Fuel tank un-porting in high pressure fuel system

[stringfellow]

Originally Posted by 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.

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?

[bobnoffs]

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.

[Toobuilder]

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.

[RKellogg]

Originally Posted by stringfellow

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

[Toobuilder]

Originally Posted by RKellogg

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.

[charosenz]

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.

[rv7charlie]

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.

[charosenz]

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