Mogas in Aviation

Hi Roger,

The ethanol in mogas remains just about constant year round, and like you I don’t have much worry about it affecting engine performance.  However, the original post has nothing to do with the amount of ethanol in fuel, it’s is about the fuels Reid Vapor Pressure (RVP), and we should not conflate the two issues. Reid Vapor Pressure, is determined by the percentage of lighter elements (mostly Butane) that are blended into the fuel, and this changes dramatically between seasons.  In the winter when evaporation is low, the manufactures (in the USA and some other countries) are allowed to use more of these lighter elements to help engines start easier in cold conditions. In the summer, the EPA wants less of these lighter elements to help keep evaporation into the environment lower. Summer fuel has an RVP very similar to Avgas, but winter fuel is the problem if a very warm day comes around.  On a side note, butane is cheap, so winter blends are cheaper to produce, and the refineries use it as long as they can before switching to the more costly summer fuel blends.

While it is true that aircraft design elements play a role in fuel vaporization, a lower vapor pressure fuel will lower the risk of vaporization in any situation.  This is why Avgas is required to have a year round vapor pressure of 5-7 psi, while auto fuel varies between 8 and 14 psi depending on the season.  I like what Ron is doing here trying to bring this issue to the forefront.  

Hello Roger,

I too am driven by science... but you have completely missed my point of the presentation.  The reasons I mentioned ethanol was to simply define it... state that it does not cause vapor lock... and point out that the EPA has allowed fuels that contain 10% ethanol to have a higher RVP by 1%.

I was in no way claiming that high ethanol content causes vapor lock.  I was cautioning pilots that high RVP (winter blend fuels) when heated and subjected to low atmospheric pressure can (and will) cause vaporization in your fuel system - to the point of significantly reduced power, or a complete shutdown.  This is based on science!  Winter blend fuel is not an increase in ethanol content.

I realize that my presentation is very long, but I suggest you watch it in its entirety and then I hope you will agree with my overall logic and message.  

It is extremely rare for me to post on forums because it can be misunderstood, but with the increased number of pilots flying with Mogas, it was important for me to do so.

Even though this is my first post to Rotax Owner, I am not new to Rotax engines.  I have built a couple of aircraft and flown (in front of, and behind) 4 Rotax 912s since 1999.  My current aircraft is my first fuel injected computer controlled Rotax 912.

To add to Jeff's comments about cheaper winter blend fuel: Gas stations and/or FBOs appear to stock up with the cheaper fuel as they approach the legal cutoff date.  The FBO where I filled up with Mogas was selling 13 psi fuel, way after the cutoff date.  They had no idea of the effects of this high volatility fuel in warm/low pressure conditions.

There are two cut-off dates to consider.
The refiners and distributors need to clear out their stock by May 1.
The FBO can top off their tanks before the May 1 cut-off but they are legally required to STOP selling it by Jun 1.
If the FBO is stockpiling more fuel than he can sell before Jun 1, he has a serious liability to consider.

- - -
How Hot, How High?
I have posted this before, but it is worth having a look again.

Hello Bill,

Thank you for your comments!  I do cover these dates in the presentation and also point out that I purchased the fuel that caused my near-engine-out on June 21st - much after the dispensing cutoff date..  The FBO would have continued selling the fuel until empty - had I not called them with what I had learned.  I believe that they locked up the tank until topping it up with summer blend.

I like the chart you attached, but before I show (or quote) it, I would like to know it's source.  Do you have that info?

Thanks,

Ron