
Flame front and rate of expansion are highly variable and influenced
by compression pressure, temperature, mixture, and fuel composition.

Because Gasoline is NOT a tightly defined compund, but (particularly
in recent years) a very wide-ranging chemical soup, it is difficult,
if not impossible, to generalize about how "Gasoline" behaves in an
engine under specific conditions.

It's been 40 years since I was taught a lot of this stuff too.
The issue date on my C of Q says December 17, 1971

I got another dose of all this in the late '70s, when I was covering
materials for _American Machinist_ and most of that was automotive; I took
the time then to re-study engine dynamics. They knew about turbulence and
lean burning, and all of that, by then.
But they have learned a lot more about both since, as well.

What's new is that they have sensors and photogrammetry that let them detect
all kinds of things. Now they can describe not only the outcomes, but also
the how and why of much of it. I was startled to see that one piece of
research identified 669 chemical species in the combustion process. g! Now
they can tell us why different fuels that deliver the same MON or RON
numbers in a standardized test, but which are chemically different fuels,
may perform radically different under real-world conditions. One test showed
a 2:1 ratio in detonation resistance, between two fuels that have the same
RON.

And just as wide a difference in detonation resistance between engines
with the same compression ratios - and many cases where high
compression engines are so much MORE detonation RESISTANT than other
engines of exactly the same design with reduced compression ratios.

None of that changes the fact that cetane and octane are close to being
opposite scales of the same thing, and that gasoline resists ignition, and
burns slower, than diesel in an engine. This explains why that MIT engineer
jokingly said 20 years ago that it's not like using those fuels to start
your charcoal grill. Flashpoint and volatility at ambient temperatures,
which are important in starting a grill, become trivial with direct
injection at high compression ratios.

For all intents and purposes, Cetane and Octane ratings are very CLOSE
to opposites, but there are subtle differences.
Most definitely, behaviour under atmospheric conditions does NOT
predict how a fuel will behave under elevated temperatures and
pressures.


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