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Mogas versus 100LL
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bakerocb



Joined: 15 Jan 2006
Posts: 727
Location: FAIRFAX VA

PostPosted: Thu Sep 21, 2006 5:58 pm    Post subject: Mogas versus 100LL Reply with quote

9/21/2006

Hello Fellow Builders, The aeroelectric list digest for 9/20/2006 contained
several postings on the subject of using mogas versus 100 LL.

Most of these messages just focused on the effect the two different fuels
would have on the engine performance.

I suggest that builders keep in mind the possible effects, some very
adverse, that mogas and unknown chemicals mixed into mogas, not just
ethanol, may have on their aircraft's entire fuel system. Seals, hoses,
tanks and elastomers may suffer.

OC -- The best investment we will ever make is in gathering knowledge


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PostPosted: Thu Sep 21, 2006 7:24 pm    Post subject: Mogas versus 100LL Reply with quote

Octane rating is not the only thing one needs to be concerned with when it
comes to fuel. I don't know the technocrat term for it, but the speed at
which the fuel burns is a definite concern. On huge displacement engines
turning low RPMs, if you were to use a racing fuel designed for high RPM
engines of the same octane you would most like run into problems. From
what the locals at the airport say, high octane mogas should not be used
in a O-540s because it burns too fast, and since the cylinders are the
same as a O-360??

On the other hand, using 100LL that is a slow burning fuel used in a 73CC
22HP (well over 11K) Yamaha YZ 80 engine that is in my self retrieving
balloon kills performance and probably raises EGT to disheartening levels.

On Rotax 4 strokes you can use 100LL, but it does raise the EGTs because
it burns slower than high octane mogas, and some of that burn makes its
way into the exhaust.

A old timer said on old auto engines that didn't use aluminium pistons,
used to loosen the distributor, and run up the engine and begin retarding
the ignition, he said he would get the exhaust glowing, hence carbon would
be burned off. Slow burning fuel does the same in a engine designed for
fast burning fuel.

Using a fast burning fuel in a engine designed for a slow burning fuel can
cause detonation. Detonation raises temperatures, let it go and God forbid
pre-ignition begins to occur.

Ron Parigoris


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Mark Phillips in TN



Joined: 10 Jan 2006
Posts: 431
Location: Columbia, TN

PostPosted: Thu Sep 21, 2006 8:10 pm    Post subject: Mogas versus 100LL Reply with quote

In a message dated 09/21/2006 9:02:17 PM Central Daylight Time, bakerocb(at)cox.net writes:
Quote:
OC -- The best investment we will ever make is in gathering knowledge

>>
Howdy OC- do you subscribe to the RV-list? We usually get punch-drunk on this one at least once a year and if yer looking for knowledge (good bad or whatever) on this issue, those archives are REALLY full of it! Cool

Most respectfully and seriously-
Mark Phillips- on the "uses both" side of the issue and do not archive
[quote][b]


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PostPosted: Thu Sep 21, 2006 10:07 pm    Post subject: Mogas versus 100LL Reply with quote

I was riding across the Arizona desert on I-10 one night some years back
and pulled up alongside a guy riding an old BSA single who's exhaust
pipe was glowing red. I pointed and gestured until he pulled over
whereupon he told me not to worry - he was just running kerosene in it
'cos it was cheaper. He had a small tank with gas in it that he used for
starting and acceleration. I don't know if he messed with the timing to
make it run, but run it did. I decided not to try it in my Z1.

Pax,

Ed Holyoke


A old timer said on old auto engines that didn't use aluminium pistons,
used to loosen the distributor, and run up the engine and begin
retarding
the ignition, he said he would get the exhaust glowing, hence carbon
would
be burned off. Slow burning fuel does the same in a engine designed for
fast burning fuel.

Using a fast burning fuel in a engine designed for a slow burning fuel
can
cause detonation. Detonation raises temperatures, let it go and God
forbid
pre-ignition begins to occur.

Ron Parigoris


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FLYaDIVE(at)aol.com
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PostPosted: Fri Sep 22, 2006 3:24 am    Post subject: Mogas versus 100LL Reply with quote

HERE WE GO AGAIN!

Ron:

STOP talking to those "locals at the airport". They don't know what they are
talking about and you are propagating a HUGE misconception.

"I don't know the technocrat term for it, but the speed at which the fuel
burns is a definite concern." That elusive term you don't know is called
OCTANE! And we are not talking about "Racing Fuels" ... Which some ARE ALCOHOL.
And from watching your Tach you should realize that the Lycoming and
Continental engines are NOT high RPM engines. They Red Line around 2700 RPM.
And to correct the other huge misconception you are spouting, " high octane
MoGas should not be used in a O-540s because it burns too fast" ... BULL DINKY!
The Higher the Octane the SLOWER the gas burns. I think that was covered in
primary flight training and I know it is covered in the 'P' training of A&P.

SO! Ron, and readers of Ron, do your homework. "Do not believe anything you
see and only half of what you read". Better yet, don't even believe me ... Do
Your Homework!
I HATE THE INTERNET
And hate is not a strong enough word. At the speed of an electron BS can be
spread all around the world. And remember BS is a fertilizer, some times it
does good but MOST of the time over the Internet it does bad.
There is a saying that goes: "If you say it long enough and loud enough even
a lie becomes true."

Barry
"Chop'd Liver"

"Show them the first time, correct them the second time, kick them the third
time."
Yamashiada
========================================
In a message dated 9/21/06 11:27:53 PM Eastern Daylight Time,
rparigor(at)SUFFOLK.LIB.NY.US writes:

Quote:
Octane rating is not the only thing one needs to be concerned with when it
comes to fuel. I don't know the technocrat term for it, but the speed at
which the fuel burns is a definite concern. On huge displacement engines
turning low RPMs, if you were to use a racing fuel designed for high RPM
engines of the same octane you would most like run into problems. From
what the locals at the airport say, high octane mogas should not be used
in a O-540s because it burns too fast, and since the cylinders are the
same as a O-360??

On the other hand, using 100LL that is a slow burning fuel used in a 73CC
22HP (well over 11K) Yamaha YZ 80 engine that is in my self retrieving
balloon kills performance and probably raises EGT to disheartening levels.

On Rotax 4 strokes you can use 100LL, but it does raise the EGTs because
it burns slower than high octane mogas, and some of that burn makes its
way into the exhaust.

[Barry] DUH Ron ... 100 LL, IS High Octane Gas ... What do you think the 100
Stands for!
Quote:

A old timer said on old auto engines that didn't use aluminium pistons,
used to loosen the distributor, and run up the engine and begin retarding
the ignition, he said he would get the exhaust glowing, hence carbon would
be burned off. Slow burning fuel does the same in a engine designed for
fast burning fuel.

Using a fast burning fuel in a engine designed for a slow burning fuel can
cause detonation. Detonation raises temperatures, let it go and God forbid
pre-ignition begins to occur.

[Barry] This was a question on the primary flight training ... Do you know
the difference between Detonation and Pre-Ignition? I don't think so, you have
been talking to "locals at the airport"!
Quote:

Ron Parigoris
==================================


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Kellym



Joined: 10 Jan 2006
Posts: 1705
Location: Sun Lakes AZ

PostPosted: Fri Sep 22, 2006 5:55 am    Post subject: Mogas versus 100LL Reply with quote

Yup, quite right Barry.....except, there are far more errors concerning
mixture and overheating and detonation on the P part of the A&P test
than most anywhere outside the internet. It contains all the OWTs of the
'50s and '60s as right answers. I had to unlearn a lot of stuff, to
ensure I knew the FAA's correct anwer, regardless of scientific facts.
KM
A&P/IA

FLYaDIVE(at)aol.com wrote:
Quote:


HERE WE GO AGAIN!

Ron:

STOP talking to those "locals at the airport". They don't know what they are
talking about and you are propagating a HUGE misconception.

"I don't know the technocrat term for it, but the speed at which the fuel
burns is a definite concern." That elusive term you don't know is called
OCTANE! And we are not talking about "Racing Fuels" ... Which some ARE ALCOHOL.
And from watching your Tach you should realize that the Lycoming and
Continental engines are NOT high RPM engines. They Red Line around 2700 RPM.
And to correct the other huge misconception you are spouting, " high octane
MoGas should not be used in a O-540s because it burns too fast" ... BULL DINKY!
The Higher the Octane the SLOWER the gas burns. I think that was covered in
primary flight training and I know it is covered in the 'P' training of A&P.

SO! Ron, and readers of Ron, do your homework. "Do not believe anything you
see and only half of what you read". Better yet, don't even believe me ... Do
Your Homework!



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PostPosted: Fri Sep 22, 2006 6:12 am    Post subject: Mogas versus 100LL Reply with quote

On 9/21/06 11:22 PM, "rparigor(at)SUFFOLK.LIB.NY.US"
<rparigor(at)SUFFOLK.LIB.NY.US> wrote:

Quote:


Octane rating is not the only thing one needs to be concerned with when it
comes to fuel. I don't know the technocrat term for it, but the speed at
which the fuel burns is a definite concern. On huge displacement engines
turning low RPMs, if you were to use a racing fuel designed for high RPM
engines of the same octane you would most like run into problems. From
what the locals at the airport say, high octane mogas should not be used
in a O-540s because it burns too fast, and since the cylinders are the
same as a O-360??

On the other hand, using 100LL that is a slow burning fuel used in a 73CC
22HP (well over 11K) Yamaha YZ 80 engine that is in my self retrieving
balloon kills performance and probably raises EGT to disheartening levels.

On Rotax 4 strokes you can use 100LL, but it does raise the EGTs because
it burns slower than high octane mogas, and some of that burn makes its
way into the exhaust.

A old timer said on old auto engines that didn't use aluminium pistons,
used to loosen the distributor, and run up the engine and begin retarding
the ignition, he said he would get the exhaust glowing, hence carbon would
be burned off. Slow burning fuel does the same in a engine designed for
fast burning fuel.

Using a fast burning fuel in a engine designed for a slow burning fuel can
cause detonation. Detonation raises temperatures, let it go and God forbid
pre-ignition begins to occur.

Ron Parigoris

hey Ron, is that why my old lawn mower is running better with some "dirty"
avgas I have at home?

Lory Ghertner
Quote:







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PostPosted: Fri Sep 22, 2006 10:22 am    Post subject: Mogas versus 100LL Reply with quote

Hello Barry

"HERE WE GO AGAIN!"

I think an education on one or both of our parts is in order.

"Show me the first time, correct me the second time on the following":

Detonation is not anywhere near the show stopper that Pre-Ignition is.
Detonation is the faster, not orderly burning of a fuel mixture.
Combustion pressures are higher and combustion temperatures are higher
during detonation operations. Ping is detonation. The octane rating of
fuel is the ability of fuel to burn in a orderly manor, in other words not
detonate. The higher the octane rating, the higher the temperature and
pressure that can be reached before detonation occurs. I am not positive
on this, but am pretty certain on Mainland USA, the pump octane rating is
not figured the same as Aviation fuel is, I forget if Avgas is higher or
lower. Anyway for a short time detonation probably will not cause any real
harm. Now if you let it go on for long periods of time, things will begin
to heat up, detonation will become more pronounced. The burning of fuel
mixture during Detonation begins with a spark at the plug. Preignition is
the real show stopper. This is where for some reason the mixture ignites
way too far advanced. Could be a glowing piece of carbon, or leaving the
helicoil tab stick a bit too much into the cylinder, or just plain too
high a temperature. Piston moving up and fighting pressure now.

OK comments here??

OK so if we are straight that octane rating is the ability for a fuel to
not detonate, I don't think that is absolute takes into consideration the
speed at which the flame front will burn in an orderly fashion. Different
brews can have the same octane rating, but can burn at different speeds.

I read an article in some aviation Magazine a while back that alluded to
this. I have not conducted any tests myself to prove or disprove this.

I think you misconstrued my example of large displacement slow turning
motors and small displacement fast turning motors. Fair compression slow
speed huge displacement engines need a specific octane rating, yes, but if
the brew burns in an orderly fashion, but a bit too fast, you can get
excessive pressures and heat build up in the combustion chamber. In other
words if we took a O-540 tuned with the same compression ratio as a stock
engine to develop max HP at 7500rpm, 100LL would not be as effective as a
fuel with the same octane rating yet having a faster orderly burn.

Do you know of O-540s and or O-360s that have gone to TBO using mogas?? (I
am not sure there is a STC??)

My example of my 73cc screamer was an example on the other side of things,
it requires high test gasoline, yet when using 100LL, 100LL does not burn
fast enough to keep up with what is necessary. 100LL is probably higher
octane than mogas octane rating of 93, it unquestionably burns slower, and
not fast enough for the screamer. I tried it to deal with moGas going
stale.

O-300s have a Mogas STC, they were initial designed for 80 octane.
Listening to hangar talk, using MoGas in a O-300 actual lowers EGTs. This
is because it was said to burn a little faster. It is also said that it
will tend to detonate a bit easier, and going a little richer on the
mixture with MoGas (larger main jet) gives a bit more leeway and long term
success can be expected.

Comments here, I am only repeating what I heard on the O-300.

Sincerely
Ron Parigoris


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LloydDR(at)wernerco.com
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PostPosted: Fri Sep 22, 2006 11:29 am    Post subject: Mogas versus 100LL Reply with quote

And what does any of this have to do with the electrical system of an
airplane?

Dan Lloyd
40269
RV10E
Especially DO not archive

--


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AV8ORJWC



Joined: 13 Jul 2006
Posts: 1149
Location: Aurora, Oregon "Home of VANS"

PostPosted: Fri Sep 22, 2006 11:37 am    Post subject: Mogas versus 100LL Reply with quote

Guys, get a copy of the October issue of Kitplanes Magazine $4,99 US,
before all of my RV-10 buddies do. Read Walter Atkinson's article
beginning on Page 35 with the Hand Grenade as a header - "Detonation and
Pre-Ignition". Or call him as the article says at (225) 925-2066. This
has very little to do with airplane electrical. Walter has done a
remarkable job on this subject. It is more clear that what was taught
in A & P school.

Repeating what is rumored or heard is far from resolution on this
important issue.

John Cox
Do not Archive

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PostPosted: Fri Sep 22, 2006 11:51 pm    Post subject: Mogas versus 100LL Reply with quote

Quote:
Time: 08:24:46 PM PST US
Subject: Re: AeroElectric-List: Mogas versus 100LL
From: <rparigor(at)SUFFOLK.LIB.NY.US>

Quote:
Octane rating is not the only thing one needs to be concerned with when it
comes to fuel. I don't know the technocrat term for it, but the speed at
which the fuel burns is a definite concern. On huge displacement engines
turning low RPMs, if you were to use a racing fuel designed for high RPM
engines of the same octane you would most like run into problems. From
what the locals at the airport say, high octane mogas should not be used
in a O-540s because it burns too fast, and since the cylinders are the
same as a O-360??

I believe you are referring to something called the Cetane rating
Ron. The only place I've ever heard this term used is in association with
DIESEL engines. Since the fuel is injected directly into the highly
heated and compressed air in the diesel's cylinders, it must burn at a
certain rate so as not to spontaneously combust and cause detonation (yes
diesels can be damaged by detonation too). I've never heard it used
in association with gasoline engines but that doesn't mean it's not
important.

Quote:
On the other hand, using 100LL that is a slow burning fuel used in a 73CC
22HP (well over 11K) Yamaha YZ 80 engine that is in my self retrieving
balloon kills performance and probably raises EGT to disheartening levels.

I could be wrong but I don't think it's the high octane that is
causing your situation here. Tetraethel lead in the fuel effectively
increases the resistance of the mixture to spontaneously combusting in the
higher temperatures of aircooled engines and higher cylinder pressures of
the old 1960s era high performance auto engines (10/11/12 to 1
compression ratios with the old design wedge heads). What may be
causing the 100LL to burn slower is the particular mix of aromatic
hydrocarbons that make up the composition of av-gas. The av-gas
mixture has a much lower volatility than mogas and that may be related to
why it burns at a different speed (assuming you are correct here).

Quote:
On Rotax 4 strokes you can use 100LL, but it does raise the EGTs because
it burns slower than high octane mogas, and some of that burn makes its
way into the exhaust.

There may be something to this. The old Ford tractor we used to mow
the runway at the soaring club was run on 100LL (always available from our
tow plane supply) for many years and although it seemed to run just fine,
eventually it needed a valve job. Not sure if that was due to 100LL or
just wear and tear. Old piston engines used softer valves and seats
and needed the cushioning effect of the lead in the fuel to get
reasonable service life from these components (mfgrs went to much
harder valves and seats when the lead was phased out many years ago).

Quote:
A old timer said on old auto engines that didn't use aluminium pistons,
used to loosen the distributor, and run up the engine and begin retarding
the ignition, he said he would get the exhaust glowing, hence carbon would
be burned off. Slow burning fuel does the same in a engine designed for
fast burning fuel.

Not sure why you'd want to do that unless the carbon was causing
detonation or pre-ignition. But yes, on an engine with severely
retarded timing the fuel would still be burning when the exhaust valve
opened and EGT would be higher (ala your "slow burning" fuel).

Quote:
Using a fast burning fuel in a engine designed for a slow burning fuel can
cause detonation. Detonation raises temperatures, let it go and God forbid
pre-ignition begins to occur.

First time I've really heard this argument Ron, I don't think the
lead is the culprit though, racing engines use lead in the gasoline to
this day as far as I know (you just can't get octanes much above 95 with
unleaded gasoline so you have to use lead or go to alcohol or some
other fuel to get there without the lead). But...it may be the fuel
formulation differences, I'll refer this one to Archie (and anyone else
who wants to chime in about the subject) on the engines list. There are
some very knowledgeable folks over there.

Quote:
Ron Parigoris

Dean Psiropoulos
Do not archive on the aeroelectric list


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PostPosted: Sat Sep 23, 2006 5:24 am    Post subject: Mogas versus 100LL Reply with quote

Good comments, Ron.  Yes, the octane rating is a measure of the resistance of the fuel/air mixture to auto-ignite, or ignite without the presence of an open flame.  It's a complex phenomenon and autoignition is a function of time, pressure and temperature.  The rating methodology, developed by Ethyl Corporation way back when, compares the resistance to auto-ignition of the test fuel to mixtures of normal heptane to iso-octane.  The octane number is the percentage of iso-octane in the mix that behaves like the test fuel (80-octane fuel behaves like a a mixture of 20% n-heptane and 80% iso-octane).  Since the actual test is really done by measuring the BMEP of a test engine, numbers over 100 are possible - 110-octane fuel will allow the test engine to operate at 110% of the BMEP allowed by iso-octane(not exactly correct, but close enough).  The method isn't exact and if the test engine is operated at different temperatures and loads the observed octane number comes out differently.  Running at light load creates an octane rating called the "research octane" number and running at high temperature/high load produces a number called "motor octane".  The law says the average octane rating of an automotive fuel must be displayed on the pump and that's why it says (R+M)/2 on the pump.  Aviation fuel, because engines are usually operated under conditions more like the the motor method, are typically rated by the motor method - I believe 100LL is more correctly called 100/120 (100 motor octane and 120 research octane).  That's why some of the past-history fuels were called 110/130 etc.
When the fuel is ignited by an open flame front there is little difference in flame speed between high octane and low octane fuel:  Flame speed is NOT the primary difference between the fuels.  Detonation is auto-ignition of the "end gases" late in the combustion process.  Because of combustion in the cylinder the end gases(the portion of the charge furthest from the ignition source) are compressed to very high pressures, which because of the gas law PV=NRT, raises their temperature.  They will only remain unburned for a limited time and if the flame front doesn't arrive soon enough, igniting them progressively, they will auto-ignite all at once, "exploding."  This will cause all sorts of problems, but will not usually result in immediate engine failure.

As Ron correctly described below, "pre-ignition" is simply ignition of the fuel charge before the spark occurs.  Because the charge burns and raises the pressure before the piston fully compresses the charge the temperature and pressure can go to extremely high levels, causing major damage very quickly.  For example (George Braly would have the real numbers) normal compression (no ignition) pressure might be 350psi, combustion results in pressures of 800 to 1,000 psi and pre-ignition could result in pressures up to 2,000 psi.  Pre-ignition can (likely) result from pre-ignition because of the extremely high pressure and temperature of the end gases.  One thing that isn't usually thought about is the rate of heat transfer from a gas to the cylinder, which is proportional to the velocity, temperature difference AND PRESSURE of the gas.  Double the pressure and double the temperature and the heat transfer will go up by a factor of 4.  Detonation results in sonic shock waves traveling through the combustion chamber, which means locally the hot gases are moving at sonic velocity.  Much of the damage from both detonation and pre-ignition is caused by the extremely high heat transfer rates, not just the pressure.  Detonation under high loads can increase the local heat transfer rate to the point that after a short time a component (edge of the exhaust valve, carbon deposit, etc) can be heated to the point it causes pre-ignition.  This has sometimes been referred to as "runaway detonation."

I hope from the above description you get the idea that detonation and pre-ignition can be a real horror story for the engine.  The best thing is to maintain a healthy detonation margin.  In general, detonation is suppressed by increasing the octane rating, lowering the compression ratio, retarding the ignition timing, lowering the inlet temperature and pressure, lowering the cylinder head temperature, increasing turbulence and reducing the distance from the spark plug to the end gases(by running on both plugs).  And you can dilute the charge by adding water, excess fuel or excess air (run LOP).  As you can see, most of these things will reduce the power output of the engine and therein lies the conundrum.  For a standard unmodified aircraft engine the worst operating case would be a takeoff from Death Valley on a dry, 115-degree day with the carb heat left on, mixture leaned and one of the mags inoperative.  As I recall it is just those conditions that the FAA requires to be demonstrated for a certified engine.

Sorry about the excessively long non-electric dissertation.

Gary Casey

[quote]
The higher the octane rating, the higher the temperature and
pressure that can be reached before detonation occurs. I am not positive
on this, but am pretty certain on Mainland USA, the pump octane rating
is
not figured the same as Aviation fuel is, I forget if Avgas is higher or
lower. Anyway for a short time detonation probably will not cause any
real
harm. Now if you let it go on for long periods of time, things will
begin
to heat up, detonation will become more pronounced. The burning of fuel
mixture during Detonation begins with a spark at the plug. Preignition
is
the real show stopper. This is where for some reason the mixture ignites
way too far advanced. Could be a glowing piece of carbon, or leaving the
helicoil tab stick a bit too much into the cylinder, or just plain too
high a temperature. Piston moving up and fighting pressure now.


OK comments here??


OK so if we are straight that octane rating is the ability for a fuel to
not detonate, I don't think that is absolute takes into consideration
the
speed at which the flame front will burn in an orderly fashion.
Different
brews can have the same octane rating, but can burn at different speeds.

Ron Parigoris [b]


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n801bh(at)NetZero.com
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PostPosted: Sat Sep 23, 2006 2:38 pm    Post subject: Mogas versus 100LL Reply with quote

Thank you Gary. That is the best explanation of octane and detonation/ preignition scenerios I have ever read. I was going to try to desrcibe it but you did so much better. The only thing I might add is the fact that in the process of diluting the mixture with extra air there is a point where before it really starts to run lean the engine flame front goes into a blow torch effect and will melt the top of the piston and or the exhaust valve. If you can transition though this setting fast enough the motor just lays down and the EGT's show a cooler number, set the air/fuel mixture at around 16-1, depending on the engine of course, and work the motor hard, you will get very high EGT's, low fuel flow numbers and hurt pistons/ valves. The best way to describe it is watch a cutting torch work, one adjusts the mixture till they get a neutral flame and when you press the lever the additional O2 will not only add to the velocity of the flame front , it will almost double the temp of the cutting flame itself. .

Ben
www.haaspowerair.com
Ben Haas
N801BH
www.haaspowerair.com

-- Gary Casey <glcasey(at)adelphia.net> wrote:
Good comments, Ron. Yes, the octane rating is a measure of the resistance of the fuel/air mixture to auto-ignite, or ignite without the presence of an open flame. It's a complex phenomenon and autoignition is a function of time, pressure and temperature.  The rating methodology, developed by Ethyl Corporation way back when, compares the resistance to auto-ignition of the test fuel to mixtures of normal heptane to iso-octane. The octane number is the percentage of iso-octane in the mix that behaves like the test fuel (80-octane fuel behaves like a a mixture of 20% n-heptane and 80% iso-octane).  Since the actual test is really done by measuring the BMEP of a test engine, numbers over 100 are possible - 110-octane fuel will allow the test engine to operate at 110% of the BMEP allowed by iso-octane(not exactly correct, but close enough). The method isn't exact and if the test engine is operated at different temperatures and loads the observed octane number comes out differently. Running at light load creates an octane rating called the "research octane" number and running at high temperature/high load produces a number called "motor octane". The law says the average octane rating of an automotive fuel must be displayed on the pump and that's why it says (R+M)/2 on the pump. Aviation fuel, because engines are usually operated under conditions more like the the motor method, are typically rated by the motor method - I believe 100LL is more correctly called 100/120 (100 motor octane and 120 research octane). That's why some of the past-history fuels were called 110/130 etc.
When the fuel is ignited by an open flame front there is little difference in flame speed between high octane and low octane fuel: Flame speed is NOT the primary difference between the fuels. Detonation is auto-ignition of the "end gases" late in the combustion process. Because of combustion in the cylinder the end gases(the portion of the charge furthest from the ignition source) are compressed to very high pressures, which because of the gas law PV=NRT, raises their temperature. They will only remain unburned for a limited time and if the flame front doesn't arrive soon enough, igniting them progressively, they will auto-ignite all at once, "exploding." This will cause all sorts of problems, but will not usually result in immediate engine failure.

As Ron correctly described below, "pre-ignition" is simply ignition of the fuel charge before the spark occurs. Because the charge burns and raises the pressure before the piston fully compresses the charge the temperature and pressure can go to extremely high levels, causing major damage very quickly. For example (George Braly would have the real numbers) normal compression (no ignition) pressure might be 350psi, combustion results in pressures of 800 to 1,000 psi and pre-ignition could result in pressures up to 2,000 psi. Pre-ignition can (likely) result from pre-ignition because of the extremely high pressure and temperature of the end gases. One thing that isn't usually thought about is the rate of heat transfer from a gas to the cylinder, which is proportional to the velocity, temperature difference AND PRESSURE of the gas. Double the pressure and double the temperature and the heat transfer will go up by a factor of 4. Detonation results in sonic shock waves traveling through the combustion chamber, which means locally the hot gases are moving at sonic velocity. Much of the damage from both detonation and pre-ignition is caused by the extremely high heat transfer rates, not just the pressure. Detonation under high loads can increase the local heat transfer rate to the point that after a short time a component (edge of the exhaust valve, carbon deposit, etc) can be heated to the point it causes pre-ignition. This has sometimes been referred to as "runaway detonation."

I hope from the above description you get the idea that detonation and pre-ignition can be a real horror story for the engine. The best thing is to maintain a healthy detonation margin. In general, detonation is suppressed by increasing the octane rating, lowering the compression ratio, retarding the ignition timing, lowering the inlet temperature and pressure, lowering the cylinder head temperature, increasing turbulence and reducing the distance from the spark plug to the end gases(by running on both plugs). And you can dilute the charge by adding water, excess fuel or excess air (run LOP). As you can see, most of these things will reduce the power output of the engine and therein lies the conundrum. For a standard unmodified aircraft engine the worst operating case would be a takeoff from Death Valley on a dry, 115-degree day with the carb heat left on, mixture leaned and one of the mags inoperative. As I recall it is just those conditions that the FAA requires to be demonstrated for a certified engine.

Sorry about the excessively long non-electric dissertation.

Gary Casey

[quote]
The higher the octane rating, the higher the temperature and
pressure that can be reached before detonation occurs. I am not positive
on this, but am pretty certain on Mainland USA, the pump octane rating
is
not figured the same as Aviation fuel is, I forget if Avgas is higher or
lower. Anyway for a short time detonation probably will not cause any
real
harm. Now if you let it go on for long periods of time, things will
begin
to heat up, detonation will become more pronounced. The burning of fuel
mixture during Detonation begins with a spark at the plug. Preignition
is
the real show stopper. This is where for some reason the mixture ignites
way too far advanced. Could be a glowing piece of carbon, or leaving the
helicoil tab stick a bit too much into the cylinder, or just plain too
high a temperature. Piston moving up and fighting pressure now.


OK comments here??


OK so if we are straight that octane rating is the ability for a fuel to
not detonate, I don't think that is absolute takes into consideration
the
speed at which the flame front will burn in an orderly fashion.
Different
brews can have the same octane rating, but can burn at different speeds.

Ron Parigoris

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[b]


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Jim Baker



Joined: 30 Mar 2006
Posts: 181
Location: Sayre, PA

PostPosted: Sat Sep 23, 2006 3:36 pm    Post subject: Mogas versus 100LL Reply with quote

Quote:
in the process of diluting the mixture with extra air there is a point
where before it really starts to run lean the engine flame front goes into a blow torch effect and
will melt the top of the piston and or the exhaust valve.

I suppose you can prove this? Not anecdotally, but with solid,
repeatable science that has already been done? And might want
to take this to the Matronics Engines list instead.....

Jim Baker
580.788.2779
Elmore City, OK


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Jim Baker



Joined: 30 Mar 2006
Posts: 181
Location: Sayre, PA

PostPosted: Sat Sep 23, 2006 3:45 pm    Post subject: Mogas versus 100LL Reply with quote

Quote:
The best way to describe it is
watch a cutting torch work, one adjusts the mixture till they get a neutral flameand when you
press the lever the additional O2 will not only add to the velocity of the flame front , it will almost
double the temp of the cutting flame itself. .

Oops...too quick on the send..didn't even see this bit of mis-info.

Tell ya what you do....take the torch, heat up the metal to start
the cut, start the cut with the extra O2 then turn off the fuel gas.
Continues to cut, doesn't it. The flame never gets twice as hot,
not even close. As a matter of record the temperature stays
roughly the same.

http://www.welding.org/newsletters/winter2002/oxyfuel.html


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Kellym



Joined: 10 Jan 2006
Posts: 1705
Location: Sun Lakes AZ

PostPosted: Sat Sep 23, 2006 6:59 pm    Post subject: Mogas versus 100LL Reply with quote

Well, other than promoting serious old wive's tales on a list where
subject is way off topic...........
by definition, any leaning past peak EGT is in fact COOLER than at
peak. Given that the oxygen content is changing less than 1 percent
between 50 ROP and 50 LOP, the torch effect you claim violates known
laws of physics.

n801bh(at)netzero.com wrote:
Quote:

Thank you Gary. That is the best explanation of octane and detonation/
preignition scenerios I have ever read. I was going to try to desrcibe
it but you did so much better. The only thing I might add is the fact
that in the process of diluting the mixture with extra air there is a
point where before it really starts to run lean the engine flame front
goes into a blow torch effect and will melt the top of the piston and
or the exhaust valve. If you can transition though this setting fast
enough the motor just lays down and the EGT's show a cooler number,
set the air/fuel mixture at around 16-1, depending on the engine of
course, and work the motor hard, you will get very high EGT's, low
fuel flow numbers and hurt pistons/ valves. The best way to describe
it is watch a cutting torch work, one adjusts the mixture till they
get a neutral flame and when you press the lever the additional O2
will not only add to the velocity of the flame front , it will almost
double the temp of the cutting flame itself. .



Ben

www.haaspowerair.com



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n395v



Joined: 10 Jan 2006
Posts: 450

PostPosted: Sun Sep 24, 2006 4:38 am    Post subject: Re: Mogas versus 100LL Reply with quote

It is my understanding that spending too much time in an FBO listening to old farts spout OWTs will fry your brain and cause you to post fuel related blather and weird misinformation on the aeroelectric forum.

DO NOT ARCHIVE


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PostPosted: Sun Sep 24, 2006 4:57 am    Post subject: Mogas versus 100LL Reply with quote

Yes, this probably should go over to the engine list, but one more short comment (can't help myself):Cetane rating is essentially the inverse of the Octane rating - a fuel with a high Cetane rating will spontaneously ignite very easily, and that is what you want for a diesel engine.  Cetane and Octane ratings, as far as I know, are never used at the same time.  It's not a different phenomena that is being rated - the scale is just reversed.  Also, someone mentioned the "cutting torch" effect of running lean - this indeed was the conventional thought in the automotive world back when I "grew up" in it.  the idea was that you always have to run rich of stoichiometric (even richer that peak EGT) or the left over oxygen in the exhaust would oxidize (burn) the exhaust valve.  Yesterday's equivalent of an urban legend, as George Braly has so often pointed out.  The valve and valve seats just aren't that hot.

Gary Casey
[quote]


I believe you are referring to something called the Cetane rating
Ron.  The only place I've ever heard this term used is in association with
DIESEL engines.  Since the fuel is injected directly into the highly
heated and compressed air in the diesel's cylinders, it must burn at a
certain rate so as not to spontaneously combust and cause detonation.... [b]


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PostPosted: Sun Sep 24, 2006 12:54 pm    Post subject: Mogas versus 100LL Reply with quote

At 07:22 AM 9/22/2006 -0400, you wrote:

Quote:


HERE WE GO AGAIN!

Ron:

STOP talking to those "locals at the airport". They don't know what they are
talking about and you are propagating a HUGE misconception.

It's been an interesting thread . . . but never did see
any words that were trading off the features of Mogas
vs. 100LL . . . I WAS interested because I don't understand
how or why they would be compared to each other.

Mogas is intended for engines that originally ran on 80
octane avgas. I know folks that have been using 90+ Mogas
in their airplanes for decades before EAA got the "Mogas
STC gleam" in their eye. We sold 91+ mogas at 1K1 and about
half the fleet anchored there uses the less expensive fuel.
I'll suggest that the Mogas experiment has been quite successful.

As for 100LL . . . every year at OSH they've been telling
us that the stuff was "going away soon" . . . that started
up 15 or more years ago. During on of my earliest trips to
OSH I met an retired GM engine-guru who was building a Longez
powered with an O-235. He still had "pull" at GM and
was running his engine in a test cell to experiment with
some alternatives to 100LL.

He observed that lower octane mogas would run fine in any
engine when peak pressures before and during combustion were
limited . . . like when operating at low manifold pressures.
He hypothesized that the only time he NEEDED high manifold
pressures was during takeoff and early climb phase. High
altitude cruise would not allow him to take advantage of
the octane rating of 100LL because the spark from mags
was too late and full-throttle manifold pressure
was too low.

He also hypothesized that the ability of electronics
for controlling a two-fluids (air/gasoline) system
could be easily adapted to controlling a three-fluids
system (air/gasoline/water+alcohol).

His goal was to see if practical techniques could be
crafted to allow burning lower octane fuels in non-
turbocharged)100 octane engines. He reasoned that when
100LL finally bit the dust, there might be a way to
avoid junking 100,000 or so airplanes that needed
that fuel.

The last time we talked, he was thinking he'd need to
carry about 10 gallons of water-alcohol to supplement
the standard Longez fuel load.

I've lost track of him . . . it would be interesting
to see what he ultimately discovered. Here are a few
interesting links that speak to the need for matching
burn characteristics of the fuel to the engine.

http://en.wikipedia.org/wiki/Detonation_internal_combustion_engine

http://www.technologyreview.com/read_article.aspx?id=16727&ch=energy

Bob . . .


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Paul LeDoux



Joined: 10 Jan 2006
Posts: 13

PostPosted: Sun Sep 24, 2006 3:30 pm    Post subject: Mogas versus 100LL Reply with quote

Back in the 1950's my Dad and I raced outboards with Mercury motors. We ran alcohol fuel and nitro methane as an additive. They did indeed melt the top of the piston if they were run too lean. Been there, done that. Never heard of it in a 4 stroke engine though.
Paul

DO NOT ARCHIVE
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