Avgas & Mogas Mix Ratios

ChiefPilot · #1 07-15-2013, 07:35 PM

Although I had run several tanks of 91UL mogas through my -6A during the winter months, I've been reluctant to do so during the summer months due to fear of vapor lock. While my plane was (and is still) in the paint shop, I thought this would be a good time to learn more about auto fuel in aircraft engines.

One aspect is detonation, and I'm still learning. Another aspect is vapor lock, and this is something that science can help solve. Pete Howell was good enough to stop by last week and demonstrate his simple rig for finding the pressure at which liquid would boil. One trip to Harbor Freight later, I had everything I needed. After talking with Pete and hearing about his and others experiences, I thought I'd test a range of mix ratios. I'd read on here that a small amount of 100ll would affect the vapor pressure of mogas disproportionately so I thought I'd check it out.

I sampled mixes of 1:1, 2:1, 4:1, and 8:1 mogas to avgas. South St. Paul sells both side by side so obtaining the samples was pretty trivial. All tests were done at an ambient temperature of 84ºF after letting the fuel adjust accordingly. Before starting, I measured 100LL boiling at 21" Hg of vacuum and the 91UL mogas boiling at 16" Hg vacuum at ambient temperature. The chart below captures my findings:

The actual pressure values aren't as important as the resulting curve. The curve clearly confirms that 100LL affects the vapor pressure disproportionally to the mixture ratio. With pure 100LL at 21" and pure 91UL at 16", I would have intuitively thought that a 1:1 mix would result in a boiling point of 18.5". The data shows however that a 1:1 mix has a pressure close to that of pure 100LL and that it takes an 8:1 mix to move the pressure to the halfway point between that of 100LL and 91UL.

I invite any comments, suggestions, or additional data on the topic. I think this is pretty cool stuff, and my thanks to Pete for the knowledge and techniques!

Disclaimer: I am not a chemist, I understand that there are various blends and such which may affect the outcomes, and I did not stay at a Holiday Inn Express last night.

Tony Spicer · #2 07-15-2013, 08:05 PM

Have you read this?

http://www.jabiru.net.au/Service%20B...l_Guidance.pdf

Tony

Kyle Boatright · #3 07-15-2013, 08:07 PM

RVP of Mogas varies widely by season and location. Depending on emission regulations in your local area, you may find summer blends of mogas that are required to have RVP within the acceptable range for Avgas.

That is the case here in the Metro Atlanta area, where (IIRC) the RVP of summer blends can't be higher than 7.0, which (again, IIRC) is also the upper limit for Avgas. Of course, that doesn't fix the ethanol issue if that is of concern to you.

A little googling will show the required RVP's by season and location for most anywhere in the US. A little more googling will show the RVP spec's for Avgas...

pierre smith · #4 07-16-2013, 04:11 AM

Brad, I'm totally lost.

What did you buy and how exactly, was this test conducted?

Thanks,

PaigeHoffart · #5 07-16-2013, 05:36 AM

Quote:

Originally Posted by pierre smith

Brad, I'm totally lost.

What did you buy and how exactly, was this test conducted?

Thanks,

http://www.vansairforce.com/communit...Vapor+pressure

ChiefPilot · #6 07-16-2013, 05:59 AM

Quote:

Originally Posted by pierre smith

Brad, I'm totally lost.

What did you buy and how exactly, was this test conducted?

Thanks,

What Paige said. I bought the same bits from Harbor Freight as Pete did.

I put .5oz of 100LL into the sample cup, attached it to the top, then used the handheld vacuum pump to evacuate the cup. When the liquid started to boil, I recorded the vacuum pressure shown on the gauge. I then added the appropriate amount of 91UL to get the 1:1, 2:1, 4:1, and 8:1 mixes of 91UL:100LL and repeated the process. I did this whole thing three times, then plotted the values along with an average to produce the graph shown above.

My goal was to see how adding 100LL affected the vapor pressure of the mogas; since I was interested in relative effects and not absolute values, I didn't care too much about the absolute temperature. Actual Reid vapor pressure is determined at 100?F; the temperatures during my tests were between 82? and 84?. Further, the pressure values were relative to ambient and not absolute. One could deduce the actual pressure fairly easily by getting the altimeter setting and subtracting the vacuum pressure shown on the gauge but I did not do so. Again, since I was interested in the relative effects, knowing the absolute pressure was not required.