clare at snyder dot ontario dot canada wrote in message
...
On Sun, 13 Apr 2008 11:28:11 -0400, "Ed Huntress"
wrote:


clare at snyder dot ontario dot canada wrote in message
. ..
On Sat, 12 Apr 2008 01:38:50 -0400, "Ed Huntress"
wrote:

Something is funny here, Tim. I wonder if those guys told your dad what
actually happened?

The cetane rating of gasoline is so low that, if you can ignite it at
all
in
a diesel, the gasoline burns so slowly that the engine knocks like
crazy,
and it's still burning past the end of the power stroke. Thus, the white
smoke that someone else mentioned. At low temperatures, a diesel fueled
with
gasoline may not ignite at all.

At the risk of oversimplifying, octane and cetane have roughly opposite
burning characteristics. A high cetane rating (as in premium diesel)
indicates that the fuel ignites easily from the heat of compression and
that
it burns quickly once ignited. In fact, the cetane rating is based on
the
time it takes for fuel ignited at high temperature and compression to
reach
a specified cylinder pressure.

An octane rating, which is very low in diesel and much higher in
gasoline,
indicates the *resistance* of the fuel to burning under those same
conditions. Higher octane allows higher compression ratios without
premature
ignition from the heat of compression. However, what octane rating
really
measures, if I recall correctly, is the speed with which a flame
progresses.
High octane, slow burning.

Your recollection is not quite correct, which is where the confusion
comes in. Octain has NOTHING to do with the temperature at which the
fuel will auto-ignite, nor the speed of burn, when you get right down
to it. It has everything to do with resistance to dissassociation, and
resultant detonation.

I don't want to turn this into a battle of citations, but what you're
saying
disagrees with the research -- particularly the recent research. Because
of
the interest in HCCI engines a lot of work is being done in this area,
because octane is suddenly of interest in compression-ignition and
spark-assisted compression-ignition engines. Here are a few remarks from
recent research papers:

================================================ ======
[The Effect of Prf Fuel Octane Number on Hcci Operation - 2004]

"The test results show that, with the increase of the octane number, the
ignition timing delayed, the combustion rate decreased, and the cylinder
pressure decreased."

[A Method of Defining Ignition Quality of Fuels in Hcci Engines - 2003]

"The higher the OI [octane index], the more the resistance to autoignition
and the later is the heat release in the HCCI engine at a given condition.
When the engine is run with a boost pressure of 1 bar and with the intake
air temperature maintained at 40\mDC, K is highly negative and fuels of
low
MON, such as those containing aromatics, olefins or ethanol, have a higher
OI and ignite later than paraffinic fuels of comparable RON."

[Note that the study above examines the burn characteristics of aliphatics
versus paraffinics -- more about that later.]

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.

Ethanol., intrinsically has a higher
octane than gasoline, yet they are stating low RON fuels containing
ethanol.

Where do they say that? MON, RON, and OI are not the same things. The most
useful octane rating, based on relative performance in an engine, is the
Octane Index. Ethanol has a higher OI than gasoline.


Aromatics and olefins DO behave differently than parafins, and MON and
RON are different - which is why the typical automotive octane ratings
(ron+mon/2) are somewhat misleading and confusing.
Kinda like aircraft octane, where you have a rich and a lean number.
Which actualy tells you more than ron+mon/2.

When you normalize for performance in an engine, you get OI.

ANyway - enough arguing
There is always going to be more research, with morefindings to be
proven or disproven when it comes to fuel and combustion technology.
Suffice it to say taht gasoline in a compression ignition engine is
NOT a good idea for reasons pertaining to the difference in combustion
characteristics alone - and then there are the issues of lubricity etc
which can be circumvented by mixing lubricating oils with the fuel lie
in typical 2 stroke (fuel lubricated) gasoline practice.

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.

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.

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.

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.



[Knock in Spark-Ignition Engines: End-Gas Temperature Measurements Using
Rotational Cars and Detailed Kinetic Calculations of the Autoignition
Process - 1997]

"It is found that calculations with different RONs of the fuel lead to
different levels of radical concentrations in the end-gas. The appearance
of
the first stage of the autoignition process is marginally influence by the
RON, while the ignition delay of the second stage is increased with
increasing RON."

[There are two stages to controlled autoignition, and the final one is
delayed as octane number increases.]


ANd this disproves or dissagrees with my explanation how? The "radical
concentrations" refer to what? Higher RON reduces the radical
concentrations, which delay the "second stage of autoignition" -
which in spark engine parlance is "detonation"

No, they're talking about controlled autoignition. Detonation is
uncontrolled autoignition that progesses to an explosive, or near-explosive
state. And they're talking about diesels, not SI engines.


In my words, high octane gasoline resists thermal dissassociation of
the hydrocarbons, resulting in lower production of hydrogen radicals
and a marked reduction in the propensity of the engine to detonate.

But it also slows down the flame front, which contradicts, apparently, what
you were taught. This appears over and over again in those research papers.

FWIW, there is a lot of research that picks apart the multiple stages of
intermediate combustion products, which are heavily involved in the rate of
combustion and which vary widely between fuels. The simplified idea that the
hydrocarbons are just dissociated and then burn doesn't cut it at the level
of engineering science they're working at today.


You are talking DIESEL ENGINE RESEARCH - where autoignition is a good
thing (particularly stage one) Second stage autoignition (also
referred to as detonation) is apparently also an issue in the engines
in question.

They aren't talking about detonation. There are two stages of controlled
autoignition.

snip

This could go on, but you'll find it interesting to pick through the current
research. They debunk a few old misconceptions, but the basics haven't
changed.

And, as you say, the important thing is, don't burn gasoline in a diesel
that's tuned to run on diesel fuel.

--
Ed Huntress