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Ethanol and wing tanks Ethanol and wing tanks

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pwmac(at)sisna.com
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Posted: Wed Aug 16, 2006 11:58 am Post subject: Ethanol and wing tanks Ethanol and wing tanks

Mike G.,

 Kind of off subject which was alcohol in the fiberglass fuel tanks. Reading on----

  I have to explain. When I said:

 "An alky engine properly designed will have more horsepower that an optimally tuned gas engine due to higher octane."

  It is a fact that an alky specific engines are always designed to have a higher compression ratio (CR) than the typical gas engine. And this is allowed because of the higher octane. There is a direct relationship between CR and efficiency and horsepower. Higher is better.

 By the same token if you put alky (any percentage) in an engine designed for gas it will have less horsepower because the engine is not taking advantage of the higher octane AND less fuel economy due to less energy per gallon. With the engine designed with a CR for alky and you find some high octane gasoline that has the same octane as the alky then the power will be the same/similar but when using alky that engine will have poorer fuel economy compared to the high test gasoline.



 This is all academic because the engines now being produced in the US are not properly designed for any percentage of alcohol. The result is lower power and lower fuel economy compared to using gas which is the design point. The engines are designed of the lowest octane that the manufacturer expects to be used.



 Note: In the US gasoline is posted as (R+M)/2 even though the short cut way to write it is R+M/2. Trivia to be sure.



  You said:

  87 octane fuel, the "regular" gasoline in the US and Canada, would be 91-95 (regular) in Europe." 

 I thought there was a constant relationship between (R+M)/2 and M but I was wrong. I just looked up some racing fuels and found one with 110M, 112R, 111 (R+M)/2 and another that was 91M, 103R, 97 (R+M)/2. Lots of chemistry in these fuels that prevents us from relating the (R+M)/2 to M.



 You said:

 "Autogas and avgas octanes are not calculated the same way. Avgas octane is measured using the "MON" method, while auto fuels (in the U.S. anyway) are typically measured using the (R+M/2) method (see below). Since octane rating represents a ratio, it cannot exceed 100. Values for octane over 100 are really what are called "performance ratings". 



 Please don't believe everything you read on the Internet.

 This contradicts what I know about the subject. The fuel companies actually do the testing to measure the octane. Yes, it is a ratio but there is no rule that says the value cannot be better than the reference. I did not take the time to look up Ethanol but I am sure it is above 100M.



 Here is some trivia for you all :

 Reno style air race fuel is 120.3M leaded using the ASTM test method. Good stuff for the high CR engines and explains the high speeds achieved with these high power engines. This compares with something like 80-83M at your local gas station 87(R+M)/2.



 Comments below



 Regards, Paul

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



 At 09:22 AM 8/16/2006, you wrote:

  	  Quote: 	
		  --> Kitfox-List message posted by: Michael Gibbs <MichaelGibbs(at)cox.net>



 Bob sez:



  	  Quote: 	
		  There are many ways to calculate "octane", but if you do it the same way autogas and 100LL are done, Ethanol is over 100 octane. 	


 Autogas and avgas octanes are not calculated the same way.  Avgas octane is measured using the "MON" method, while auto fuels (in the U.S. anyway) are typically measured using the (R+M/2) method (see below).  Since octane rating represents a ratio, it cannot exceed 100.  Values for octane over 100 are really what are called "performance ratings".



 Paul sez:



  	  Quote: 	
		  An alky engine properly designed will have more horsepower that an optimally tuned gas engine due to higher octane. 	


 There may be some rules of thumb for common fuels, but technically, octane rating has no direct correlation with engine power or efficiency.



 "Octane" is a measure of the fuel's resistance to pre-ignition and knocking.  The Research Octane Number (RON) is determined by running the fuel through a specific test engine with a variable compression ratio under controlled conditions, and comparing these results with those for mixtures of isooctane and n-heptane.



 The Motor Octane Number (MON) or the aviation lean octane rating, is a better measure of how the fuel behaves when under load. MON testing uses a similar test engine to that used in RON testing, but with a preheated fuel mixture, a higher engine speed, and variable ignition timing to further stress the fuel's knock resistance. 	


 You are correct, M method is the best measure of octane. M stands for Motor



  	  Quote: 	
		  Depending on the composition of the fuel, the MON of a modern gasoline will be about 8 to 10 points lower than the RON. Normally fuel specifications require both a minimum RON and a minimum MON. 	


 I noted in my research that your 8 to 10 is not a good predictor. But, I have read that 4-5 is a good difference for Europe vs NA between (R+M)/2 and M. So I guess that the 91 that Rotax specifies is 87 at sea level here in the US? Any comment on that?



 

 [quote]In most countries (including all of Europe and Australia) the "headline" octane that would be shown on the pump is the RON, but in the United States and some other countries the headline number is the average of the RON and the MON, sometimes called the Anti-Knock Index (AKI), Road Octane Number (RdON), Pump Octane Number (PON), or (R+M)/2. Because of the 10 point difference noted above, this means that the octane in the United States will be about 4 to 5 points lower than the same fuel elsewhere: 87 octane fuel, the "regular" gasoline in the US and Canada, would be 91-95 (regular) in Europe.



 To learn more than you ever wanted to know about aviation fuel octane, check out < http://www.chevron.com/products/prodserv/fuels/bulletin/aviationfuel/9_ag_specsandtest.shtm > or < http://www.prime-mover.org/Engines/GArticles/octane.html>.



 Mike G.

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MichaelGibbs(at)cox.net
Guest

Posted: Thu Aug 17, 2006 12:12 am Post subject: Ethanol and wing tanks Ethanol and wing tanks

Paul sez:



You're right Paul, we have wandered off into the realm of trivia. 

  You're also right about the relationship between compression 

ratio and efficiency.



 	  Quote: 	
		  Please don't believe everything you read on the Internet. This 

contradicts what I know about the subject. The fuel companies 

actually do the testing to measure the octane. Yes, it is a ratio 

but there is no rule that says the value cannot be better than the 

reference. I did not take the time to look up Ethanol but I am sure 

it is above 100M.


	


But...it's not about what I read on the Internet, it's about the 

definition of octane rating.  Octane is the ratio of iso-octane to 

heptane that matches the pre-ignition behavior of a given fuel 

sample, with iso-octane rated at 100 and heptane rated at 0.  A 

sample that matched 100% iso-octane would be rated as having 100 

octane.  There is nothing higher than 100%.



Values above 100 are referred to as "performance ratings."



I would think the folks at Chevron would have a handle on such 

things: 

<http://www.chevron.com/products/prodserv/fuels/bulletin/aviationfuel/9_ag_specsandtest.shtm%3E>



 	  Quote: 	
		  ...(R+M)/2 even though the short cut way to write it is R+M/2


	


It is shorter, but mathematically the result is different.  



Mike G.

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Float Flyr

Joined: 19 Jul 2006
Posts: 2704
Location: Campbellton, Newfoundland

Posted: Thu Aug 17, 2006 4:55 am Post subject: Ethanol and wing tanks Ethanol and wing tanks

There are holes in your argument.  First we are talking about a rating not a

ratio.  There fore it is possible to have an octane rating of more than 100

without actually having any octane in the solution at all.  Ethanol is a

good example of this.  Pure ethanol has an octane rating of around 110.  too

bad the stuff has no power in it, has such an affinity to water and loves to

munch on composite fuel tanks.



Noel



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