My goal with the fuel system is to have a system that is compatible with automotive fuel that is available at the pumps, which in today's world implies mogas with 10% ethanol (also known as E10). When the Sportsman is eventually on floats, I also want to be able to refuel at a boat marina, which is straight mogas, or could also be mogas with 10% ethanol depending on the state.

I contacted as many people as I could find that were doing research into using ethanol based fuels, and put out general inquiries on the mailing lists I am on asking for input regarding mogas and ethanol. I have a summary of the replies I recieved and other research notes on this web page.

Please note that the information on this website is my own opinion only, and I am not an expert in aircraft fuel systems. I *believe* that I can create a fuel system for the Sportsman that is safe to use mogas containing ethanol, but I can't guarantee it. If you decide to use any of this information, you do so AT YOUR OWN RISK. I can't guarantee the accuracy or safety implications of any of this information. If you have information related to designing this fuel system, please contact me, and if there is something that seems wrong, PLEASE contact me! :-) I'm still learning, and am trying to gather as much information as I can.

There appears to be 4 major items to address when wanting to use automotive fuel containing ethanol in an aircraft.
1) Vapor Lock (Wikipedia Link)
2) Corrosion (Wikipedia Link)
3) Materials Compatibility (with petroleum and ethanol)
4) Phase Separation (water and ethanol separating from the petroleum)

Vapor Lock
With the Eggenfellner Subaru package, one place for a chance of vapor lock is at the inlet to the high pressure fuel pumps. The "sucking" of the fuel pumps creates a low pressure area by the inlet of the pumps.

Being a high wing aircraft with fuel tanks in the wings, the Sportsman has an edge over low wing kits. Gravity pulling the fuel down from the wings to the lower parts of the fuel system helps to increase the pressure at the inlet to the pumps. It is also recommended to use at least a 4 gallon header tank, and place the fuel pumps as close to the header tank as possible. I intend to place my fuel pumps directly underneath the header tank located behind the front seats, using a very short feed line (probably 3-5 inches), which is just about the best position possible aside from having pumps inside the header tank itself.

The Subaru package uses a full flow and return fuel system. This system uses high pressure fuel pumps to pump fuel to the fuel rail on the engine, and then back through a pressure regulator and back to the header tank. The fuel is constantly circulating, and any heat absorbed by the fuel when it is in the engine compartment is cooled when it returns to the header tank, which in turn is cooled by the fuel coming from the wings. The pressure regulator has a small pin-hole bypass to allow any vapor in the fuel rail system to be easily purged from the system and returned to the header tank, which then passes out the vent of the header tank.

With this type of fuel system, the chances of vapor lock are very low, if not nearly impossible.

Corrosion
From what people have told me in their replies, mogas combined with ethanol combined with water is a very corrosive combination. It is recommended that any aluminum parts be alodined to help prevent corrosion, which includes the insides of fuel tanks and fuel lines.

On a related note, several people have written to me saying they have been using E10 now for a few years, and have not seen any evidence of corrosion in their aluminum fuel tanks.

Materials Compatibility
Mogas, ethanol, and the combination of the two can be destructive to various types of rubber. It is very important to make sure any seals, gaskets, O-rings, etc that are in the fuel system are compatible with mogas containing ethanol. Jan Eggenfellner has assured me that all parts shipped with the engine package are compatible with this fuel since they are the same or similar parts as used in the automobile. That leaves any parts used in the airframe that are part of the fuel system. I have not yet run any tests personally, but I have been told by a few different people that fuel system "rubberized" parts made within the past 20 years should be fine. When I get some time (and before actually building the fuel system), I am going to drop various parts in glass jars containing mogas/ethanol and see what happens.

Phase Separation
Phase separation seems to be the largest worry that most people mention when talking about using E10 fuel. If any water gets into the fuel, it is absorbed by the ethanol. You may remember buying cans of "Dry Gas" to help remove water that may get into the fuel tank of your car. Dry Gas is essentially just alcohol (ethanol). The water molecules combine with the ethanol molecules, and the water is essentially "absorbed" into the fuel. This water will simply be burned during combustion in the engine and presents no danger.

If enough water is added to the fuel, it can cause the water to "phase separate", which basically means that the water will separate from the rest of the fuel and settle to the bottom of the tank, and can take the ethanol with it. If this happens, you get fuel at the top, and a nasty ethanol and water combo at the bottom of the fuel tank. Since the fuel pickup is also at the bottom of the tank, this is a pretty bad thing.

In addition to the amount of water, temperature and atmospheric pressure also play a role. In other words, you can have X amount of water in E10 that will stay mixed at one temperature and atmospheric pressure, but if you change the temperature or atmospheric pressure, it may be enough to cause phase separation. Most critics of using E10 in aircraft are worried that the fuel may be fine at ground level, but will phase separate once you have taken off and increase altitude, which will of course change the temperature and outside atmospheric pressure. This is a very valid concern.

I offer this research paper for your perusal, located at https://deej.net/sportsman/reference/waterphs.pdf

Essentially, you’d have to get more than a cup of water in a typical aircraft fuel tank before it would phase separate on a typical spring/summer/fall day. You will note that as the temperature drops, E10 will "hold" less water, implying that going from warmer temps to colder temps can cause the phase separation. However, IMHO if you have that much water getting into your fuel system, you are going to have problems no matter what fuel you are using (unless you can burn water, of course… :-). Note that with regular mogas (E0) it takes even less water before it will phase separate.

Water being absorbed via the fuel vent from the humidity in the air is essentially a non-issue as it would take several months at 100% humidity to get enough into the fuel to worry about, so we are primarly concerned with water getting into the fuel tanks through some sort of leak during a rainstorm, or perhaps when you are washing the plane. This implies we want to make sure there are no leaks in our fuel caps, and perhaps either keep the plane hangared year round, or use waterproof wing covers year round, and be careful with the water hose when washing the plane.

Taking these reasonable precautions, it would seem that the phase separation issue can be safely dealt with. In addition, with the fully recirculating fuel system of the Subaru, any water that does phase separate would be quickly moved through the fuel rail and sent back to the header tank, which would then be mixed back into the fuel. The fuel moves through the fuel system at approximately four times the amount necessary to run the engine at full throttle (ie, less than 1/4 of the fuel in the fuel rail is used by the engine, and the rest is sent back to the header tank to mix with the existing fuel there). This rapid recirculation adds another layer of protection when using E10.

Some interesting articles and discussions on the use of E10:
http://www.airportbusiness.com/online/article.jsp?id=42495&siteSection=1
http://k0lee.com/2007/08/can-ethanol-be-an-aviation-fuel/
http://www.vansairforce.com/community/showthread.php?t=35677&highlight=ethanol
http://www.vansairforce.com/community/showthread.php?t=62947