Returnless FI?

[svyolo]

Another sticking point here, is how do you do a valid fuel flow test on EFI, that complies with the regs to the letter, but more importantly, simulates how the system is actually running?
Whee stated that his boost pump fed fuel at 40 gph, and that works for his application. His boost pump has to supply fuel, under very little pressure, to the inlet of his engine driven pump. He probably just took the boost pump outlet, and stuck it in a gas can.

That is not a valid test for EFI, although it might be for the FAA regs. EFI pumps have to pump a certain amount of fuel (125% for the FAA), but they have to do it AGAINST the operating pressure of the fuel injection system. The electric pumps we use will have very different outputs pumping against atmospheric, versus 3 BAR. The pumps we use are rated at 155 Liters per hour at a certain PSI. I will have to check, but I believe it is 40 psi. Higher pressures will lower the output, very low pressure might double the output.

So whatever fuel system logic we use, we need to test it at the correct flow rate. I think I will set up the test according to FAA specs, and check gravity flow to the pump inlet (for my own information). But to test the system, I will check, and measure the output of the bypass side of the fuel pressure regulator. With the pump on, and the engine off, 100% of the pumped fuel will go through the bypass section of the regulator. And the test will be done under pressure. I will do this with the regulator mounted at the pump outlet, and with it mounted in the complete system.

For me, I could care less about the 125%, other than showing that the system exceeds that. What it needs to do in the real world is feed whatever the pumps are outputting (45 gph), reliably, without sucking in air. Nose high or nose low, in a turn, low on fuel, only 1 fuel line is feeding. With the fuel valve in BOTH, 3 lines are unported. Can the pump scavenge fuel from this one line, at 45 gph, without drawing in air from the 3 other lines? It will have a better chance of working, if fuel is not returned to the tanks, and the tanks and lines only have to feed engine consumption. Return-less or returning to a header tank.

Can I test the fuel system, to see if it will supply engine consumption only? (125% of this for the FAA). I think you can. The pump volumetric output can be reduced by increasing the fuel pressure. The regulator is adjustable from 35-70 psi. Raising the pressure regulator to 60 or 65 psi (whatever gives you about 32 gph) would verify that the tanks and lines can supply at least the 32 gph, if they fail the test at 45 gph.

Returning fuel from the SDS fuel block to a vented header tank can deal with some air. But I am not sure how much.

[Archer39J]

Gasoline RVP is about 9 psi (winter blend). Running 6X that pressure in the system, accounting for a higher temp of what, 200F (RVP is 100F) and that's quite a hot fuel line... Linear correlation would give 3X margin, though I doubt it is exactly. I'm not sure vapor lock post pump is really something to worry about. And I really don't see how that happens in a diesel, but I didn't check.

[Archer39J]

TVP scales exponentially with temperature but it's hard to find a definite value, so some reduction in those margins is warranted. There's probably a reason we see 45-50 psi as the target for gasoline fuel injection.

[svyolo]

Dave;
I am starting to doubt my system. I reread a couple of the fuel flow tests. I think the gravity feed part of mine should be higher than those tests, as I only have to gravity feed to a header tank, which is directly under the tank. 20 degrees nose up will have much less affect on the tank inlet, than on a carburetor 5 feet farther forward. I think my header tank feed will have about double the vertical distance from the tank, than the inlet to a carburetor. That should help a lot.

What concerns me is they only showed about 15 gph from one rear line, and very little more when running in both and having both lines feed. To me that means that with gravity only, the T's and the fuel valve are restricting the flow. If I run a header tank, even a bigger one, if it is being gravity fed at only 15 gph, I can run my header tank out of fuel.

I remember you used Y's instead of T's for your tank junctions. I also found these:

2 AN6 in, one AN8 out.

[Russellmn]

Gravity feed (pre-pump) lines should be at least one size larger than the pressurized line(-8 feed/-6 pressure). This helps the pump maintain clean fuel flow without cavitation, which is what is most likely to introduce air/fuel vapors into your system. That Y-fitting WOULD be sufficient except for the concern about unporting one pickup and running off of a single line. I would look for a 2x-8 in/single -8 out minimum. If you can find it, -10s in would be better.

[Cguy]

That is an interesting fitting, who knew?!

I see SDS mentioned several times in this thread, all I can add is I had a 14 gallon custom ferry tank fabricated by them 20 years ago, and most recently, 12 years ago anyway, the header tank for my current plane (not a Bearhawk, Rotax powered). Point being, the tapered header tank was seemingly beyond my ability to reliably translate into a drawing that didn't look like a cartoon drawn by a 5 year old, despite repeated efforts. It was all tapers and angles, so after calling them first, I UPS'd them the actual cardboard mockup I had used to fit the thing up, complete with the fittings located with a marker pen. It came within 10 days, and was perfectly accurate, and the price was very reasonable, about $200.00 I think it was. Good company! My header FWIW, is tapered so ALL fuel is useable, and it has a sight gauge. Inflight tests of my non FI engine show that the engine only quits when 100% of the fuel is gone, both wing tanks always on, in fact no valves between them just the main shut off before the firewall, simple and effective works for me.

[Cguy]

Oops, it was RDS, not SDS, my bad. http://www.rdsaluminum.com/

[whee]

I remember you used 1/2" fuel lines. When you did your fuel flow test, did you check the gravity feed, to the boost pump inlet? For a single fuel line feedIng?

I used 1/2" lines only on the rear ports. Front lines are still 3/8". This is the part where I didn't keep good notes or thoroughly test things in a way that would be meaningful to others. I checked the flow rate to the inlet of my boost pump at 3 point attitude with a couple gallons in each tank (front line unported so all the fuel is being supplied by the rear fuel line). No "Both" position on my fuel valve so all the fuel is being supplied by a single line. The flow rate was somewhere in the 50s gph. I wrote the results in one of my notebooks, but I can't find it.

The Continental fuel injection system is sorta similar to EFI in that it has a high flow rate and returns a large amount of fuel through the return line. In cruise flight it flows in the neighborhood of 30gph but the engine is only consuming 10gph or so. Operating pressures are much less though, 20psi max. During flight testing I was attempting full power stalls which puts the airplane in an attitude of about 20deg nose high. During some of those test I selected the tank that was low on fuel because I wanted to know if the fuel system had any issues with unporting or otherwise. Several times I held the airplane in the nose high attitude on the verge of stall for a minute or so and never had any problems. I don't know what the overall flow rate was during those tests but the engine was only consuming 16gph.

Another sticking point here, is how do you do a valid fuel flow test on EFI, that complies with the regs to the letter, but more importantly, simulates how the system is actually running?
Whee stated that his boost pump fed fuel at 40 gph, and that works for his application. His boost pump has to supply fuel, under very little pressure, to the inlet of his engine driven pump. He probably just took the boost pump outlet, and stuck it in a gas can.

That is not a valid test for EFI, although it might be for the FAA regs. EFI pumps have to pump a certain amount of fuel (125% for the FAA), but they have to do it AGAINST the operating pressure of the fuel injection system. The electric pumps we use will have very different outputs pumping against atmospheric, versus 3 BAR. The pumps we use are rated at 155 Liters per hour at a certain PSI. I will have to check, but I believe it is 40 psi. Higher pressures will lower the output, very low pressure might double the output.

So whatever fuel system logic we use, we need to test it at the correct flow rate. I think I will set up the test according to FAA specs, and check gravity flow to the pump inlet (for my own information). But to test the system, I will check, and measure the output of the bypass side of the fuel pressure regulator. With the pump on, and the engine off, 100% of the pumped fuel will go through the bypass section of the regulator. And the test will be done under pressure. I will do this with the regulator mounted at the pump outlet, and with it mounted in the complete system.

For me, I could care less about the 125%, other than showing that the system exceeds that. What it needs to do in the real world is feed whatever the pumps are outputting (45 gph), reliably, without sucking in air. Nose high or nose low, in a turn, low on fuel, only 1 fuel line is feeding. With the fuel valve in BOTH, 3 lines are unported. Can the pump scavenge fuel from this one line, at 45 gph, without drawing in air from the 3 other lines? It will have a better chance of working, if fuel is not returned to the tanks, and the tanks and lines only have to feed engine consumption. Return-less or returning to a header tank.

Can I test the fuel system, to see if it will supply engine consumption only? (125% of this for the FAA). I think you can. The pump volumetric output can be reduced by increasing the fuel pressure. The regulator is adjustable from 35-70 psi. Raising the pressure regulator to 60 or 65 psi (whatever gives you about 32 gph) would verify that the tanks and lines can supply at least the 32 gph, if they fail the test at 45 gph.

Returning fuel from the SDS fuel block to a vented header tank can deal with some air. But I am not sure how much.

I agree. You need to figure out what you want tested and how to accomplish that. The goals of my tests were #1, make sure my engine driven pump was getting 125% of 30gph so I was sure it wasn't having to suck fuel from the tanks and #2 satisfy the DAR by showing a fuel test was completed. Indeed I just took the fuel line off the inlet of the engine driven pump and stuck it in a container. This didn't test the function of the boost pump though. It needs to act as a backup to the engine pump so it needs to maintain a certain fuel pressure . I tested this by turning on the pump while flying.

[zkelley2]

Oh that's real interesting about the 50gph out of 1/2" lines. That's through the fuel selector(which one)? I figured that would be the choke point, more so than the lines.

[Archer39J]

Dave;
I am starting to doubt my system. I reread a couple of the fuel flow tests. I think the gravity feed part of mine should be higher than those tests, as I only have to gravity feed to a header tank, which is directly under the tank. 20 degrees nose up will have much less affect on the tank inlet, than on a carburetor 5 feet farther forward. I think my header tank feed will have about double the vertical distance from the tank, than the inlet to a carburetor. That should help a lot.

What concerns me is they only showed about 15 gph from one rear line, and very little more when running in both and having both lines feed. To me that means that with gravity only, the T's and the fuel valve are restricting the flow. If I run a header tank, even a bigger one, if it is being gravity fed at only 15 gph, I can run my header tank out of fuel.

I remember you used Y's instead of T's for your tank junctions. I also found these:

2 AN6 in, one AN8 out.

I noticed the same thing re pitch angle. Figuring my pump inlet was at STA B it’d take a 45 pitch to be level with the rear tank pickup, looking at an extreme case. I may move it back to STA C but I’ll have to look at it. I too wonder how this affects people with engine driven pumps being farther forward, as long as we can improve on that baseline I think we’re good, but it doesn’t seem to be a problem for them.

Looking back I found Mark G. saying he got about 24 gph on a single tank, and Jared using a fuel transducer got 25 GPH though he doesn’t state it was on a single tank (I assume he was doing the test correctly though). Given that we don’t hear about Bearhawks sucking air into their lines and losing power, recall how common unporting tanks apparently is, it must be that most installs are fine in real world conditions. Better head pressures and better flows for our systems, seems like a good way to go.

Yeah I used Y with -6 ports all around. Since we’re mainly interested in single line feeding (nose high) I don’t know if that -8 outlet fitting would be beneficial, going to a -6 fuel valve too, and the complication of ½ lines after, wouldn’t hurt though.

[Archer39J]

"The pitch angle has nothing whatsoever to do with the pressure due to gravity."

So this is false then?

[zkelley2]

Saying pitch angle changes the pressure demonstrates a misunderstanding of the physics involved. Changing the pitch does change the pressure, but only because it changes the distance the carb/whatever you have is below the fuel tank. In the equation for figuring out the fuel pressure due to gravity at the carb inlet, there is no place for pitch angle. Only the height difference.

[Archer39J]

Nope, its merely acknowledging that systems pivot around the wing, which changes the z height. C'mon now... John and I have been discussing this without having to spell that out to each other...

[Archer39J]

Looks like a minimum .26 psi loss at necessary flow rates with the stock gascolator. That's just from the geometry so it's definitely going to be higher. This precludes keeping it in the system as is just for a water drain so I'll be doing something else there.

No restrictive gascolator or fuel transducers though so that's good. That adequate flows are reported including those is telling.

[svyolo]

Dave;
I have a Newton valve. It has O ring fittings, I think also called ORB. You can buy AN6 or AN8 fittings for the same valve, so maybe that means the valve is ported for AN8 flow rates, an all you have to do is change the fitting size.

All that being said, as long as your system, and mine, only have to feed MAX engine feed X 1.25, I don't see the need. If we had to feed 45 gph, continuously, I would want a different solution.

This is experimental aviation, and one of the goals of that is to learn. I am learning a lot. Probably more than a few are questioning why we don't just slap a carb on it. For me because I am stubborn, but I am also learning a lot more. I don't look at it as a problem, but as a way to figure out a solution.

[Archer39J]

Yeah I forgot about the other port options you're right, that point is moot then.

Everyone will make their own decisions commensurate with their comfort level. I am also very grateful for those who are willing and capable of designing and testing new ideas, without that we wouldn't even be flying.

[Mark Goldberg]

I have followed this discussion with only partial understanding of the points you guys are making.

I only want to interject that fuel starvation accidents are right up at the top of what brings down experimentals. While the spirit and fun of being free to do anything in our builds is great, it is awfully sad when a newly finished plane ends up bent after forced landing. So be careful how far you go off the reservation. Mark

[svyolo]

Mark, I can't speak for Dave, but believe it or not, we are trying to stay on the reservation.

You can take a carberated BH, remove the carb, put on a Bendix style FI, and make no changes to the BH fuel system upstream of the fuel valve or gascolator. My intention is the same. I want to take the BH fuel system, at the fuel valve, and hook up the EFI fuel feed so that no changes are required of the BH fuel system, including how that fuel system functions.

If I take the EFI and hook it up with a duplex valve, and return fuel to the main tanks, I have changed how the BH fuel system operates, and forces it to do 2 things it was not intended to do.

So, I agree with you, and my intention is to make my fuel system, stay on the reservation, because the BH fuel system works as designed.

[Archer39J]

Better said than me. It looks like there's potential for a very robust system which is worth exploring.

[Archer39J]

I have followed this discussion with only partial understanding of the points you guys are making.

I only want to interject that fuel starvation accidents are right up at the top of what brings down experimentals. While the spirit and fun of being free to do anything in our builds is great, it is awfully sad when a newly finished plane ends up bent after forced landing. So be careful how far you go off the reservation. Mark

This is true. Folks definitely need to understand what they're doing.

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[rodsmith]

I am wondering what the advantages are of fuel injection systems that require a header tank or return line vs Bendix or Airflow Performance. Personally I wouldn't want to be dependent on electrically driven pumps or have to redesign a proven system.

[whee]

I know nothing about Airflow Performance. I had a lengthy conversation with Mr. Peterson, the guy that has developed many of the mogas STCs for certified planes, during which we discussed why he never got any fuel injected lycoming engines certified for mogas. He tried. Short version is the lack of a return line in the Bendix system caused vapor lock issues. Yes the engines would run and the test planes flew but they were constanly as risk of failure. If you don’t want to run mogas then a return line has no advantage.

[zkelley2]

Aside from the mogas as whee mentioned, literally nothing. There's not even fuel savings from EFI compared to a tuned bendix system. The fuel savings in the EFII comes from the ignition.

[Archer39J]

I am wondering what the advantages are of fuel injection systems that require a header tank or return line vs Bendix or Airflow Performance. Personally I wouldn't want to be dependent on electrically driven pumps or have to redesign a proven system.

I appreciate the question, but there are many, many threads both here and on VAF that discuss the pros and cons of EFI in general. I'm not really interested in rehashing those discussions myself in this thread. Each person needs to do their own research and determine what their mission is and what they're comfortable with modifying.

[svyolo]

One of the reasons I chose EFI was I understand it, and Bendix FI to me has a couple of "black boxes" that I don't understand. I did spend a couple hours the last few days trying to learn a little more. I still have a couple of questions about individual brands.

One thing I don't see with the Bendix style is a return line. I did see a bleed screw (bled to atmosphere) in the fuel divider, probably to purge air out of the system when it is first used. I don't see any way to purge air, or vapors out of the system once it is running, other than forcing through the injectors. Near as I can tell the system runs at a fairly low pressure, I believe 30 psi or less. I believe AFP has some kind of bypass to purge vapors/hot fuel out to help with hot starts.

The electric boosts pumps, Andair, EFII, and AFP, all return excess fuel from the pump outlet, right back to the pump inlet. EFII, and maybe AFP, use a similar pump to what EFII and SDS use for high pressure pumps. In the case of the EFII boost pump, it might be the same Walbro part number. I haven't asked.

Some or all of the engine driven pumps also have an internal bypass, or return. I also saw a return line from the fuel servo back to the engine driven pump in some. But I see no way of purging air out of any of them, once the system is running.

The fuel feed portion of EFI is bloody simply by comparison. 2 electric pumps, in parallel for redundancy. One running at a time. From there, fuel goes to the injectors. The electric pumps put out a constant fuel flow based on PSI and voltage. The ones we use it is about 40 - 45 gph. Without a computer controlled pump, the excess fuel has to go somewhere. I haven't looked at the psi that EFII uses, but SDS recommends 45-50 psi. Fuel vapor should be much less of a problem than with Bendix style, which is why SDS said they use the higher pressure.

The simplest way to deal with that excess fuel is simply reintroduce it back to the pump inlet. Place the fuel pressure regulator at the fuel pump outlet, and return excess fuel 8 inches back to the inlet. Andair, AFP, and EFII all do something similar with their boost pumps. Since those boost pumps seem to work fine on BH's with Bendix style fuel injection, I can't see any reason why you couldn't do the same, and feed electric injectors, instead of a mechanical fuel pump and servo. If Bendix style can work without a return from the injectors or fuel divider, why can't EFI?

The only issue I can think of is heating of the fuel pump. In-line fuel pumps run warm, and fuel going through them actually cools them. If you keep recycling 75% of the fuel back through the pump, would the pump slowly heat the fuel, and would the pump keep getting hotter and hotter? I don't know.

I am not planning on using this type of return-less system, but I am planning on trying it.

Now that I know a little bit about Bendix style FI, wow, is it complicated. I know it is proven, and I know that it works. Except for the boost pump, it requires no electricity. That last part is a big plus if you don't want a big electrical system. But the mechanical pump, and servo, are both parts that I don't really understand, and probably would not take the time to learn how to service myself.