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100LL fuel burn


S3flyer

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Does 100LL versus ethanol have lower fuel burn? It seems to but I never really gave it much thought.

 

I just got back from Fort Walton Beach where I pretty much burned pure 100LL on the return to Dallas. I had a leg at 4500' that went exactly 2.1 hrs (from starting the engine to stopping) with an RPM in cruise no less than 5250. I put in 9.0 gallons. It would seem I'm getting about 10% better burn. Is this what y'all see?

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Hi Dave,

 

That's a pretty tough question. Your real close to regular fuel consumption on your figures already. There may be too many variables to do it in the real world verse a lab. The problem is figuring in outside air temps, winds no matter how light, aircraft trim no matter how small, exact fuel/engine run times. These could easily account for a 10% difference.That's why this is such a tough question.

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  • 3 weeks later...

The fuel octane rating has nothing to do with fuel efficiency on carbureted engines. The octane rating only has to do with the stability of the fuel under compression. The Lead also has no effect of consumption. On more sophisticated fuel injection engines, octane may have a small effect on consumption but this is only because the engine is monitoring incoming air pressure and EGTS and using a programmed fuel map to very the amount of fuel injected into the cylinders

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In doing further research, the oxygenation does affect the fuel burn. Ethanol has 35% oxygen by weight and about 1/3 the energy level of gasoline. Consumer Reports (and several other labs) have done side-by-side testing of E85 vs. straight unleaded. In using a Chevy Tahoe as the test vehicle, the unleaded delivered 26.5% better fuel economy. I would expect 100LL to be analogous. Chemistry may not be linear in this case but removing 50% of the ethanol to come up with E90 would yield a 17% degredation of fuel economy. Given that in my area, mogas is UP TO 10% ethanol, the ~10% better fuel burn with 100LL might not be off the mark.

 

That being said, I'm not going to plan for it. Just an interesting observation (to me :-)).

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We are supposed to have 10% here in Tucson too, but we have never seen over 7%. You may want to actually test your fuel. Not many are up to a full 10%. That said it would be more costly in the long run to burn 100LL with all the extra maint cost and added fuel price.

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I tested my local fuel source and came up with 6%. Agreed the economics would never work out (using 100ll over ethanol) given that 100LL is about 40% more expensive locally than 93 octane and the additional maintenance that would be needed on the Rotax.

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In theory E10 should have less energy, and it does have ~3% less for similar autofuel, but AVGAS is a fairly low energy fuel on it's own. http://en.wikipedia.org/wiki/Fuel_efficiency

 

 

http://www.bp.com/liveassets/bp_internet/aviation/air_bp/STAGING/local_assets/downloads_pdfs/a/air_bp_products_handbook_04004_1.pdf

 

Really the energy content of E10 and 100LL are within 2%

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Just for clarity, E10 means 10% maximum ethanol, E85 means 85% maximum ethanol. 100% gas has about 114K BTU's. E100 has about 76K BTU's. On a recent trip to Brazil they had two grades of gas: E25 and E 100. E25 cost about 1/3 more than E100. All cars there can run on either one. The new agriculture spray planes there run on E100 because of avgas steep prices. Here is a picture of the tail of one. post-156-0-75050700-1306724438_thumb.jpg post-156-0-23639200-1306725237_thumb.jpg

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