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Mogas versus 100LL
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bakerocb

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

Posted: Thu Sep 21, 2006 5:58 pm Post subject: Mogas versus 100LL

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

rparigor(at)SUFFOLK.LIB.N
Guest

Posted: Thu Sep 21, 2006 7:24 pm Post subject: Mogas versus 100LL

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

Mark Phillips in TN

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

Posted: Thu Sep 21, 2006 8:10 pm Post subject: Mogas versus 100LL

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!  

  

 Most respectfully and seriously- 

 Mark Phillips- on the "uses both" side of the issue and do not archive

  [quote][b]

_________________
From The PossumWorks...

bicyclop(at)pacbell.net
Guest

Posted: Thu Sep 21, 2006 10:07 pm Post subject: Mogas versus 100LL

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

FLYaDIVE(at)aol.com
Guest

Posted: Fri Sep 22, 2006 3:24 am Post subject: Mogas versus 100LL

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

==================================

Kellym

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

Posted: Fri Sep 22, 2006 5:55 am Post subject: Mogas versus 100LL

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!

_________________
Kelly McMullen
A&P/IA, EAA Tech Counselor # 5286
KCHD

nghertner(at)verizon.net
Guest

Posted: Fri Sep 22, 2006 6:12 am Post subject: Mogas versus 100LL

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:

rparigor(at)SUFFOLK.LIB.N
Guest

Posted: Fri Sep 22, 2006 10:22 am Post subject: Mogas versus 100LL

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

LloydDR(at)wernerco.com
Guest

Posted: Fri Sep 22, 2006 11:29 am Post subject: Mogas versus 100LL

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

airplane? 



Dan Lloyd

40269

RV10E

Especially DO not archive 



--

AV8ORJWC

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

Posted: Fri Sep 22, 2006 11:37 am Post subject: Mogas versus 100LL

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



--

dean.psiropoulos(at)veriz
Guest

Posted: Fri Sep 22, 2006 11:51 pm Post subject: Mogas versus 100LL

 	  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

glcasey(at)adelphia.net
Guest

Posted: Sat Sep 23, 2006 5:24 am Post subject: Mogas versus 100LL

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]

n801bh(at)NetZero.com
Guest

Posted: Sat Sep 23, 2006 2:38 pm Post subject: Mogas versus 100LL

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]

Jim Baker

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

Posted: Sat Sep 23, 2006 3:36 pm Post subject: Mogas versus 100LL

 	  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

Jim Baker

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

Posted: Sat Sep 23, 2006 3:45 pm Post subject: Mogas versus 100LL

 	  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

Kellym

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

Posted: Sat Sep 23, 2006 6:59 pm Post subject: Mogas versus 100LL

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

_________________
Kelly McMullen
A&P/IA, EAA Tech Counselor # 5286
KCHD

n395v

Joined: 10 Jan 2006
Posts: 450

Posted: Sun Sep 24, 2006 4:38 am Post subject: Re: Mogas versus 100LL

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

_________________
Milt

glcasey(at)adelphia.net
Guest

Posted: Sun Sep 24, 2006 4:57 am Post subject: Mogas versus 100LL

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]

nuckollsr(at)cox.net
Guest

Posted: Sun Sep 24, 2006 12:54 pm Post subject: Mogas versus 100LL

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 . . .

Paul LeDoux

Joined: 10 Jan 2006
Posts: 13

Posted: Sun Sep 24, 2006 3:30 pm Post subject: Mogas versus 100LL

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

  [quote][b]

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