Returnless FI?

[svyolo]

Finally found a pic of the header tank. It is mounted but not plumbed. I still might replace it with a bigger one with internal pumps. I will eventually.

[Archer39J]

Right on, that's a nice looking little tank.

[Battson]

Nobody has noticed the factor of 2 error in the BH fuel flow discussed throughout the last page?? save Zkelley who was ignored.

The 260hp engine consumes no more than 24 USgal/hr at full power. You are required to get 125% of 24 gal with an injected system. Not sure where the 45 gal came from, but it's as concerning as the whole premise of this discussion.

[Archer39J]

Nobody has noticed the factor of 2 error in the BH fuel flow discussed throughout the last page?? save Zkelley who was ignored.

The 260hp engine consumes no more than 24 USgal/hr at full power. You are required to get 125% of 24 gal with an injected system. Not sure where the 45 gal came from, but it's as concerning as the whole premise of this discussion.

No, the SDS system returns at least 45 gph if you do the full mains return. This discussion is about using a return loop, one of the benefits of which is the Bob system only has to supply the amount the engine burns, which has been mentioned several times.

No need for concern, I think we're tracking

[Battson]

No, the SDS system returns at least 45 gph if you do the full mains return. This discussion is about using a return loop, one of the benefits of which is the Bob system only has to supply the amount the engine burns, which has been mentioned several times.

No need for concern, I think we're tracking

Look again.

So your discussion is about a closed loop return system, not a mains return system. Thus fuel not used by the engine just goes around in a circle, and your question is about having no header tank - thus the fuel just flows around in a circle back feeding itself. No matter how fast that re-circulation loop fuel flow happens, it will not affect the flow rates in the rest of the system.

Bottom line. The draw through all fuel lines (apart from the loop) will never exceed 24 gal/hr steady state. Regs relate to the engine's demand for fuel.

This didn't make sense to me.

I read a few threads on BH's doing fuel flow tests, and none of them that I recall approached the required fuel flow. I think, reading the regs, we would need 125% of 45 gph, which is 57 gph.

If that 45 gal/hr is the fuel return rate, then that is huge, almost 2:1 re-circ volume in normal cruise flight. Much more in a long descent. The reasons for returning to a tank are obvious in that case. Refer to comments about fuel heating.

Have you determined why hot fuel is a problem? That would be interesting.
Do you established whether unported tanks are a problem or not? Unporting is not a problem in the standard Lycoming fuel injected system.

[Archer39J]

Look again.

So your discussion is about a closed loop return system, not a mains return system. Thus fuel not used by the engine just goes around in a circle, and your question is about having no header tank - thus the fuel just flows around in a circle back feeding itself. No matter how fast that re-circulation loop fuel flow happens, it will not affect the flow rates in the rest of the system.

Bottom line. The draw through all fuel lines (apart from the loop) will never exceed 24 gal/hr steady state. Regs relate to the engine's demand for fuel.

This didn't make sense to me.

Exactly, so there's no confusion there then?

I mean, I noticed when he said that too and I recalled the "regs" say something about engine consumption anyway... it was just a tangent I didn't really want to get into because it's moot for what we're discussing. He's not using a full return either, he was talking about what he thought might be required if he was.

Big caveat, I have no idea how this system works

Then I don't understand why you're telling us what we shouldn't be doing?

[Battson]

Then I don't understand why you're telling us what we shouldn't be doing?

The answer is contained within the original question.

I’ve been vacillating on this for quite a while and I’m still undecided, but I wanted to post my thoughts and hopefully benefit from the experience of the group.

To expand a little - surely, if it was easy to make this system work the way you envisage, the designer would have designed the system to do exactly what you are describing. It would be better in every way, but it's not that way. So there must be a problem. Generally speaking, smart people are design and sell this gear, and they have thought through the same things which you are thinking through - except they are years ahead in their thinking - having got a head start.

Surely that kind of deductive reasoning is easy for someone such as yourself (yes I saw your comment but no I don't mind)

[Archer39J]

The answer is contained within the original question.

You've made your opinion clear and you state you don't know anything about the system we're talking about, as well as don't seem to be following the discussion. The things you brought up were either already covered or obviated by the design of the SDS system.

I seriously welcome pleasant discourse about the issue at hand, but I don't need or want to engage with this kind of attitude.

Go look at where I say I talked to the designer and he agreed with what I wanted to do...

[Battson]

You sound surprised? You asked why I was raising concerns, what did you expect me to say to a question like that?

If you have the designers approval, what do you want from the forum? If not our input and our experience?

Besides, I was done with the discussion two pages ago. I just wanted to point out the fuel flow number was incorrect in case it was pertinent, which you have acknowledged.

Best of luck.

[Archer39J]

You sound surprised? You asked why I was raising concerns, what did you expect me to say to a question like that?

Nah I was wondering how you presume to critique a design for a system when you admit to knowing nothing about it, that's all.

If you have the designers approval, what do you want from the forum? If not our input and our experience?

Again, previous posts address this too. My suspicion that the requirements weren't actually "required" was correct.

Besides, I was done with the discussion two pages ago. I just wanted to point out the fuel flow number was incorrect in case it was pertinent, which you have acknowledged.

Best of luck.

Yeah you said that. Thanks.

[Battson]

Nah I was wondering how you presume to critique a design for a system when you admit to knowing nothing about it, that's all.
.

On the contrary, I haven't questioned the design system, have I? My intent was to highlight assumptions about how it functions.

e.g.
Have you determined why hot fuel is a problem? That would be interesting to know.
Did you establish whether unported tanks are a problem or not? The standard Lycoming fuel injected system can tolerate this.

[Archer39J]

Anyway. From this discussion and those on other forums (as well as talking to the system designer himself) this sounds like a reasonable direction to go. Depending on your skill set and comfort level obviously. I'll for sure post updates, though it may be some time.

svyolo I'd love to hear how your testing goes, if you need help with anything let me know, I'm just down the freeway. You'll definitely be running well before me




Never be afraid to ask questions, even when you think you know the answers already...

[svyolo]

Jon;
The 45 gph is the steady, constant flow rate from the electric fuel pumps. If they are powered, and supplied with fuel, they will put out 45 gph, whether the engine is demanding 25 gph, 10 gph, or even zero, with the engine off. The only type that don't do this are some car pumps (most?) are now computer controlled, and only deliver fuel that the engine is demanding.

The simplest sounding system using these pumps is returning 0-45 gph back to the main tanks. If the engine is at max power, 20 gph are returned. With the engine off, but the pump powered, all 45 gph are returned right back to the tanks. Where fuel heating is a concern, this is recommended. Earlier EFI cars mostly did this.

Can the BH fuel system, as designed, feed 45 gph, all day, tanks full or tanks almost empty,15 degrees nose up or down, 45 degrees angle of bank. My 100% definitive answer is, I simply don't know. It was a nagging question in my mind. I had 1/4 NPT bungs welded into both tanks to function as return ports, but luckily didn't buy the duplex valve. If I was planning on expecting the BH fuel system to do this, I would definitely ask Bob.

A returnless system, whether using a header tank or not, only requires the BH fuel system to feed 25 gph to the engine, which is what it was designed for. I am much more comfortable with that.

EFII's fuel feed system to the injectors, is like a cars'. It is a fuel rail across the heads that does heat the fuel. If using a header tank, they recommend at least a 5 gallons (I checked their online docs) to mitigate fuel heating. SDS uses a fuel block, or spider, not that dissimilar to Bendix style. Unburned fuel is normally (but can be blocked off) returned from this fuel block. Much less heat is put into the fuel that returns.

If I used a full return to the mains, with a duplex fuel valve, I would want to test it. 20 degrees nose high, and another run nose low. 10 gallons, in one tank only, the other empty. Duplex valve to "Both". Turn the pump on, until it is not flowing fuel anymore. See how well it works. Maybe it works perfect, I don't know.

For me, the header tank made that nagging question mark in my mind, go away.

Dave is thinking about doing return-less. I don't know that much about how to design an aircraft fuel system, but I did learn a great deal about how to take that fuel and feed it to the injectors. Lots of spare time sitting in a coffee shop or home, researching how aftermarket and OEM's do it. I thought about doing return-less myself. To try it on mine, I just have to block off the return line (or port at the fuel block), and put the fuel pressure regulator at the fuel pump outlet, and run a MAP line to it. Return the fuel 6 inches to the header tank. I guess I could also try returning it to the inlet side of the pump. I probably have an extra T lying around to do that, but a "Y" might be better.

I am going to break my engine in on the ground. Just for my own knowledge, I will try some of this. I am pretty sure it will work fine. Returning fuel directly to the inlet side of the pump, I am not as sure. It is probably fine, maybe a small manifold or accumulator to dampen pressure spikes might help. I am not sure how well the pump outlet will play with the pump inlet.

[svyolo]

BTW I have hard lines to the fuel valve, Aeroquip hose after that. The reason I can swap stuff around easily is that it flexible line. It should be easy to do.

[Archer39J]

I would think the being exposed to the tank head pressure would help even out any pressure spikes going from idle to WOT and WOT to idle, but I can’t say for sure. I will be using a Y fitting like I have elsewhere. I’m looking at modeling this up to see what the pressures do at those different rates, the system is pretty simple so it’s not hard. Question now is whether I want to return before the gascolator or after, the gascolator remaining on the non-pressurized side of the pumps either way. Despite not being necessary for FI I’ll still be using one for a low point drain and filter, especially since we’re talking standard flow rates before the loop.

[schu]

Compared to a bendix or carb, the EFI systems have one major difference that requires careful consideration of your fuel system:

They pump FAR more fuel than is needed and bleed off everything above a specific pressure.

This causes two problems:

1. Extra fuel must go somewhere.

2. The extra fuel draw can be faster than the gravity system can provide when the engine is at low power settings.

Two very specific solutions have been developed to address these issues:

1. Return the fuel to one or both of the tanks.

2. Have a header tank or ensure the gravity system can provide enough fuel.

You could combine both and return the fuel to the header tank, but if you do that, you need to have enough mass of fuel to not get hot.

What I have read your post to mean is that you don't believe you need those solutions because:

1. You can return the excess fuel to a T in the system.

2. Your fuel will be pressurized so you don't believe the fact that it will get hot will result in vapor lock.

3. The bearhawk fuel system can provide enough fuel to not cause the fuel pumps to bring air into the system.

If I'm correctly picking up what you are laying down, then I respectfully disagree and would suggest that those actually are big issues and that ignoring them could be very dangerous.

If I was installing SDS, I would do one of the following:

1. Use a duplex valve to return to tank so that the fuel is cooled, then use 1/2 fuel lines to the pumps to ensure that things won't unport.

2. Use a standard valve and return to one tank, then use 1/2 fuel lines to the pumps, the SOP would be to run on the return tank until it's 1/2, then standard tank until it's empty, then back to return tank.

3. Use a standard valve and a 2-3 gallon header tank. Return fuel to header tank, and vent header tank to main tanks.

From what I understand, all of the above setups have been used by some certified airplane.

I think #3 above has the least possibility of failure because you pretty much couldn't unport a tank, while #1 above is probably fine and as I understand it, in a flying bearhawk.

My solution, battons solution:

Airflow performance.

[svyolo]

Schu, of you last 3, I settled on #3. But I agree on 1 and 2 requiring 1/2" fuel lines. But the other problem with #1 is the aviation duplex valves aren't ported to support 1/2" lines, so even with the bigger lines, you might not get the flow you think.

I read a couple of BH fuel flow reports. Nose high, with one 3/8" feeding yields about 7 gph due to gravity. A boost pump got that up to 15.

[Archer39J]

Compared to a bendix or carb, the EFI systems have one major difference that requires careful consideration of your fuel system:

They pump FAR more fuel than is needed and bleed off everything above a specific pressure.

This causes two problems:

1. Extra fuel must go somewhere.

2. The extra fuel draw can be faster than the gravity system can provide when the engine is at low power settings.

Two very specific solutions have been developed to address these issues:

1. Return the fuel to one or both of the tanks.

2. Have a header tank or ensure the gravity system can provide enough fuel.

You could combine both and return the fuel to the header tank, but if you do that, you need to have enough mass of fuel to not get hot.

What I have read your post to mean is that you don't believe you need those solutions because:

1. You can return the excess fuel to a T in the system.

2. Your fuel will be pressurized so you don't believe the fact that it will get hot will result in vapor lock.

3. The bearhawk fuel system can provide enough fuel to not cause the fuel pumps to bring air into the system.

If I'm correctly picking up what you are laying down, then I respectfully disagree and would suggest that those actually are big issues and that ignoring them could be very dangerous.

It sounds like you understand my main points. Though I respectfully disagree I’m absolutely not ignoring these issues, I’m specifically designing a system that mitigates them.

Looking at others who have installed SDS and tested vapor lock specifically they don’t have problems with vapor lock even at ambient temperatures in excess of 112F. As Ross says the volume in the pressurized lines FWF is a matter of a few CCs and consumed a few seconds after start, it’s also at 45-50 PSI which is higher than standard mechanical FI (close to what autos use iirc), so vapor lock in the pressurized side of the system should be avoided. That leaves vapor locking the fuel pump itself if the recirculating fuel gets too hot, but given the positive head pressure from the tanks and the isolation of the recirculation loop from the hot engine I think this won’t be a problem. Pump heating is a thing too, so obviously there is more analysis to be done there, then testing, then if it is a problem like I said passive or active cooling measures can be implemented.

As for sucking air into the stock BH system with just engine consumption fuel flow rates, that’s what looks like isn’t possible with the gravity fed system, again without a concerted effort and assuming you designed your fuel system to exceed engine required flow with the AC43.13 margin of safety. This doesn’t seem to be a problem for any BH running the stock system I’ve heard about. If anyone knows different please point to the example.

[whee]

I'm sure your digging deeper than just taking to Ross and reading the RV forum, both are great sources of info, but rarely does the manufacturer fully disclose all info and people rarely post about issues they caused by deviating from the manufacturers recommendations. During my efforts to design my system and then later resolve some issues I spoke to several builders that had fueling issues with their EFI systems that they caused by installation deviations. None of them had posted any info on the internet; I found them though friends . All of them resolved the issue by removing the EFI because either it would be too difficult to make the required changes or they had a reason for not installing per the manufactures instructions and were still unwilling to do so.