Premium Gas in a base RSX?

I'd guess it's because of the poor combustion characteristics of
gasoline under diesel-level pressures. For maximum efficiency (in a
diesel) you want ignition at the forefront of the vapor wave as it
enters the cylinder, with combustion proceeding smoothly back through
the cloud. Gasoline vapor would tend to spontaneously ignite at
unpredictable points within the vapor cloud, resulting in turbulent
combustion, poor oxygenation of the flame, and inefficiencies.

According to Wikipedia, some diesel engines can run on E95, which
is 5% gasoline and 95% ethanol; the high "octane" (ignition
temperature) of the ethanol prevents the vapor from igniting at the
wrong places.

Surely you're not denying that higher compression means higher gas
temperature (volume being constant)? And that higher temperature
increases the likelihood of ignition? I'm not sure what exactly
you're taking exception to in the quoted paragraph.

Indeed it is, but I fail to see how the comparison is relevant.

Perhaps so. I'll retract my claim about why high-compression engines
are more expensive.

At any rate, the basic point is that the "octane" rating of a sample
of gasoline refers to its ignition temperature, and that higher
octane means less knock, and that the appropriate octane for an
engine will maximize gasoline performance for that engine, with no
additional benefit for going higher.

--
Michael Wojcik

Against all odds, over a noisy telephone line, tapped by the tax authorities
and the secret police, Alice will happily attempt, with someone she doesn't
trust, whom she can't hear clearly, and who is probably someone else, to
fiddle her tax return and to organise a coup d'etat, while at the same time
minimising the cost of the phone call. -- John Gordon

 

Perhaps so. In the fifteen years or so that I've been posting to
Usenet, I've certainly written messages I've later regretted, and
many others that could have been better. I don't feel this was
one of the former, but it probably falls into the latter category.

 

Would someone please define "adiabatic heating temperature"? Please provide
your source for this definition. What is the approximate number, in degrees
F, for the "adiabatic heating temperature" (AHT) of gasoline at the
pressures with which we are concerned here?

Or is Jim just being sloppy?

I know what "adiabatic heating" is, but the statement above seems to be
implying the AHT phrase altogether is part of engineering or scientific
parlance.

Rather than impetuously call this statement "rubbish," he or someone gets a
chance to tweak it.

 

Not so fast. It's a fact that diesels are higher compression engines, and
they do require greater cylinder wall thicknesses, in some proportion to the
higher pressures they see, for one. The material cost of a higher
compression engine will, in general, be higher.

I would wager that this is true for higher compression "power performance"
gasoline engines vs. lower compression gasoline engines, too.

But don't use, say, dealer price for a "performance" car to gage the cost of
materials to build the engine. Too many other variables. So we can't go to
the net and compare edmunds.com prices for car D with compression ratio x to
car G with lower compression ratio y.

A higher octane does not correspond to a higher ignition temperature. Higher
octane means "more resistance to self-ignition, particularly, ignition due
to high pressure instead of a spark."

Unless you regularly deliver (carefully prepared) lectures on the subject,
this is a complicated topic that you should not attempt to summarize off the
top of your head. You're butchering a number of points.

Optimize...

 

Sort of. Both gasoline and diesel in a diesel engine will autoignite
(detonate), exactly as the diesel cycle expects them to. The difference is
in the preflame reaction. Cetane fuels (diesels) have a much shorter
preflame reaction time than heptane fuels (gasolines). Once the preflame
reactions have occurred though, both go up instantly.

The above gleaned from this Google search:
http://tinyurl.com/8h9hj

Additonal reasons gasoline is not used in diesels set up for diesel fuel:
1) Low viscosity results in excess fuel delivery
2) Gasoline has insufficient lubricity to protect fuel pump.

It will result in very low power, lots of smoke, and high probability of
fuel pump damage. Other than that, according to what I get from the search
above, it won't do any damage in the short term. Long term usage /could/
coke up the rings.

 

thanks for bothering to dig up such a great citation. your summary is a
little light in that it's important to distinguish the circumstances
under which you can run compression ignition [ci] engines in "multifuel"
mode, i.e. even higher compression ratios than required for normal
diesel operation. i can tell you from experience, a normal diesel just
won't run on gas.

the military spent a lot of time & a lot of money researching multi-fuel
vehicles. ateotd, i think intent was more propaganda in an effort to
divert soviet resources than the desire to impliment it in practice
because very few vehicles could run in multi-fuel mode, and those that
could were run on diesel pretty much all the time.

 

"squeeze" temperature.

my old lecture notes.

you're not compressing gasoline, you're compressing the air charge in
the case of a diesel or the air/fuel charge in the case of spark
ignition. for diesels final compression temp is 500-600C. for spark
ignition engines, it's in the range 250-350C approx. iirc, diesel fuel
ignites in the 400C range, gasoline in the 600C range approx.

 

kinda, but as i understand it, it's more along the lines of diesel
having individual droplets with boundry layer combusion, whereas gas is
full vapor that can be detonated by compression waves anywhere.

diesels can't really ignite at the wrong places because once fuel is
injected, it's starting to burn. it doens't get a chance to evaporate
into a detonatable vapor. with gas engines, you're trying to get a
smooth flame front progressing from the center of the chamber [more or
less]. combusion chamber irregularities [think old detroit iron] cause
local pressure spots & detonation ahead of the flame front because the
fully evaporated vapor is already there.

of course not.

of course not.

"So high-compression engines need fuel with a higher ignition point,
which means a higher octane rating."

strictly speaking, that's detonation point, not ignition.

you're saying that higher compression gas engines are build stronger to
take that compression. that's not true. output pressure dwarfs
compression pressure. the build difference [if any] is ability to cope
with the output.

kinda, but it's more detonation sensitivity than just plain temp. rate
comes into the equation as well.

 

to cope with combustion pressure, combustion temperature & mechanical
output, not initial compression.

output, not compression.

 

That was a good one, citing many concrete refernces from credible
textbooks.

I printed the citation off to read it better to find out why.

Apparently this is because diesel is injected into a diesel engine at about
the same time the spark plug fires in a spark engine (16~30deg BTDC). Since
it takes gasoline so long to "light off" (detonate), the engine is well
past TDC before it can. Therefore, if it ever does detonate, the piston is
pretty near the bottom of its power stroke. And usually it never detonates
at all, but just gets pumped into the exhaust.

Actually, I read that wrong. I printed off that citation so I could read it
better, and discovered that gasoline's low viscosity results in
INSUFFICIENT fuel delivery for two reasons:
1) Gasoline's lower density packs less punch per volume, and
2) Some of it squeezes back past the injection pump's plunger instead of
going into the injectors the way it's supposed to.




Another mistake here: In that thread, somebody suggested using WD-40 in a
diesel. Somebody else said this would coke up the rings in the long term.
During my on-line read, misread it as being /gasoline/ as the coker-upper.