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.
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.

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. 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.

So they aren't exactly opposite characteristics, but they're pretty close.
FWIW, ethanol is even harder to ignite with compression than gasoline. But
ether, which has a cetane rating around 80 (compared to petroleum diesel's
rating of 40 to 55), ignites much easier than any of them, with either
compression or a spark.

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