What/how does compression ratio affect an engine?

That depends on the BTU content of the fuel. The higher octane will allow
higher compression and tolerate heat better before knocking. Part of that
heat is the release of BTU energy. When I used to roadrace motorcycles long
ago in a mis-spent youth, I used a fuel made by ERC in Hayward. CA. The air
cooled bikes used stuff around 106 octane with about 18K BTU's. The water
cooled stuff could tolerate more BTU as the cooling system removed heat
before it became destructive. The fuel used for drag racing was rated at 22K
BTU. The engines would tolerate this heat release as it was for very short
times with an engine that was not fully warmed up. If this fuel was used in
an endurance event with one of our air-cooled engines, it would result in
holes in the pistons and seizures.

Now, what would 22K BTU stuff do in an 7 1/2 to 1 aircraft engine? Probably
nothing as the lower compression would limit the amount of work actually
being done. But it will still burn hotter than a fuel with a lower BTU
content. Might have to richen the mixture a bit if CHT or EGT were being
monitored, but knock and/or detonation won't be a problem.

One of the main benefits of using higher compression is the amount of power
available at part throttle. You can get more power out of a higher
compression engine at very low power settings than a lower compression
engine. The result of this is a lower fuel burn, unless the mixture is
richened to cool the cylinder. But cooling at part throttle shouldn't be an
issue all things being equal.

Dale Alexander


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.

"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.

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.




--
"He is not only dull himself; he is the cause of dullness in others."
-Samuel Johnson

Octane and BTU are two different qualities of fuel. In a very simplistic
view, the longer the molecule, the more energy it takes to break it. By
needing more energy to break, it is more resistant to knocking or detonation
(not the same but still applicable). This would be octane.

BTU is the amount of energy that the chemical reaction will produce AFTER
the reaction has started. Compression, or lack of it, would reduce the
amount of pressure thus heat that the reaction would ultimately make. In a
system that is optimized (tuned) all of the heat energy possible may be
extracted. But even in a lower compression engine, an increase in heat would
be available as there is more potential in the first place. So no, the only
difference is NOT octane.

As I said, very simplistic but serves to illustrate the point.

By the way, I seem to remember a research paper that stated that
conventional motor fuels (not alcohol or nitro based, just for
clarification) burn at the same rate no matter the octane. I think that
confusion exists here in that a previous poster stated that higher octane
fuels burn slower. A lower octane fuel may "seem" to burn faster, but what
may be happening is that the normal flame front increases the pressure in
the remaining unignited mixture to the point of self-ignition and the
resulting second flame front advances to meet the first in a reduced period
of time. Still burning at the same rate but from two different starting
points and meetin gin the middle. This is abnormal combustion though and in
a normal combustion event, the fuels would burn at the same rate.

"Bob Fry" wrote in message
...
"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.

Yes, the fuels used were different and proprietary blends that I was not
privy to. I was just a comsumer, not a business partner. But the fuels were
blended for specific purposes using various chemical qualities to achieve
end results. This I stated previously.

It seems that ERC is still in business. See

http://www.ercracingfuels.com/sxs1.htm

for a comparision of various blends. It would appear that their ERC MUL/A is
the fuel that I used in my motors and the ERC 1-19A is the 120 octane stuff.
But look around the website and find that various fuels do not have a direct
relationship between octane and BTU. So something other than octane is
definitely going on here. Read some of the descriptions of the bases and
blending to see what qualities they build the fuel for.


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

See above




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.


You are correct that optimization of the engine as you mention will result
in more power, mileage etc. But we learned that you have to watch out for
the BTU as well. And an engine can generate too much heat and power to the
point of reducing power without ever suffering from pinging, knocking or
detonation. But that is a subject for another day.

I love this group! It would be great to sit down to a dinner conversation
with many of you. A lot of accumulated knowledge in this group..

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.



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

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. --

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