What/how does compression ratio affect an engine?

On Mon, 10 Dec 2007 20:38:01 -0800, Bob Fry
wrote:

"DA" == Dale Alexander writes:

DA Now, what would 22K BTU stuff do in an 7 1/2 to 1 aircraft
DA engine? Probably nothing as the lower compression would limit
DA the amount of work actually being done.

If I can reword your statement to "what would higher octane stuff
do..." then I can answer not probably, but definitely, it will not do
anything different, if the only difference between the two fuels is
octane.

DA But it will still burn
DA hotter than a fuel with a lower BTU content.

Where did the difference in "BTU content" (i.e. unit chemical energy
content) come from? Not from a mere octane enhancer. Perhaps the
fuels used in your road racing experience had not only different
octanes, but also different unit energies.

Racing fuel, generally, is not even CLOSE to gasoline. It is a very
complex witches brew, and generally significantly higher specific
gravity, as well as higher BTU per gallon (not necessarily more BTU
per lb, hence the higher SG)

Eh? Where's this "extra heat" come from? There is no
practical unit energy difference between different octane
fuels. --


Exactly. "Octane" is, by definition, a measure of a fuel's
resistance to knock under specific conditions. "High Octane"
fuel does not burn any hotter, generate any more power, or
improve your fuel economy (note: see exception below). Higher
octane fuel lets the engine designer use a higher compression
ratio, or more spark advance, etc. without triggering knock. It
is the compression / spark changes that result in more power,
etc.



--
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On Tue, 11 Dec 2007 16:19:46 -0500, "Morgans"
wrote:


"Jim Logajan" wrote

BUT - one is more likely to burn unevenly and/or more quickly (i.e.
"knocking" or "pinging") which leads either to unplanned overpressures or
localized hot-spots (i.e. _high heat concentrations_). So sure - no
_final_
difference in released energy, but time and space concentrations can
result
in metal melting or breaking in one case and not the other.

So, in your example, the high octane would be the cooler burning fuel,
because it burns more evenly, and slowly, so it prevents knocking.

High and low octane fuel burn at the same rate. Hydrogen burns much
faster. Detonation involves "cracking" of the fuel, releasing unstable
hydrogen radicals which explode instead of burning.

Other scientific data showes that the higher octane fuel, the lower the /btu
content. That's fact. Not a lot of difference, but a difference, none the
less.

High octane fuel does not BY DEFAULT have a lower BTU content.
Straight run high octane would be as high or higher in BTUs than
straight run regular. HOWEVER the additives used to increase the
octane in MOST high octane pump gas today are significantly lower in
BTU content than the gasoline they are added to. Look at Ethanol, as
an example. E10 has 10% ethanol, and ethanol has roughly half the btu
value of gasoline, so E10 has roughly 5% less BTU per gallon.
It is also less dense, so the BTU per lb may not be that much lower -
but we do not measure our fuel by weight.




--
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On Tue, 11 Dec 2007 23:36:24 -0000, Jim Logajan
wrote:

"Morgans" wrote:
"Jim Logajan" wrote

BUT - one is more likely to burn unevenly and/or more quickly (i.e.
"knocking" or "pinging") which leads either to unplanned
overpressures or localized hot-spots (i.e. _high heat
concentrations_). So sure - no _final_
difference in released energy, but time and space concentrations can
result
in metal melting or breaking in one case and not the other.

So, in your example, the high octane would be the cooler burning fuel,
because it burns more evenly, and slowly, so it prevents knocking.

Except that it appears what I wrote is not correct. :-(

The octane rating appears to be a measure of the activation energy of
combustion, not a measure of the combustion rate.

And not the activation energy of NORMAL combustion. Octane relates
STRICTLY to the resistance to thermal dissassociation of the fuel in
the combustion chamber. Under high heat and pressure gasoline/air
mixtures can "come unglued" letting the hydrogen "flake off" so to
speak. The hydrogen, which normally likes to hang around in couples
(as H2) starts running around as H1- an unstable and promiscuous
radical that will couple with anything available - in this case excess
O2 particals - and when this happens, ALL HELL BREAKS LOOSE inside the
cyl..

--
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On Tue, 11 Dec 2007 18:08:03 -0700, "Bill Daniels"
bildan@comcast-dot-net wrote:


"Jerry Wass" wrote in message
.net...
GeorgeB wrote:
On Mon, 10 Dec 2007 09:23:52 -0800, Richard Riley
wrote:

If you're flying a homebuilt you can burn whatever you want - but the
alcohol restriction wasn't put there at random, it increases vapor
lock problems dramatically,

How do the planes which do fly on ethanol handle that problem,
pressurized tanks?

and is incompatable with many of the
materials commonly used in aircraft fuel systems.

The sealant sloshed in the tanks is one, I think ...

Do automobiles with flex-fuel capability do anything to minimize the
vapor lock issues? I'm sure the materials were selected to be ok.

Ole Jerry said: most of the automobiles in the last 20 yrs or so have the
GAS PUMP inside the GAS Tank. Vapor lock is usually induced by Sucking on
the fuel at some point in the fuel system.

Maybe off topic but this involves fuel pumps in tanks. They fail way too
often and when they do, you aren't going to fix it on the road side. After
five failures at $800 a pop, I removed the pump from the tank and put an
aftermarket pump in the fuel line where I can replace it with a screw driver
for $50. No vapor locks yet.

Strange. I have 362000km on my current vehicle (pump in tank - still
original and 14 years old) and had 240,000 on each of my last two
vehicles - 14 and 18 years old - all on the original in-tank pumps.

I NEVER got that kind of mileage out of a mechanical pump, and frame
mounted electric pumps virtually ALL rusted out in less than 10 years,
requiring replacement. This is why, to a large degree, most
manufacturers went to in-tank pumps a long time ago.
If you don't change fuel filters, you WILL burn out pumps. If you
always run on the bottom 1/4 tank you will LIKELY burn out pumps - but
the top 1/4 doesn't cost any more to keep full than the bottom 1/4.

--
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Yeah, I might have gone over the top there with that comparison. But the
subsequent posts started into high octane not burning any hotter and I used
the info that I was familiar with. Everybody was so damned sure that high
octane didn't burn any hotter and I knew of instances where that was not
true. I listed my sources with a website for the manufacturer of the fuel I
was using as a reference. You could put an end to this by showing a source
that definitively showed that in every instance (with regard to aviation
fuel), there is no difference in heat output or flame speed between the
various fuels.

By the way, other than vapor pressure, do you believe that there is a
difference between winter and summer blends of automotive fuels?

And if you don't mind my asking, what is your training/schooling in this
field? The statements I have seen attributed to you show a disciplined mind.

Dale Alexander

clare at snyder.on.ca wrote in message
...
On Mon, 10 Dec 2007 20:38:01 -0800, Bob Fry
wrote:


Racing fuel, generally, is not even CLOSE to gasoline. It is a very
complex witches brew, and generally significantly higher specific
gravity, as well as higher BTU per gallon (not necessarily more BTU
per lb, hence the higher SG)

Eh? Where's this "extra heat" come from? There is no
practical unit energy difference between different octane
fuels. --

OK, so us poor bastages with A-65s with no mixture control might be
screwed anyways Anybody have any ideas on how to get a mixture
control onto an A-65? If I recall, my carb is an NAS3A1 Stromberg. I
THINK there might be a plate on top of the carb where a mixture control
may have been present at one time???

Scott





Aggressive leaning at less than peak power will also help
reduce this lead build-up.

"Scott" == Scott writes:

Scott OK, so us poor bastages with A-65s with no mixture control
Scott might be screwed anyways Anybody have any ideas on how
Scott to get a mixture control onto an A-65? If I recall, my
Scott carb is an NAS3A1 Stromberg. I THINK there might be a
Scott plate on top of the carb where a mixture control may have
Scott been present at one time???

Can't answer that question, but you might try adding TCP to your fuel
to help scavenge the lead out.

--
We choose our joys and sorrows long before we experience them.
~ Kahlil Gibran

I will check into that...



Bob Fry wrote:
"Scott" == Scott writes:


Scott OK, so us poor bastages with A-65s with no mixture control
Scott might be screwed anyways Anybody have any ideas on how
Scott to get a mixture control onto an A-65? If I recall, my
Scott carb is an NAS3A1 Stromberg. I THINK there might be a
Scott plate on top of the carb where a mixture control may have
Scott been present at one time???

Can't answer that question, but you might try adding TCP to your fuel
to help scavenge the lead out.


--
Scott
http://corbenflyer.tripod.com/
Gotta Fly or Gonna Die
Building RV-4 (Super Slow Build Version)

On Tue, 11 Dec 2007 20:18:54 -0800, "Dale Alexander"
wrote:

Yeah, I might have gone over the top there with that comparison. But the
subsequent posts started into high octane not burning any hotter and I used
the info that I was familiar with. Everybody was so damned sure that high
octane didn't burn any hotter and I knew of instances where that was not
true. I listed my sources with a website for the manufacturer of the fuel I
was using as a reference. You could put an end to this by showing a source
that definitively showed that in every instance (with regard to aviation
fuel), there is no difference in heat output or flame speed between the
various fuels.

By the way, other than vapor pressure, do you believe that there is a
difference between winter and summer blends of automotive fuels?

Definitely. In most cases the formulation is quite different.

And if you don't mind my asking, what is your training/schooling in this
field? The statements I have seen attributed to you show a disciplined mind.

Dale Alexander

I am a long time (now semi-retired) auto mechanic, former auto shop
instructor at both secondary and trade level, and quite widely read on
the subject. I tend to be "curious" and research things quite
thoroughly when trying to get a handle on something.

I am also a partner in a Pegazair project which will be flying a
lightly modified Corvair engine.

clare at snyder.on.ca wrote in message
.. .
On Mon, 10 Dec 2007 20:38:01 -0800, Bob Fry
wrote:


Racing fuel, generally, is not even CLOSE to gasoline. It is a very
complex witches brew, and generally significantly higher specific
gravity, as well as higher BTU per gallon (not necessarily more BTU
per lb, hence the higher SG)

Eh? Where's this "extra heat" come from? There is no
practical unit energy difference between different octane
fuels. --




--
Posted via a free Usenet account from http://www.teranews.com

On Dec 12, 5:03 am, Scott wrote:
OK, so us poor bastages with A-65s with no mixture control might be
screwed anyways Anybody have any ideas on how to get a mixture
control onto an A-65? If I recall, my carb is an NAS3A1 Stromberg. I
THINK there might be a plate on top of the carb where a mixture control
may have been present at one time???

That's it. Finding the bits and pieces might be a hassle. I
made mine, since it's a homebuilt, and it works OK. It's a back-
suction type of mixture control, where the lever rotates a valve under
that cover. There's a hole into the float bowl, another into the dead
airspace behind the venturi, and another into the venturi itself. The
area behind the venturi is at roughly atmospheric pressure, and the
venturi, of course, is much lower than that. The mixture control
valve, when rotated toward "Lean," gradually shuts off that port to
the dead airspace, allowing the venturi port to start sucking against
the fuel in the bowl. That decreases the differential pressures at the
fuel nozzle and flow decreases.
It won't work as an idle cutoff. There's so little flow through
the venturi at idle that nothing happens when you pull the mixture all
the way out. Carbs with idle cutoff are using a valve inside the carb
that varies the flow of the fuel itself from the main jet in the
bottom of the bowl, so that if it's pulled full lean, all flow,
including idle fuel, stops.

Dan