Re: OT Diesel engines


"SteveB" <pittmanpirate@xxxxxxxxxxxxx> wrote in message
news:2ot5d5-ado2.ln1@xxxxxxxxxxxxxxxxxxxx

Steve, thanks for the insult.

I do understand the difference between a diesel cycle and a spark ignited
engine. While my thermodynamics classes were decades ago we did study
the
matter.

I am sure that gasoline would also self-ignite quite well during a diesel
cycle. Maybe the compression ratio would have to be reduced thereby also
reducing stress on the engine parts.

Try me with your longer answer and see if I can comprehend.

Ivan Vegvary


What you are questioning involves two different petroleum distillate
products with entirely different properties. These properties are
differing densities, differing lower flammability levels, differing lower
explosive levels, different specific densities, and countless differences
in lubricating properties. The engines designed to burn these products
are designed metallurgically and mechanically to take advantage of the
properties of each fuel and to be able to handle their consumption through
a controlled explosion. While it is true that diesel engines can be
started with gasoline in some engines specifically designed to do so, the
extra design components are not present in the common engines of either
variety.

One engine uses simple compression pressure to ignite the fuel (diesel)
and an impulse spray (or two sprays in some newer models) at the precise
moment, while the other one utilizes a spark at a precise moment that is
calibrated by the timing gears and connector, be they a chain, or common
gears. The fuel delivery systems vary considerably, even in the gasoline
engines, with common aspiration by suction through a carburetor to
computer controlled fuel injection being the common types. Carburetors
may be either updraft or downdraft. And then, the fuel mixtures may be
given an additional boost in pressure by a turbocharger powered by exhaust
gases or a supercharger powered by a mechanical connector ultimately
ending up at the crankshaft but possibly running through intermittent
components. Superchargers are and were commonly used for diesel engines,
and fathered drag racing's development through the 4-71. 6-71, and the
grand daddy 8-71 superchargers of the early engines, commonly called
"blowers". They are still in use, although the new ones are a few light
years ahead of surplus superchargers the early drag racers used.

The compression ratio on a diesel engine is higher than a gasoline engine.
This is a rating of XX:1 to rate how high a pressure is created by the
upstroke of the piston when compressing the fuel/air mixture when the
piston reaches top dead center, and full compression is achieved.
Gasoline has properties that cause it to ignite when exposed to sudden
bursts of pressure, and cannot be as predictably controlled as diesel
fuel. Therefore, if gasoline is run with high pressure as an ignition
source, detonation occurs before the piston has reached top dead center,
much like any gasoline engine, yet detonation occurs so far before the
three or four degrees before top dead center of the piston travel that the
preignition knock can cause damage to the piston, cylinder walls, rings,
connecting rod, connecting rod pins, or all the above. This has been a
problem with gasoline engines for decades, particularly when engines used
a points system on the distributors with a vacuum driven mechanical
advance to adjust the spark according to driving conditions and how much
you pressed the gas pedal. Preignition knock was the cause of
catastrophic failure for an untold number of gasoline engines,
particularly in the era of the transition from leaded to unleaded
gasolines, and all the additives and fuel concoctions and devices to help
the public (and in some cases to merely relieve them of cash) didn't
really work very well.

If gasoline were to be able to be run in a diesel engine, even with the
addition of spark plugs, the operating temperatures would surely also
cause erosion of metal due to excessive pressures, and the piston rings
would soon lose their seal, making the engine stop running.. Excessive
temperature may cause melting of the components or simple seizure of the
piston in the cylinder. Your sureness that gas would run in a diesel
engine is flawed.

Differences between gas and diesel engines are many. Fuel flow, fuel
delivery, piston rings, compression ratios, metallurgy, sulfur vs.
non-sulfur fuel, gas/ethanol mixes vs. diesel fuels, properties that could
cause detonation of gasoline in diesel injector systems before the fuel
even reaches the injector nozzle, vast differences between the explosive
volatility of diesel fumes and gasoline fumes, exhaust systems combined
with emissions controls devices, changing fuels and the effect of that on
anti-pollution devices sensors and systems, catalytic converters, computer
controlled sensors valves and gates that would be pushed outside their
operational parameters with the introduction of a foreign fuel that the
system was never intended or designed to burn ........ it goes on.

Diesel fuel in a gasoline engine would not work because the compression
ratio (remember that, we touched lightly on that in another paragraph) is
not high enough to make the diesel fuel ignite, and either with a fuel
injection system, or a carburetor, it just wouldn't work, or work for very
long even with the spark plug firing. If it DID fire and run, it would
not do so for very long before spark plug fouling. Sorry I can't give you
a longer answer than that.

I may be wrong on some of the small points, and I'm sure that people here
who know far more than I do will pick apart my mistakes. This is only a
feeble attempt to explain what you requested, and no claim of perfection
is made or implied.

But I do know my ass from a hole in the ground and not to put diesel in a
gas engine, or vice versa.

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.

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.

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.

--
Ed Huntress


.

Relevant Pages

  • Re: OT Diesel engines
    ... gasoline (or diesel) never has a chance to "preignite". ... engine may not run at all. ... That's entirely different from mixing the fuel ...
    (rec.crafts.metalworking)
  • Gasoline FAQ
    ... environmental reasons for recent and planned future changes to gasoline ... This FAQ is intended to help readers choose the most appropriate fuel ... Does low octane fuel increase engine wear? ... The molecular structure of the hydrocarbons and other compounds present ...
    (rec.travel.europe)
  • Re: OT Diesel engines
    ... The cetane rating of gasoline is so low that, if you can ignite it at all ... time it takes for fuel ignited at high temperature and compression to ... An octane rating, which is very low in diesel and much higher in gasoline, ... and the later is the heat release in the HCCI engine at a given condition. ...
    (rec.crafts.metalworking)
  • Re: OT Diesel engines
    ... I do understand the difference between a diesel cycle and a spark ignited ... I am sure that gasoline would also self-ignite quite well during a diesel ... reducing stress on the engine parts. ... One engine uses simple compression pressure to ignite the fuel and ...
    (rec.crafts.metalworking)
  • Re: OT Diesel engines
    ... The cetane rating of gasoline is so low that, if you can ignite it at ... octane and cetane have roughly opposite ... time it takes for fuel ignited at high temperature and compression to ... and the later is the heat release in the HCCI engine at a given condition. ...
    (rec.crafts.metalworking)