On Fri, 30 May 2014 14:30:00 -0400, "Ed Huntress"
wrote:
"jim" wrote in message ...
Ed Huntress wrote:
Ethanol will not gum up a carb or an engine. But they often mix it with
low-grade gasoline (under 91 octane, among other, bigger issues) and
that gas *can* crap your engine. It does seem more prone to varnishing
the carb jets, but that isn't because of the ethanol.
ethanol is *almost always blended* with 84 octane fuel.
Your engine will run like **** without the ethanol.
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[Ed]
Yeah, the old SAE sources I had just said "under 91 octane," and I assume
it's well under.
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Ethanol will not do damage to a carburetor, large or small. *Methanol*
will do damage to aluminum or zinc (or brass, I think) if it's left in
the carburetor bowl too long. Race cars that burn methanol generally
drain the carbs, and often the tank, between races. The ethanol-damage
myth probably is a carryover from admonitions about methanol, dating
back to the 1930s.
I don't think race cars that use methanol use carburetors.
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[Ed]
But they did, for decades. It wasn't just Hilborn injectors at Indy. It was
even some carbureted sports-car classes, back in the '50s, and even go-carts
in the '60s.
The old story about methanol damaging carbs has been legend in racing for a
very long time, and I suspect that someone just assumed that ethanol caused
the same problems.
Methanol corroded the metal of the carbs - as well as bearings,
camshafts, crankshafts, and cyl heads. Common practice was to flush
EVERYTHING after a run - including changing the oil, to preserve the
"hard parts".
Ethanol, on the other hand, has a low corrosiveness to metal but is
very hard on many "soft parts" like gaskets and seals due to it's
solvency. Corrosion with ethanol is mostly due to the hygroscopic
nature of ethanol, and phase separation which allows the saturated
ethanol to drop out of suspension, so the water trapped in it can
cause corrosion.
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The MIT report on efficiency with ethanol was misrepresented in the
posts here. I read all 61 pages of it, and the story is that up to 20%
or so ethanol will allow enough BMEP from boosted compression to
increase efficiency in a high-speed highway cycle, with long runs above
60 mph and peak over 80 mph, if you are comparing a very small turbo
engine with a much larger normally-aspirated one. That engine cycle is
not used in EPA city/highway cycle comparisons.
Which MIT report and what was misrepresented?
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[Ed]
This was the statement: "This MIT study found that maximum thermal
efficiency can
be achieved with 20%-35% ethanol blends."
This is the MIT report:
http://dspace.mit.edu/bitstream/hand.../858869910.pdf
It requires careful reading. This is the "on the one hand, but on the other
hand" kind of conclusions it reaches:
MIT has published dozens of articles on ethanol
efficiency. The only MIT article I saw posted was intended
as a reference to counter a particular false claim that
was made.
In normal driving, the
MIT report says, there is almost no difference -- and required boost can
be achieved with spark retardation that is so low it has almost no
effect on performance.
retarding spark is a compromise that
decreases efficiency. Fuel that burns
late in the power stroke produces mostly
heat out the exhaust.
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[Ed]
Sure, but look at the first paragraph on page 58 of the report. The result
is not what you might expect.
And look at the bottom graph on page 57. Surprise!
Also, look at table 9 on page 56. Five degrees of retard results in only
1.55% loss of efficiency in the highly-turbocharged engine. And 5 deg. buys
you a lot of allowed boost with gasoline. As I said, the curves cross in
normal driving, but the upshot is that you actually can get HIGHER
efficiency (in terms of fuel volume/mi.) with the higher-caloric-content
gasoline in normal driving conditions.
Overall, it's a very close call -- unless you go for a pipsqueek engine
running at near hand-grenade-level peak effective pressures at nearly full
throttle. Hmmm...
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At some point, the lines of volume efficiency
cross, where the lower caloric content of ethanol is compensated by the
very high turbo boost that ethanol allows. The report is worth reading.
And the other potential efficiency gain from ethanol blends is
that you can get the same power from a much lighter engine
Removing a large amount of dead weight allows for lighter
suspension and chassis.
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{Ed}
It's like most engineering jobs: "On one hand, this improves results. On the
other, it makes them worse..."
Running tiny engines at over 13 bar of turbo boost is not a recipe for a
long and happy life. And building a tiny engine that will handle it means
the engine has to be built stronger -- and heavier -- than a less-stressed
engine.
And note that ethanol injection or blending is not the only way to prevent
detonation in a supercharged engine of any type. For example, they didn't
compare this test engine with a water-injected one.
What they were evaluating was potential fuel-volume/mile capabilities of a
few engine types. It's a good report, and it shows that you don't have to
give up (volume-based) efficiency when you use a lower-caloric-content fuel,
like ethanol, when you can compensate with lots of turbocharging and the
higher effective RON (octane) of ethanol.
But the net result is that you only get an improvement with ethanol at
conditions of very high loading of the engine -- a high-speed cycle that
isn't even measured in the EPA city/highway calculations, and which doesn't
represent *anyone's* long-term average driving conditions.
I'd say it's about a wash. Now, let's see what they can do with a
homogeneous-charge, compression-ignition (HCCI) engine g
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But there is no incentive for automakers to
design cars that perform better on ethanol blends
as long as the EPA requires fuel economy testing
to done with straight gasoline without ethanol.
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[Ed]
Or perhaps there's little incentive to build expensive and complicated tiny
turbos when the advantages, if any, are small.