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.]

[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.]

[Experimental Investigation into HCCI Combustion Using Gasoline and Diesel
Blended Fuels - 2005]

"Gasoline and diesel, the two fuels with very different characteristics and
with wide availability for conventional engine use, were blended as a HCCI
engine fuel. Gasoline, with high volatility, easy vaporization and mixture
formation, is used to form the homogeneous charge. Diesel fuel, which has
good ignitability and fast combustion at the conditions predominating in the
HCCI environment, is used to dominate the auto-ignition and restrain the
knocking combustion."

================================================== ======

The flash point of gasoline is lower than the flashpoint of diesel
fuel. The vapour pressure is higher. Therefore it actually takes less
heat to light gasoline than diesel. However, the flamability range of
gasoline is lower - too lean or too rich and it won't light.

The flashpoints of gasoline and diesel have little to do with ignition in a
high-compression engine that ignites the fuel by autoignition. The
autoignition temperature for gasoline is 246 deg. C; for diesel, it is 210
deg. C. So diesel ignites faster in a compression-ignition engine.

As for the amount of heat required to ignite either, since the initial
in-cylinder conditions are the same in both cases, the amount of heat is the
same. The quicker ignition of the diesel relates to its lower autoignition
temperature, not to the amount of heat involved.


When you inject gasoline into a diesel, if you get it injected in a
proper cone, at the right time, some of the fuel will ignite
immediately and will burn very quickly, causing a knock.

What's your reference for this? The resources I've checked (I read over 200
abstracts during the past 24 hours), at least those that address the
question, say that gasoline is slow to ignite in a diesel.

The rest of
the fuel is dissipated in an overlean mixture, and does not burn.
(severe lean misfire)This goes out the pipe as white "smoke" and also
causes the flame in the exhaust if it ends up lighting on the way out.

If the throttle is advanced enough to get sufficient fuel into the cyl
to get the mixture rich enough to burn more completely, SEVERE
"detonation" shock occurs - the engine knocks badly, smokes profusely,
and makes very little power. High octane fuel will not behave much
differently than low octane fuel in a deisel.
A diesel depends on s relatively slow even burn, and a reasonably long
calibrated injection period provides the fuel to keep building cyl
pressure constantly untill the piston is almost at BDC.

Again, diesel burns more quickly in an engine environment than gasoline
does.

--
Ed Huntress