"Tim Wescott" wrote in message
...
On Fri, 11 Apr 2008 22:19:36 -0400, Al Patrick wrote:

Ivan Vegvary wrote:


If any of the above is true, why don't people, today, put gasoline into
their diesel engines, considering the higher cost of diesel fuel?


I own a diesel F-250 and wouldn't dare do that. I previously, years
ago, owned a Mercedes 300-D and the manual actually stated that - during
cold weather - (I think it was so qualified) one could mix UP TO 30%
gasoline with the diesel fuel. I suspect this was to help prevent the
diesel fuel from congealing.

I have heard stories, unverified, that a diesel will "run away" on
gasoline and over rev the engine, possibly disassembling itself,
internally if not externally. A diesel operates at much higher
pressures than a gasoline engine. The gasoline is far more explosive
than diesel fuel. The diesel uses no spark plug to ignite the fuel.
Instead it uses pressure to explode the fuel.

Take this for as much verification as you think it's worth...

On at least one occasion my dad came home from the fire station with a
story of a truck engine running away when the guys put gasoline in the
tank instead of diesel fuel.

I don't see why this should happen _in theory_ as a diesel is throttled
by controlling the amount of fuel injected, but I could certainly see it
happening in practice if the lower viscosity of gas let it flow at a too-
high volume.

IIRC one shuts down a runaway diesel by shutting off the air. There is
(was) supposed to be an emergency shut-off valve in the intake of every
diesel motor, but in a pinch one could use articles of clothing stuffed
into the intake.

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.

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.

--
Ed Huntress