Today's Tech Trivia #2

[USMCmech]

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... The only reason it is still around is because it is the only successful way they have found to raise the octane rating to the levels needed in some high performance engines.

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I wonder about this. Automotive gasolines are available in unleaded formulations up to at least 104 octane. Of course, that's pump method (R+M)/2, so it probably doesn't square up exactly with aircraft lean/rich ratings.

I know there are other problems with automotive fuels (vapor pressure, alcohol, water, etc.) but it seems like the technology out to be out there for a no-lead aircraft fuel.

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The octane numbers for auto and aviation gas are based on diffrent scales. So replacing 87 octane avgas (which isn't made anymore) with 87 octane unleded you get at your local 7-11 probably won't work.

There are several systems in development that hope to allow high performance engines to run on no lead avgas. The one that shows the most promise is an ignition timing system from GAMI in Oklahoma. They are the ones who make GAMIjectors which improve fuel injections systems on many aircraft engines.

I wish I had some money to invest with those guys, 'cause when they get it perfected and 100LL disapears they are going to be in very high demand.

Read this article about leaded gas, it's really informative!
http://www.avweb.com/news/columns/182149-1.html

[USMCmech]

This is a copy of a question Roundout sent me as a PM.

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hey there mr. Mech,

i have a quick question regarding octane ratings. my girlfriend (commecial applicant) called me regarding her systems homework. she was talking about 100 octane avgas being the leanest performance and 130 being the richest performance. first thing that struck me is "wtf is she talking about?" i had her read it to me straight out of the book, and sure enough, the book made some reference to lean and rich performance based on the octane rating of the gas you're burning. now i'm thinking "what is this book talking about?"

i consider myself decent at systems, but this flew completely over my head. i can get you a direct quote out of the book if you'd like it.

TIA for the help, if you have any idea what this book is trying to say, or if i'm blatantly missing something.




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No fair, you made me look stuff up! [img]/ubbthreads/images/graemlins/cool.gif[/img]


Currently only 100LL is being produced, so it's really a dead issue. If anybody asks your GF about any other type, tell her to say "I don't care, since they aren't in production anyway."

There were two other grades for aircraft, but they have gone out of production.

80-87 octane (colored red). Mainly used in very old airplanes, WW1 era.

100-130 octane (colored green) This was basicly an expirment that didn't take off. It was an aromatic blend that had diffrent levels of additives (mainly Benzol) which would boost the "octane" rating.

100/130 is a diffrent way of describing the "octane" rating where there isn't any isooctane in the fuel. These are called the performance numbers

100 is lean best power, and 130 is rich best power.



100/130 avgas was very damaging to any rubber parts it contacted. It would eat away at rubber fuel baldders, and hoses. This is probably why it disapeared.



I think I will add this to the "Tech Trivia" thread with your permision. I think it will be informative to others.

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[roundout]

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Currently only 100LL is being produced, so it's really a dead issue. If anybody asks your GF about any other type, tell her to say "I don't care, since they aren't in production anyway."

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btw, that's a UND-type answer [img]/ubbthreads/images/graemlins/smile.gif[/img]

edit: (as i sit in class wearing my UND epaulets and ID card holder...) double [img]/ubbthreads/images/graemlins/smile.gif[/img]

[ananoman]

Don't forget the mighty 115/145 octane Purple. It was what the WW2 fighters and big prop liners were made to run on.

USMC is correct, these are all actually 'Performance numbers'. Octane ratings are based on a fuels resistance to detonation. The highest actual octane rating possible is 100. This indicates a fuel with the same detonation resistance as a reference fuel consisting of 100% iso-octane. If a fuel gives the same detonation resistance as a reference fuel consisting of 80% iso-octane, then it is given an octane rating of 80. You get the idea.

In the 1930's much research was done on increasing the detonation resistance of fuels for use in aircraft. This increased during WW2, and had a direct effect on the war. The more compression or boost you can run, the more power your aircraft engines can make. This was obvoiusly a topic of great interest at the time!

Fuels were developed which had greater resistance to detonation that the standard fuel containing 100% octane, and the standard was changed to reflect this as a 'performance number'.

There are many variables that can cause detonation. Compresson ratio, Cylinder bore size, ignition timing, intake air temperature, engine temperature, etc. The list is probably endless. For this reason, a standard engine is used in a lab, they have a variable compression ratio and are operated under set conditions. In a previous post someone mentioned the (R+M)/2 method. This is the method used to determine the octane of pump gas for your car. It is actually the average of two methods. I have no idea if this is the same method used for aircraft fuel.

One of the variables that will affect a fuels resistance to detonation is the mixture. If a mixture is run rich, it will resist detonation because it will burn slower and slow the pressure rise in the cylinder. The same thing happens if it is run very lean. At the extremes, if you run too lean, you get a lean misfire, and if you run too rich, the engine will flood and quit.

Between these two extremes there is best power (slightly rich) and best economy (slightly lean). Since the fuel air mixture burns at different rates, depending on the mixture setting, and since this will affect pressure rise in the cylinder, which will affect how much compression you can run before detonation is encountered, the 'octane' rating is directly affected by the mixture. (That is a run-on sentence if I ever saw one!)

It is my guess that although 100LL does not list these numbers, they are probably included in the fuel specification. It is easier for everyone this way, and like you guys said, it is the only available avgas anyway.