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Fuel tank un-porting in high pressure fuel system

stringfellow

Active Member
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?
 
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.
 
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/IM_UL520i-UL520iS-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.
 
Two cents

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

Thanks Ross. I've never run a fuel tank dry in my 20 years of being a pilot, just too conservative, but this is also the first system I will have owned that is L or R tank only, no BOTH selection. Even the Cubs I fly most of the time now have BOTH, unlike the older Pipers.

I talked to a guy the other night who build an aircraft with SDS and said he had experienced an unporting event, and while fuel pressure went down, the engine didn't stumble or die. It just kind of continued while down on power. Any theory on why that would be?
 
i too am designing a fuel system right now. to keep my fuel cool i am running the return line into the main tank. as long as the holes in the baffles allow adequate fuel flow[so baffled area doesn't fill up] the tank should drain into the header faster than the engine can use it. at this point the size of the header need only concern unporting in the header tank. fuel will stay cool sloshing in 20 plus gallon main tank.
i don't consider dual pickup lines necessary as i can't imagine a manuever that would unport the main tank long enough to use all the fuel in the header tank. and i don't ever plan to push my luck so far as to empty the main tank.
 
I'm doing the SDS EFI install on the Rocket right now and am completely overhauling the fuel system. I'm designing with the ability to run auto fuel and unlike some others, I INTEND to run one tank dry on a max effort cross country. The utility of running one tank dry while up high in cruise and consolidating all remaining fuel in a single tank for landing has been debated on this forum before so there is no need to rehash that aspect, but for the installation I'm looking at the following:

- Pumps mounted as low and to the rear as possible. In the case of the Rocket, this is in the center cavity, below the stick, stuffed parallel with and against the spar, on the floor. Its tough to get the pumps below the fuel level in an RV, but with the slight dihedral, this is about as close as one can get without punching the OML of the airframe.

- All suction lines as short as possible, with no fittings sharper than 45 degrees ( ideally, all are 180). To this end, I'm moving the fuel selector from the normal position ahead of the stick to the rear and side of the stick. This is going to make for some tight plumbing but unlike many on this site, I really enjoy making hard lines.

- As a final design consideration just for this unique circumstance of the low vapor pressure of auto fuel and the likelihood of running a tank dry as normal ops, I'm installing a pair (one per tank) of Facet "cube" pumps upstream of the selector. These will be used just like a conventional boost pump, but are there to push a slug of fuel to the main pumps in the case of an unporting event. Flight test will determine if these are required, but I consider them good insurance for initial flights.

I want to make it VERY clear that the above is not to be considered "advice" or design plans. I'm experimenting and I know the risks. Fuel system design is CRITICAL to safety of flight and this is an area that brings down a bunch of homebuilts. Tread very carefully.
 
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header tank vent line...

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.

Would connecting a vent line from the top of the header tank back to (either) of the high-wing tank resolve this issue? No need to add an additional external vent port, and the header tank sees whatever pressurization is created by the pitot effect at the main tank vent port.

- Roger
 
Would connecting a vent line from the top of the header tank back to (either) of the high-wing tank resolve this issue? No need to add an additional external vent port, and the header tank sees whatever pressurization is created by the pitot effect at the main tank vent port.

- Roger

Plenty of good info found in FAR part 23. here's a link to some easy reading.
 
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I ran across this thread, and while it is getting a bit dated, the topics are still very important.

There are several distinct different topics discussed here.

One, header tank verses return lines. And ?. air in the lines, from either, un-porting or (possibly from vapor pressure).

When it comes to "bubbles" (Not un-porting fuel starvation) I have wondered, what is to say that bubbles are expected to pass back through the smaller return lines vs travel to the fuel rail which is, arguably the path of less resistance? Is there research to suggest this actually happens?

The downside to return lines is the requirement to only use one tank at a time, and double the number of fittings and lines in the passenger compartment of the plane.

Absent the need for return lines, using both tank feed lines offers two advantages. Half the lines and fittings in the passenger compartment. And, simpler fuel management.

While double the fuel fittings and lines are not terrible, it is worth valuing. There have been some rather infamous in flight fires from a variety of fuel and hardware problems...so fewer points of failure can be a good thing.

So what are the downsides to putting the return line "upstream" of the pump? (and not routing extra lines through the passenger compartment?) Back to the bubbles issue. Keep in mind this serves the main function of the regulator purpose - to maintain proper pressure at the rail(s).

For the sake of testing I have ran two engines on a test stand set up this way as I wanted to see if there were tell tale signs of trouble. Some have thought the fuel lines get warm during the fuel flow. Not at all. No issues at all. Not to say this is the end all of research, but for my experience - no issues.

And more importantly, are the risks validated with credible research and testing.

It is worth discussing and hopefully the discussion will be supported with some data.

Great discussion. (Many of you out there have a lot of credible experience so I hope you share some of that wisdom).

Charlie
Honda running on a test stand.
RV6a airframe done.
 
popcorn time... :)

Charlie,

There are endless debates about this subject. Ross will tell you that you can't return bypassed fuel to the pump inlet ('upstream of the pump') because of the risk of heated fuel vapor locking the pump. And that's not an insignificant risk, if the regulator is after the engine fuel rail.

On the other hand, all the 'boost' pumps (using automotive pumps) currently being sold for Bendix style injection systems, *and* the old school certified boost pumps, loop bypassed fuel directly from the pump assembly's output directly back to its input. Now, those guys *do* say to never run a pump dry. My unsophisticated opinion is that the reason for that, is that if you ever did get air to that style pump, and the engine did stop, it would recirculate the air around the pump until injectors could pass enough volume to make the flow easier going out, than around. It should eventually clear, but it might take a while.

In any case, I can't see how returning bypassed fuel upstream of the pump avoids extra lines in the cabin, unless you're thinking of mounting the pump on the hot side of the firewall. If that's your thought, don't. You want the pump as close to tank, and as low in the routing, as is reasonable to achieve, to minimize pressure drop and fuel heating in the supply line. *That's* the biggest danger for vapor lock in an electronic injected engine.

I do know of one RV-6 that's flown successfully for more than a couple of decades with a small header tank and injector pumps on the firewall, but he has an always-running Facet low pressure pump close to the tank selector, feeding the header. It is a fairly complicated install, for no good reason. I think one of his original goals was to keep high pressure fuel out of the cockpit, which, at least for me, has no practical advantage. If I have a high pressure leak, I'd rather it be in the cockpit where I'll know right now, instead of in the engine compartment, where it could saturate everything before lighting off. Anyway, he found something that 'works', but it was before there was a readily accessible body of knowledge on what would have been better/simpler.

I did find it interesting that in the original thread's discussion, no one even asked what the designer said to do with the fuel lines from the tanks. Hopefully, the designer would know better than us internet trolls how best to route the fuel from his wings. (Though that's not always the case; see 'BD-4 fuel system'.)

The other alt engine Charlie

edit: I should have added that the environment on a test stand is nothing like what surrounds a tightly cowled engine on a flying a/c.
 
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RVCharlie,

Thanks for the reply, there are very good topics being discussed that I think will be of value to a lot of readers.

Most the discussions in the past I have read mainly surround the discussion about unporting and the need, or not, to use a header tank. (and to the OP I apologize if my comments are deemed to be too far off topic). For me, unporting is most likely poor fuel management, or a poorly designed system or a combination of both. No offense to anyone intended.

The bubbles issue is a bit less discussed and I that is what I was kicking around. Also, IMHO, high pressure systems that necessitate return lines is not what was on Van's mind when most of the RV series was designed. Of course in recent years more and more are moving on to high pressure EFI , and with that comes the need for more hardware.

I really respect Ross' experience and all he has done to support experimental aviation, and I do not disagree with his approach at all. If anyone should be considered the go-to guy, he would be my choice.

As far as fuel lines, fittings, pumps in the cockpit, sure sometimes it is a necessary but no question it is far from ideal. Anytime safety can be adopted we should work towards that goal. That in mind... I would rather have a high pressure fuel leak/fire outside of the cockpit than inside it. Just me. There have been a few inflight fires of leaks in the cockpit and it is not a pretty thought at all.

As far as the bubbles issue goes, I agree with you that there are some significant benefits to having pumps close to the tanks. Most who have experience and or researched the issue will probably agree that the chance for bubbles are much greater on the suction side of the pumps. Equally important is the concept of bypass/return lines or integral channels before the rail is so beneficial. Must less likely of fuel starvation by trapped air in the rail.

So, how about air being a concern on the pressure side of the pump? We know that most of the auto industry has gone to in-tank fuel pumps, with of course the pressure lines going the full length of the car from the tank to the engine, with no return lines. Not that I know about all cars on the road, but from what I know this is the mainstay of current auto technology. For me this suggest there is not much concern about bubbles in the pressure side of the system.

We must respect tried and true validated concepts and welcoming lessons learned, at the same time be willing ask questions and be open to take advantage of innovation and improvements when they present themselves. Look how far experimental aviation has come!?

All that said, isn't there some opportunity to learn from the millions of cars who uses systems use in tank fuel pumps, no header tank and no return lines, and all lines routed outside the cabin. Of course dual pumps that could feed from each tank is nothing to scoff at right?!

Thanks for "listening"....

Good flying to all!~

Charlie
 
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