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Bleed off and return to tank with AFP

akschu

Well Known Member
Patron
I'm in the final stages of picking out my fuel system for my io-540 powered bearhawk.

Due to the fuel system not really flowing more than 50GPH, and my desire to not add header tanks and such, I'm thinking I want to stay with mechanical injection. A returnless system won't ever overrun the factory fuel plumbing, while an EFI system returning 50-75GPH to the tank with the engine at idle could suck air. This combined with the dependence on electricity has me thinking that AFP is the right tool for the job.

The issue with mechanical injection of course is the limitation of 100LL due to vapor lock. Not that running 100LL is the end of the world, and it will surely see a lot of it when I'm traveling, but at my home airport there isn't any fuel, and there is a source of eth free mogas 2 miles away, and I have a 72Gal fuel transfer tank in the bed of my truck, so mogas is WAY easier to get most of the time.

I have read very post I can, and in this thread:

http://www.vansairforce.com/community/showthread.php?t=87674

N427EF talks about putting a vapor return line in and using an .020 orifice to bleed some fuel back to the tank for the purpose of cooling the mechanical pump and giving vapor a place to go.

This sounds a lot like what the continental engines do, and probably why they get STC's to run mogas. If a T'd in orifice, cooler around the mechanical pump, and a return line through a duplex valve is all that's needed to reliably run mogas, then that sounds great.

Has any one else done this? I messaged N427EF but he didn't respond.

Another question is if this would also make it more reasonable to run the FM-200A system without the purge valve given that there is a place for the vapor to go. It would be nice to not have another knob on the dash.

Any thoughts for those running injection and mogas would be welcomed!

schu
 
I'm in the final stages of picking out my fuel system for my io-540 powered bearhawk.

High wing with wing tanks?

Vapor lock is primarily the result of heat and pressure reduction, with potential contribution from details like 90 degree fittings, and restrictive fuel piping and valves.

Wing tanks mean you already have some head, thus less pressure drop leading to the mechanical pump.

The next design task to to minimize any opportunity to pick up heat. In a perfect world you'll have a very short, insulated line between a bulkhead fitting and the mechanical pump. No lines running all over the engine compartment, and none subject to radiant heating.

Eliminate all flow restrictions. Use straight fittings and have all flex lines built with 90 degree tube ends where necessary.

You get the idea. Build a system with good fundamentals and you may not need anything special.

As for the purge valve, you want one if the AFP throttle body has a drum mixture valve. If it is a disc valve model, the purge is optional.
 
I think you need to ask these questions of AFP, they are experts in the field. My low-wing RV-6A has AFP injection but uses a boost pump for pressurization during takeoff and landing. The engine was built to be mogas (non-eth) friendly but I run 100LL. The injection is not the limitation but I was told high-compression cylinders would prevent me from using mogas. My Superior XPIO-360 is plenty powerful at 180HP. The system has a purge which is plumbed back to the right tank. Shutdown procedure is to pull the purge (not mixture). Restart is to run the boost pump with the purge open to circulate hot fuel out of the system, then close the purge and start normally. The mixture will already be correctly set (at 7000 feet in Flagstaff, you should not start full rich). A couple of times on extremely hot days at low altitudes, I have had to employ a hot-start procedure (mixture out, throttle forward until engine catches, then throttle to idle and mixture in).

But my RV is not your Bearcat. I have no idea how an AFP system would work without a purge or a boost pump. On the other hand, AFP has seen installations in all kinds of aircraft and can probably answer questions relating to your particular needs. They were very responsive to me when I was figuring out the return system on my aircraft and again when I was trying to learn to use it properly. I liked them enough that my RV-10 engine also has an AFP system.
 
I appreciate the responses, but none of it is new information. I'm aware that the 200A system doesn't require a purge valve, but the 200 system does because it uses a drum valve. I also know for sure my engine would run fine on mogas, and that the issue is that returnless injection doesn't typically like mogas due to vapor lock. Several VAF members have posted that they have a solution to running mogas on returnless FI so I'm wondering who else has implemented this work around or is running mogas with an AFP system.

I suppose I can call Don and ask him, but wanted to find out all I could before pestering him.
 
So where exactly would you connect this return line?

According to this and other posts:

http://www.vansairforce.com/community/showpost.php?p=671454&postcount=13

It appears he is running the mechanical pump and boost pump in a parallel bypass with check valve (the arrows and red dots) configuration instead of in series as normal, then he puts his pressure sensor and fuel flow on the cold side of the firewall.

The bleed off appears to be a T on the high pressure side of the mechanical pump with an .020 orifice which apparently bleeds 6gph back through a duplex valve, to the source tank.

The theory of operation is that the mechanical pump is cooled with blast air, but also has some movement of fuel all of the time so the fuel cooling works better, as well as giving any vapor a place to go.

According to the author, it works so well that he is running E10 mogas in California for hundreds of hours.

This seems to make sense to me, because in a lot of ways it's is the continental fuel injection system that is known to work better with mogas and even has STCs to run mogas in certified airplanes.

When I'm ready to order my AFP stuff, I'll certainly talk to Don, but I fully expect him to say install per the manual, which is what I would do if I was in his shoes.

BUT, in the spirit of experimental aviation, which when driven by data and sound theory progresses GA forward, I'm very interested in doing it this way. I can't really see 6GPH adding enough load to the system to cause issues, I can't really see an orifice failing, I can't really see extra fuel flow/cooling hurting anything, and I can't see having a vapor escape at the mechanical pump a bad deal, so I figure I won't fall out of the sky, and it's fairly easy to figure out how safe it is by simply putting 100LL in one tank and mogas in the other and trying to make it vapor lock at safe altitudes or on the ground.
 
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