On Sat, 12 Apr 2008 02:41:13 -0400, "Ed Huntress"
wrote:
Well, assuming that's really the bottom line, maybe it's safe to pick apart
some of your answer without obscuring the point. g
That certainly was a long answer, and it really runs around the horn, but
some of your concepts in there just aren't right. First, gasoline's tendency
to burn from high compression would be an issue in a diesel except that the
gasoline (or diesel) never has a chance to "preignite". It's injected long
after preignition could take place. The environment it's injected into (high
heat, high pressure) burns the fuel progressively and its cetane rating
determines how fast it burns. Conversely, its octane rating reflects its
resistance to burn rapidly from the heat of compression. Thus, if you inject
gasoline into a diesel engine it will burn, but it does so slowly, and the
engine may not run at all. That's entirely different from mixing the fuel
with the air before it gets into the engine. At a compression ratio of 20:1
or so, gasoline/air mixes would burn in a way that you could describe as an
explosion (although there was contention about this as of a couple of
decades ago -- the high-speed flame-front versus acoustic shock wave
theories of engine detonation they were then studying at MIT's Sloan labs --
I never read how it was concluded) but that's really a side issue here and
not worth discussing. The point is, there is no gasoline (or diesel fuel) in
the cylinder until it's injected, so there is no pre-ignition, no
detonation, no explosion, and the gasoline actually burns slower than diesel
fuel does in that environment. Gasoline actually can be hard to ignite at
all when it's sprayed into a diesel cylinder, at least at low temperatures,
despite what we know about the tendency of gasoline to detonate when it's
pre-mixed with the air charge in spark-ignition engines with excessively
high compression ratios. When you go to direct injection, you're changing
some of the crucial dynamics of the whole process.
The flamability range of gasoline and air is rather narrow - so only
some of the fuel in the highly stratified charge in a deisel will
ignite. Putting an air throttle on a deisel would enhance the running
of the engine on gasoline
Another point: there is nothing that would cause gasoline to detonate in the
injector system. Some of the current common-rail, direct (cylinder)
injection gasoline engines use pressures similar to those of common-rail,
high-pressure injection in modern diesel engines. And that's very high
pressure indeed.
Gasoline will not burn hotter in a diesel engine than diesel fuel does. In
fact, diesel has somewhat higher caloric value per unit volume ( 11% - 15%,
depending on who's measuring) and the diesel fuel will produce more heat in
the cylinder. More importantly, it will produce higher peak cylinder
temperatures because (again, due to its higher cetane rating) it burns
faster.
You may be aware that there are, or were, dual-fuel spark-ignition engines
that run on gasoline or kerosene (once they're heated up), so the volatility
of fuel oils of that grade is not so low that you can't spark-ignite it at
gasoline-engine compression ratios. They were industrial and agricultural
engines that enjoyed a reasonable operating life. I reported on a line of
such engines, made in Italy back in the '70s, that were widely used for ag
jobs throughout Europe at the time.
These dual fuel engines, almost without exception, would not cold
start on Kero, and would knock, smoke and generally complain loudly if
switched to kero (or "distillate" before fully warmed up.
I listened in to an online discussion about this very subject around 20
years ago, by some very knowledgable engineers from MIT and Carnegie Mellon,
and one of them pointed out that there are a lot of incorrect assumptions
people make about these fuels based on our experience using them to start
charcoal fires. g The properties of fuels at atmospheric pressure, when
you throw a match into them to light a fire, are very different from their
behavior in an enclosed environment at high pressure and with different
systems of ignition. The idea that diesel fuel can burn faster in an engine
than gasoline does is one of the things that runs counter to our sensible
experience.
The fact that diesel will burn at a much leaner mixture than gasoline
helps here. At the lean combustion limit, gasoline burns much faster
than at the rich combustion limit. Diesel generally runs at a leaner
overall mixture in the cyl than gasoline does.
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