Fuel tank un-porting in high pressure fuel system

[stringfellow]

My question is more related to high-wing aircraft than RVs, but it may be germane to both, and I expect some learned folks here who may have thought of this at some point.

I'm designing a fuel system layout to support the SDS/FlyEFII system where the wing tanks have ports both fore and aft. An additional bung has been added for a return fuel line. An EFII system runs fuel in a continuous loop through the rail at high pressure, and to prevent vapor lock, recirculates the fuel to keep it cool. There are two ways of doing this, though they are not mutually exclusive: 1) Fuel is routed back through a duplex valve into either the L or R tank, or 2) a header tank is used.

In this case, only a duplex valve is being used and fuel is being run back to the fuel tank.

My question is how best to deal with the potential for "unporting" either of the fuel tank ports/pickups when fuel level in the tank is low. As you can imagine, in a high pressure system, this could suck air into the line and some bad stuff could happen. For STOL aircraft that often experience high AOA, this could be a real potential issue.

One solution I've had suggested is to only use the aft port, and just use the forward port for the return line. That sounds okay, but what if pitch-low scenario causes the same issue?

Would the addition of a very small header tank help? Once air is sucked in, even if the tank pickup is re-primed, the air is in there. Could one use a header tank like the opposite of a gascolator? I don't see any way to separate and vent air though safely.

Set aside the concept of EFII for a moment-- how does any fuel system that uses a boost pump deal with this issue? Is it just one of the perils of low fuel management?

[Randy]

The fuel pressure regulator should have a provision for allowing any air to escape through a bypass. The bypass can be external or internal but it must have a fixed size bypass orifice that allows some fuel or air to keep circulating even if the regulator is not open.

Without the bypass I believe it is possible for the air to become trapped at the regulator and possibly cause a delay in re establishing fuel flow. Robert tells me the EFII regulator has this bypass machined into the inside of the regulator. I am not sure about the Borla regulator that Ross uses but I suspect it has a similar feature as this is a known issue we all learned from early in Subaru development days.

You might consider testing this on the ground by placing the aircraft in the attitude of concern with a minimal amount of fuel in the tank and purposefully run the tank dry and then check recovery time.

The fuel pickup in RVs does a nice job of drawing all but a tiny amount of fuel from the tank in normal attitude. Still, on approaches, I always leave one tank with more than enough fuel in it. By this I mean I will run the other tank to a very low point if I have been cruising to near the end of my fuel range, then when I switch to the fullest tank I am committed to using it to land with.

[DanH]

You can't use two tank pickups, fore and aft, without a header tank.

With a header, the tank pickups are gravity flow to the header, and it doesn't matter if one unports.

If you want to pump directly from a tank and use a duplex, it's single pickup only, and plumbed like an RV. Depending on tank design, it maybe possible to install a flop tube or swivel pickup to accommodate more pitch, but that adds its own issues.

I'm guessing your aversion to a header tank is physical size. Just Aircraft has a good header tank system for the SuperSTOL with ULPower. You can probably buy a tank from them. It's just a wee bit larger than the header shown in ULPower's literature (page 67 below), which is only about 1.5 quarts...

http://ulpower.com/content/downloads...S-UL520iSA.pdf

....certainly nothing like the 5 gallon header Robert specifies with his EFii. On the other hand, ULPower doesn't route the fuel rail alongside the exhaust pipes.

[Robert Anglin]

I agree with Randy's assessment. Not knowing what your air frame and engine is I would only be speculating, but I can till you what we did. We have an installation in an RV-8 with an ECI engine driven pump and the Air Flow in frame bust pump in the frame not the tank. The tanks of the RV type aircraft as Randy has said pick up down to almost the last drop very nicely, so. We put an Andair duplex valve in for tank switching and ran the return line back to the highest point in the back baffle of both tanks. The tank we are drawing from at the time gets the return fuel and any vapor, if any. I hear a lot of guys trying to justify making complex installs out of concern for low fuel level pick up at pitch or roll set points. I think you may find that when you are very low on fuel you will be making every effort to stay as level as you can till you get to the runway and can put more fuel in. If I am about to run a tank dry the last thing I want to do is dip a wing or the nose. I like to reduce power and just sink as flat as I can and used rudder turns to keep the wings level. I have only had to sweat this a couple of time down through the years and it gave me a grate deal of respect for the reserve fuel FAR.
Don't know if this will help, but they are not copper coins anymore either.
Yours, R.E.A. III #80888

[rv6ejguy]

Being wary of the same concerns, I did some testing on my -6 when I switched my fuel system from a header tank setup to twin floor mounted Walbros many years ago now.

With about 3-4 gallons in the selected tank, I did a series of somewhat uncoordinated turns with bank angles up to 30 degrees in both directions. Surprisingly, the engine never missed a beat. My aircraft has the standard, rigid line pickups, Andair Duplex valve and an OEM, rail mounted Subaru regulator.

It does seem prudent not to run fuel below these levels when maneuvering down low and also to keep the ball near the center.

Our view on fuel pressure regulators is that no "bypass" orifice is required with EFI systems since the regulator is already a bypass device. Hundreds of customers have been flying our systems for years without any mods to standard OEM, Bosch and Borla regulators.

[stringfellow]

Great responses, thank you everyone. My aircraft is a 4-place Bearhawk with an IO-540 (EFII) but I have a friend who is also building one using a Continental IO-360 which, although not EFII, utilizes high pressure and return lines, so we're both in design mode. I enjoy the RV forums here because the breadth of experience is so great and there are a lot of forward-thinking folks technologically.

I'm also a little starstruck to see Ross from SDS in here...cool.

I am reluctant to run a header tank for the volume. I want to keep everything under the floorboards if possible. My plan has been to run twin electric pumps, twin ECUs, twin batteries. Everything is a backup. I have an Andair FS22 duplex valve. All fuel lines are 3/8".

Robert at Protek is who advised me to use only the aft port on the tank and just use the forward port for the return line after seeing my two fuel lines run down the door frame and join before going into the fuel selector valve. While I think that sounds like an elegant solution, would I not sacrifice some performance and/or redundancy from having 2 ports? Robert has been very helpful but I seek to understand all my options.

I have to admit I don't know much yet about pressure regulators. I did not know it was possible to bypass (or purge?) air with one.

Ground testing in an attitude to recreate the problem is a sure plan, but it will be a while before I'm there. I just want to plumb it as best I can in the first go. I'm already adapting this from my earlier intentions to just use a gravity feed and carb.

And finally, the question of whether or not to use my shiny Steve's Aircraft gascolator, mainly for pre-start water collection. For those who've been down this path, what have you done about low point drain? Do gascolators have any other benefits in this config?

[219PB]

You may want to keep the gascolator in your system due to the high wing configuration. On the RV the lowest point is the wing tank drain. With the EFII system the fuel is always flowing and it will collect at this spot in the tank.

[DanH]

Originally Posted by stringfellow

I'm also a little starstruck to see Ross from SDS in here...cool.

Ah heck, he's everywhere

Robert at Protek is who advised me to use only the aft port on the tank and just use the forward port for the return line after seeing my two fuel lines run down the door frame and join before going into the fuel selector valve.

He is entirely correct, and you should buy him a bottle of good whiskey for catching it before you crashed your airplane.

While I think that sounds like an elegant solution, would I not sacrifice some performance and/or redundancy from having 2 ports?

Very little, if any...but if you don't like the single pickup answer, install a header tank.

And finally, the question of whether or not to use my shiny Steve's Aircraft gascolator, mainly for pre-start water collection.

No gascolator. Install the filters Robert provided. And tank drains.

[rv6ejguy]

With a high wing design and pumps mounted many feet below the tanks, it should be even more difficult to suck air into the pumps than on an RV.

That said, 0.5 gallons in a feeding tank with a massive bank/climb/dive angles is not the smartest thing to do down low.

With EFI, you don't really ever want to run a tank dry. The engine will stop right now and it's not the best thing for the pumps either although they have a dry run rating of a minute or so if I recall correctly. There is always a few CCs of line or injector volume, meaning this air must be processed through the engine to purge it. The engine won't run for a few seconds until that's done.

Regulators work to hold fuel pressure constant by controlling the amount of fuel returned to the tank. A idle, 99% of the fuel pumped is returned to the tank. At full power, perhaps 50% is still returned to the tank while the engine burns the other 50%. As such, any air or fuel in the bypass line/ regulator is simply shoved right back into your tank.

In our view, gascolators are not very useful from a dynamic water removal perspective since the high return fuel rates quickly mix up any water present with the fuel. A fuel drain at the tank low points should be sufficient for that purpose.

It's good practice to mount the return fittings in your tanks as high as possible and as far away from the pump pickups too to avoid picking up any bubbles. EFI is designed to meter liquid fuel only, not combinations of air and liquid.

[stringfellow]

Originally Posted by DanH

Very little, if any...but if you don't like the single pickup answer, install a header tank.

Could you elaborate a little? I'm not arguing, just seeking to understand. Do you consider the header tank mutually exclusive with return lines? If combined, where do you see the header tank being installed in the stream?

I wouldn't be opposed to using a small header tank if it was just for the purpose of weathering an unporting event, but I do not want to use a header tank that's large enough to accomplish the fuel cooling requisite for the system. I think Paisley said it would need to be approx 5 gallons, IIRC, if you wanted to forego return lines altogether. That's just too big and won't fit under my floor.

I don't mind the return lines at all, just trying to optimize.