Questions about the Rotax describing how much 100LL needs to be added to automotive fuel

Your response offers an interesting perspective on mixing 93E10 with 90E0 that I hadn’t considered.  So, thanks for your response.

First, my biggest concern with ethanol is that the fuel ages / degrades significantly faster than non-ethanol fuel. I understand that the best resolution is to fly the airplane regularly so that the fuel doesn’t get old.  But, this isn’t always possible / predictable as demonstrated by my plane recently being down for 5 weeks.  Namely, 1 week to perform a Service Bulletin and then unexpectedly for 4 weeks to resolve an electrical issue with the battery not being charged. 

The attached picture shows the effect of adding 93E10 to 90E0 (also called REC90).

A desirable advantage of this approach is that both unleaded fuels could be purchased at the same location, mixed and then brought to the hangar.  A significant simplification from the process needed to add 100LL.

The question that I would like to better understand is the concerns about ethanol being in the fuel reduced with decreasing amount of ethanol in the fuel or does all of the ethanol need to be removed to address these concerns.  In other words, is reducing but not eliminating the amount of ethanol in fuel any better using 93E10?

Thanks. 

Brett

I don't have as much concern about lead and ethanol as I do about octane and RVP (Reed Vapor Pressure).  While lead does form deposits over time, and ethanol may reduce the storage life of the fuel, neither of these factors have the immediate risk of high RVP or low Octane fuels. 

Winter fuel in many areas has an RVP of about 13 PSI, and if you get one warm day using that fuel you can be at risk of vapor lock. And we know the harmful effects that low octane fuel can have on your engine. By mixing a 50/50 blend of 100LL with automotive fuel both problems are solved.  You have already done the math for AKI, and RVP can be calculated in a similar way.  A 50/50 blend of winter auto gas (RVP=13) with 100 LL (RVP=7) yields the average of the two (final RVP=10 PSI).  

My formula is simple: When summer fuel is being sold, I use 100% premium 91 (R+M)/2) auto fuel.  When winter fuel used, I mix premium auto fuel 50/50 with 100 LL just to lower the RVP.  I have never had any storage problems with automotive fuel, especially when stored in high quality metal gas cans which keep the lighter elements from evaporating (keep the vent closed). Until high octane unleaded aviation fuels are widely available, you must make the best choice you can and move on.

I am not sure that ethanol is a problem at all for these engines, really.  Of course if it starts eating away at your composite fuel tank, that's a different story.

When you say the E10 "degrades", what do you imagine is actually happening?  I have not found it to be a problem at all, and Rotax specifically authorizes its use.  I last flew my aircraft on 3 January, it's been sitting in sub-freezing temps for most of the last 2.5 months, and Saturday my engine fired right up, no issues...

As to ethanol percentages, if you find a certain substance to be a "bad thing", I would assume having less of that substance would be considered "a good thing"  ?  In my estimation, lead is a much worse thing than ethanol for these engines.

(PS I do agree that it's not a good idea to be running winter auto fuel in the summertime, with or without the E)

Thanks for the responses.  

I was focused on meeting the octane requirement while minimizing lead and ethanol.  I hadn’t been considering vapor pressure.  

With Spring here and things warming I can now better appreciate that the transition from winter automotive fuel to the warmer temperatures is an issue for the higher winter vapor pressure fuel. Thus, the justification for adding a higher percentage of 100LL. I will make the change. 

I agree that the detonation and vapor lock issues are more detrimental than the longer term lead deposit issue.  

Brett

As follow up questions about adding Decalin when using a mixture of automotive fuel and 100LL:

1.) When using a mixture of automotive fuel and 100LL, should Decalin still be added to reduce the impact of the lead?

2.) If so, should the amount of Decalin added be based on just the volume of 100LL added or on the volume of automotive fuel / 100LL mixture added?

Thanks. 

Brett