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mogas vs 100LL

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vectorwarbirds(at)aol.com
Guest

Posted: Mon Nov 12, 2007 8:13 pm Post subject: mogas vs 100LL

According to Aeroshell:

 

  72 octane is the highest grade of gasoline that can be  manufactured without [additives].   

  80/87   has    0.50 ml  TEL [tetra-ethyl lead] per gallon and is dyed RED  

  91/96       2.00 ml TEL per gallon BLUE 

  100LL      2.00 ml TEL per gallon BLUE 

  100/130   3.00 ml TEL per gallon GREEN 

  115/145   4.60 ml TEL per gallon PURPLE 

 

  

 

  

 

  That makes the lead content of 100LL only 4 times higher than 80/87 octane not 8.

  

 

  As for the idea that all gasoline is gasoline that it not true.

 

  

 

  What is true that 80/87 produces more power than 100LL given the engine does not require a higher octane for anti knock reasons.  The reason for this is that 80/87 has a higher volitility rating than 100LL, so in simple terms it burns faster.  In engines made to burn lower octane fuel the timing and other engine parameters were set for this factor.  It might seem odd that fuels with higher octane ratings burn less easily, yet are popularly thought of as more powerful.  This misunderstanding is caused by confusing the ability of the fuel to resist compression detonation as opposed to the ability of the fuel to burn (combustion).  A simple explanation is that carbon-carbon bonds contain more energy than carbon-hydrogen bonds.  Hence a fuel with a greater number of carbon bonds will carry more energy regardless of the octane rating and 80/87 carries a greater number than 100LL.

 

 

  [b]If you take that same fuel that worked well in a high compression engine and use it in a low compression engine with it's lower cranking compression and lower combustion chamber temps, it will, without a doubt, burn much too slowly at those lower temps and pressures and reach maximum cylinder pressure too far after TDC for best power.[/b] 

  [b]Things generally burn slower when they are cooler and vice versa. Peak cylinder pressure will occur much past TDC - decreasing the power produced if you keep the same ignition timing.[/b]

   [b]  You can advance ignition timing to try to recover power, but that will cause the air/fuel mixture to burn earlier in the crank stroke and spend, percentage wise, more of the energy produced by the expanding, burning mixture, pushing back down, trying to prevent the piston from rising up to the power stroke - robbing power.[/b]

   [b]If you MUST use a slow burning fuel, which USUALLY has a high octane rating, advancing the ignition timing will lessen the power loss, but the best results are usually obtained with the quickest burning fuel obtainable, that, of course, doesn't "knock".

  

  All other factors being the same, except for burn rate - use the quickest burning fuel that doesn't "knock", light the spark in the middle of the combustion chamber, adjust ignition timing to reach peak cylinder pressure ~TDC and keep your mixture correct. When the ignition timing is correct, the engine will make best power for that fuel.[/b]

  

 [b][/b]

  [b]Also according to the EAA Peterson STC you  can put mo-gas in the tank 75% (TEL 0.002 ml) and 25% av-gas  (100LL at 2.00 ml TEL) and you have the same 0.50 ml TEL that 80/87 had.  Another way is to run about three tanks of mo-gas and  the fourth is a tank of av-gas (100LL)[/b]

  

 

  So I think the bottom line it this, our engines were designed and built to burn 70 octane Chinese or Russian fuel not 100LL Avgas. And until someone can show me the stats on the lead content in Chinese 70 oct I will continue to use mogas and avgas at the shows in mine.

  

 

  One thing I do know, its cheaper!!

  

 Thanks for your thoughts.

  

 

  GBunn

  Vector Aviation USA

 

  

 

   Email and AIM finally together. You've gotta check out free AOL Mail! 

  [quote][b]

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brian-1927(at)lloyd.com
Guest

Posted: Mon Nov 12, 2007 9:12 pm Post subject: mogas vs 100LL

On Nov 12, 2007, at 8:12 PM, vectorwarbirds(at)aol.com wrote:



 	  Quote: 	
		   According to Aeroshell:



 72 octane is the highest grade of gasoline that can be manufactured  

 without [additives].

 80/87 has 0.50 ml TEL [tetra-ethyl lead] per gallon and is dyed RED

 91/96 2.00 ml TEL per gallon BLUE

 100LL 2.00 ml TEL per gallon BLUE

 100/130 3.00 ml TEL per gallon GREEN

 115/145 4.60 ml TEL per gallon PURPLE

 That makes the lead content of 100LL only 4 times higher than 80/87  

 octane not 8.


	


Well, I said, "about six times". I didn't look it up before  

commenting. Shame on me.



 	  Quote: 	
		   As for the idea that all gasoline is gasoline that it not true.



 What is true that 80/87 produces more power than 100LL given the  

 engine does not require a higher octane for anti knock reasons.   

 The reason for this is that 80/87 has a higher volitility rating  

 than 100LL, so in simple terms it burns faster.


	


Hmm, my understanding is different. I was under the impression that  

octane rating does not affect the speed of combustion, only the  

resistance to detonation.



Reference: http://en.wikipedia.org/wiki/Octane_rating



To quote:



"Octane rating has no direct impact on the deflagration (burn) of the  

air/fuel mixture in the combustion chamber. Other properties of  

gasoline and engine design account for the manner at which  

deflagration takes place. In other words, the flame speed of a  

normally ignited mixture is not directly connected to octane rating.  

Deflagration is the type of combustion that constitutes the normal  

burn. Detonation is a different type of combustion and this is to be  

avoided in spark ignited gasoline engines. Octane rating is a measure  

of detonation resistance, not deflagration characteristics."



 	  Quote: 	
		   In engines made to burn lower octane fuel the timing and other  

 engine parameters were set for this factor.  It might seem odd that  

 fuels with higher octane ratings burn less easily, yet are  

 popularly thought of as more powerful.  This misunderstanding is  

 caused by confusing the ability of the fuel to resist compression  

 detonation as opposed to the ability of the fuel to burn  

 (combustion).  A simple explanation is that carbon-carbon bonds  

 contain more energy than carbon-hydrogen bonds.  Hence a fuel with  

 a greater number of carbon bonds will carry more energy regardless  

 of the octane rating and 80/87 carries a greater number than 100LL.


	


Hmm, most gasoline comes from the same base stock at the refineries.  

Again, I was under the impression that the differences mostly stem  

from the additive packages that go into the base gasoline after  

refining.



As for carbon-carbon bonds vs. carbon-hydrogen bonds: it seems to me  

that we would have a different reaction requiring a different amount  

of O2 to ensure complete combustion. Since all these fuels operate at  

the same mixtures by mass, it seems to me that the gasoline is  

therefore inherently the same.



 	  Quote: 	
		   If you take that same fuel that worked well in a high compression  

 engine and use it in a low compression engine with it's lower  

 cranking compression and lower combustion chamber temps, it will,  

 without a doubt, burn much too slowly at those lower temps and  

 pressures and reach maximum cylinder pressure too far after TDC for  

 best power.


	


This also seems odd. If so, we would notice a difference in  

performance between mogas and avgas with performance suffering from  

the use of avgas. I am not aware of anyone noticing this. I am also  

not aware of any warnings that an engine designed for 72 or 80 octane  

fuel will produce less power when operated on 100LL. I suspect that,  

were this so, the EAA and the FAA would let us know.



 	  Quote: 	
		   Things generally burn slower when they are cooler and vice versa.  

 Peak cylinder pressure will occur much past TDC - decreasing the  

 power produced if you keep the same ignition timing.


	


Hmm, another interesting point would be EGT. If what you are saying  

is true, there would be a general increase in EGT with 100LL as the  

combustion endpoint would be later leading to less heat transfer to  

the cylinder and resulting in higher absolute exhaust gas  

temperatures. I certainly have never noticed a difference in absolute  

EGTs when operating on mogas vs. 100LL.

 	  Quote: 	
		   You can advance ignition timing to try to recover power, but that  

 will cause the air/fuel mixture to burn earlier in the crank stroke  

 and spend, percentage wise, more of the energy produced by the  

 expanding, burning mixture, pushing back down, trying to prevent  

 the piston from rising up to the power stroke - robbing power.

 If you MUST use a slow burning fuel, which USUALLY has a high  

 octane rating, advancing the ignition timing will lessen the power  

 loss, but the best results are usually obtained with the quickest  

 burning fuel obtainable, that, of course, doesn't "knock".


	


Hmm, the airframe and engine manufacturers do not specify different  

ignition timings for different fuels. If what you say is true, I  

would expect them to require a change in spark timing if different  

fuels are being used.



 	  Quote: 	
		   All other factors being the same, except for burn rate - use the  

 quickest burning fuel that doesn't "knock", light the spark in the  

 middle of the combustion chamber,


	


Wouldn't two points of ignition result in two flame fronts and more  

complete combustion? Isn't that why we get more power and lower EGTs  

with both mags on?



 	  Quote: 	
		   adjust ignition timing to reach peak cylinder pressure ~TDC and  

 keep your mixture correct.


	


I could be wrong but I was under the impression that you want peak  

cylinder pressure to occur substantially after TDC where the crank  

position provides more favorable mechanical advantage.



 	  Quote: 	
		   When the ignition timing is correct, the engine will make best  

 power for that fuel.


	


I agree with that.



 	  Quote: 	
		   Also according to the EAA Peterson STC you can put mo-gas in the  

 tank 75% (TEL 0.002 ml) and 25% av-gas (100LL at 2.00 ml TEL) and  

 you have the same 0.50 ml TEL that 80/87 had.  Another way is to  

 run about three tanks of mo-gas and the fourth is a tank of av-gas  

 (100LL)


	


Yes.



Also, a tiny amount of 100LL added to mogas results in a substantial  

increase in octane/AKI (anti-knock index), more than one would expect  

by the amount of TEL introduced.

 	  Quote: 	
		  

 So I think the bottom line it this, our engines were designed and  

 built to burn 70 octane Chinese or Russian fuel not 100LL Avgas.  

 And until someone can show me the stats on the lead content in  

 Chinese 70 oct I will continue to use mogas and avgas at the shows  

 in mine.



 One thing I do know, its cheaper!!


	


Yup, I agree with that.



 	  Quote: 	
		  

 Thanks for your thoughts.



 GBunn

 Vector Aviation USA



 Email and AIM finally together. You've gotta check out free AOL Mail!

 www.matronics.com/Navigator?Yak-List _- 

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