Returnless FI?

[whee]

A couple things to consider:

Pumping the fuel will cause it to heat. I recall my thermodynamics instructor having us work a problem to determine how long it would take to heat his new "heater-less" hot tub to a set temp and what max temperature it could achieve. It was a interesting problem to work mostly because we all got to make our predictions and he came back with real life results a week later. My SWAG is that this is not an issue but I think it should be considered.

At 45gph you'll have turbulent flow. Where is all the air caused by that flow going to go? How long will it take to build up enough air to cause an issue? Air in the fuel will cause injector damage, tuning issues and power loss. I can't remember if it was the SDS or the EFII manual that specified a minimum distance of separation between a return port and a supply port to ensure the vapor was not drawn back into the supply line.

I should have taken better notes when testing my fuel system so I could provide accurate empirical data to the community. Unfortunately I was lazy. What I can say is my EFII Boost Pump Module supplies fuel to my engine at a rate of 40gph. At a 19deg nose high attitude (front lines unported) my rear 1/2" fuel lines flow enough fuel to keep the boost pump flooded with fuel (no suction). I am confident my system would work with EFI unchanged. However, had I wanted EFI I would have installed a header tank and returned the fuel there.

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

I totally agree but would add that asking a question is pointless if your not willing to listen and give thought to the answers. The opposing answers should be given especially thorough discovery for validity or even partial validity.

[schu]

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.

Respectfully, that's a lot more "I think"s, and "should"s then I would feel comfortable with.

Why not do what everybody else has done and return to tank or use a header tank large enough to cool the fuel?

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.

Battson said earlier in this conversation that he has unported a tank. Given that he could do that with a non-return fuel system, and given that sucking air is MUCH more likely with a return system that is flowing double the fuel, I'd be very concerned.

Did you look at the other fuel system design threads?

https://bearhawkforums.com/forum/bearhawk-general-discussion-news/37981-header-tank-design-and-location

https://bearhawkforums.com/forum/bearhawk-4-place-plans-build/21615-fuel-system-questions

https://bearhawkforums.com/forum/bearhawk-4-place-quickbuild/3160-fuel-system-design-questions

I'm not here to argue with you, but I also want you to be safe, as we all do.

[schu]

Dave,

Here is another SDS powered bearhawk:

https://www.ntsb.gov/_layouts/ntsb.a...20190327X81141

It says EFII, but I'm pretty sure it was the older EFII that used SDS. Regardless, the issue here was almost certainly fuel related.

I'd look into that and ask Ross about it, and completely and totally understand what happened before moving forward.

schu

[Archer39J]

Respectfully, that's a lot more "I think"s, and "should"s then I would feel comfortable with.

And that's perfectly acceptable for your aircraft. I am comfortable so far with the design and discussion to continue investigating this possibility. I'm absolutely suspicious of seemingly simple solutions to complex problems though, which is why I'm not designing in a vacuum.

Why not do what everybody else has done and return to tank or use a header tank large enough to cool the fuel?

I didn't, and still don't, see the need. And the designer opined that with the Bob system it may not be necessary either. So this is something I'd like to investigate.

Battson said earlier in this conversation that he has unported a tank.

Unporting a tank /= sucking air into the system, with the caveats I've explained. And header or loop the Bob system doesn't need to supply 45gph, just what the engine consumes.

I'm not here to argue with you, but I also want you to be safe, as we all do.

You're good, I really appreciate the discussion and links.

[Archer39J]

A couple things to consider:

Pumping the fuel will cause it to heat. I recall my thermodynamics instructor having us work a problem to determine how long it would take to heat his new "heater-less" hot tub to a set temp and what max temperature it could achieve. It was a interesting problem to work mostly because we all got to make our predictions and he came back with real life results a week later. My SWAG is that this is not an issue but I think it should be considered.

At 45gph you'll have turbulent flow. Where is all the air caused by that flow going to go? How long will it take to build up enough air to cause an issue? Air in the fuel will cause injector damage, tuning issues and power loss. I can't remember if it was the SDS or the EFII manual that specified a minimum distance of separation between a return port and a supply port to ensure the vapor was not drawn back into the supply line.

I should have taken better notes when testing my fuel system so I could provide accurate empirical data to the community. Unfortunately I was lazy. What I can say is my EFII Boost Pump Module supplies fuel to my engine at a rate of 40gph. At a 19deg nose high attitude (front lines unported) my rear 1/2" fuel lines flow enough fuel to keep the boost pump flooded with fuel (no suction). I am confident my system would work with EFI unchanged. However, had I wanted EFI I would have installed a header tank and returned the fuel there.

Yeah that’s my guess about the fuel pumps too, though I’m not going to bet my life on just an assumption lol.

I’m not sure I follow the point about air forming at that flow rate, I assume you mean fuel vapor. At areas of higher velocity/lower pressure it may cavitate sure, but the fleet history (drawing that flow rate UP from a tank) suggests this isn’t a problem at those rates.

I haven't heard of injector damage as you described, but I'll look into that. With either EFI system they state you'd just have to process the air through the engine, which can take "up to 30 seconds", but there are a lot of variables that would affect that time. I'm familiar with this potential issue.

I totally agree but would add that asking a question is pointless if your not willing to listen and give thought to the answers. The opposing answers should be given especially thorough discovery for validity or even partial validity.

Absolutely, and I hope it's apparent I'm receptive to constructive feedback (Brooks correction for example).

It's the "You shouldn't even be asking this question" before even attempting to understand the system and what I'm doing that I don't have time for.

I'll add explaining the same concept over and over again gets old, but that's not unique to this thread

[whee]

Sorry I didn't describe the air thing very well. I am taking about fuel vapor that is created by the pump and post pump turbulent flow then entrained in the fuel. This issue I'm thinking about is the ever increasing amount of entrained vapor. A tank provides a place for this vapor to settle out. Without a tank the vapor will be returned directly back into the system so the amount of entrained vapor will constantly be increasing. Maybe it will never increase to the point of being an issue, but maybe it will.

Injectors being damaged by vapor contaminated fuel is a common thing but in my mind is only a secondary issue since you can just monitor and replace the injectors as necessary. All brands of vehicles have had issues with it. They have almost eliminated the issue on gas vehicles and are pretty much there on diesel vehicles as well. The entrained air erodes the ball/seat that acts as the valve in the injector and eventually you end up with leaky injectors. All the experimental EFI manufactures use off the shelf injectors so don't think it is not a potential issue.

I recall one vehicle brand had a recall due to premature injector failure. Their solution was to move the return line away from the in-tank pump to prevent vapor from being returned to the system. My diesel pickup has a design flaw where the injection pump is also the main fuel pump so it sucks fuel from the tank.The low pressure on the suction side allows vapor to form in the fuel and like clockwork the injectors start leaking after 75K miles. Their solution was to simply issue a recall that extended warranty coverage on injectors to 100K miles. If you were unlucky your injectors lasted past the warranty and you got to part for replacement out of pocket. I'm on set #3.

Modern EFI vehicles use significantly higher fuel pressure than early EFI vehicles partially to help reduce the amount of vapor in the fuel but mostly to increase the efficiency of atomizing the fuel in the cylinder. But then again my diesel pickup runs at 4500-5000psi and it still has issues with vapor in the fuel.

[Battson]

Dave,

Here is another SDS powered bearhawk:

https://www.ntsb.gov/_layouts/ntsb.a...20190327X81141

schu

Now that is worth its weight in gold. Or it's weight in unbent Bearhawks, whichever is more valuable...

Well done Schu, good find.

[Archer39J]

There are several reasons I'm not going with EFII. Before selecting your system be sure to research your options throughouly...

[schu]

Kathryn's report has pictures:

http://www.kathrynsreport.com/2019/0...cident_30.html

I wouldn't post this unless the owner walked away, and I don't do it lightly, but fuel system design is absolutely critical.

Interesting to note that the airplane had a header tank. I wonder how it was vented and if it was full when the engine died. There are a lot of things that could have caused this. Electrical connector, crank position sensor, short somewhere, vapor lock.... it's hard to say.

Anyway, dave asked for feedback, and here it is.....

[Archer39J]

Modern EFI vehicles use significantly higher fuel pressure than early EFI vehicles partially to help reduce the amount of vapor in the fuel but mostly to increase the efficiency of atomizing the fuel in the cylinder. But then again my diesel pickup runs at 4500-5000psi and it still has issues with vapor in the fuel.

My 2013 Frontier runs at 43 psi, apparently. The goal is to avoid boiling the fuel all together, which it looks like can be done.

Woah, at 4500 psi it's got to be outside gas getting in at that point I would think.

[whee]

I’d think so too but obviously it isn’t. GM and bosch acknowledge the issue and GM changed the design. I’m on my 3rd set of injectors but this time bosche claims they used new injector technology that holds up better to air contaminated fuel and that I’ll never need to replace injectors again. We’ll see.

[Archer39J]

Kathryn's report has pictures:

http://www.kathrynsreport.com/2019/0...cident_30.html

I wouldn't post this unless the owner walked away, and I don't do it lightly, but fuel system design is absolutely critical.

Dave asked for feedback, and here it is.....

From https://bearhawkforums.com/forum/com...0396#post50396

Anyway, I'm not sure it's fuel related in the traditional sense as there was fuel pressure (so we know at least one pump was running), and enough fuel in the system. I suspect the EFI box cut out or the crank position trigger failed or some wiring came loose or there was a short with the injectors.

Is this still your opinion or is there new information out there?

Also keep in mind old EFII systems are not the same as current SDS ones (spider vs fuel rail as svyolo explained.)

[schu]

There are several reasons I'm not going with EFII. Before selecting your system be sure to research your options throughouly...

This airplane had an SDS based EFI unit in it.

[Archer39J]

For folks reading later. The EFII system in the referenced Bearhawk crash that is "SDS based" uses a different fuel delivery system than the current SDS system I'm designing. EFII uses a fuel rail, and routes it by hot exhaust tubes I might add, whereas the contemporary SDS system uses a remote mounted fuel block and dead head lines running to each injector separately. Lest we make comparisons we shouldn't.

[schu]

As far as I understand the fuel rail is different, but the controller box, wiring, fuel system requirements, crank position trigger, power requirements, and fuel pumps remain the same.

Please tell us what you don't like about the EFII system. Is it just the fuel plumbing?

[Archer39J]

Apologies, but I don't want to get into that here and now, and it's largely based on my personal opinions as someone who designs commercial aircraft components and systems for a living.

[schu]

Is this still your opinion or is there new information out there?

I've been reading a lot about fuel systems to make sure that the information I'm posting isn't totally wrong. Given a few things I've learned today, I would add vapor lock and venting to the list.

[schu]

Woah, at 4500 psi it's got to be outside gas getting in at that point I would think.

Totally common for diesel engines these days. Some are now pushing 30,000psi. The injectors/pumps combined with the complex emissions gear are the reasons I abandoned diesel and went back to gas. They are so complex that I don't think they are more reliable anymore. I don't tow 12,000lbs, so 385hp/430ftlbs is enough.

[svyolo]

Sorry I didn't describe the air thing very well. I am taking about fuel vapor that is created by the pump and post pump turbulent flow then entrained in the fuel. This issue I'm thinking about is the ever increasing amount of entrained vapor. A tank provides a place for this vapor to settle out. Without a tank the vapor will be returned directly back into the system so the amount of entrained vapor will constantly be increasing. Maybe it will never increase to the point of being an issue, but maybe it will.

Injectors being damaged by vapor contaminated fuel is a common thing but in my mind is only a secondary issue since you can just monitor and replace the injectors as necessary. All brands of vehicles have had issues with it. They have almost eliminated the issue on gas vehicles and are pretty much there on diesel vehicles as well. The entrained air erodes the ball/seat that acts as the valve in the injector and eventually you end up with leaky injectors. All the experimental EFI manufactures use off the shelf injectors so don't think it is not a potential issue.

I recall one vehicle brand had a recall due to premature injector failure. Their solution was to move the return line away from the in-tank pump to prevent vapor from being returned to the system. My diesel pickup has a design flaw where the injection pump is also the main fuel pump so it sucks fuel from the tank.The low pressure on the suction side allows vapor to form in the fuel and like clockwork the injectors start leaking after 75K miles. Their solution was to simply issue a recall that extended warranty coverage on injectors to 100K miles. If you were unlucky your injectors lasted past the warranty and you got to part for replacement out of pocket. I'm on set #3.

Modern EFI vehicles use significantly higher fuel pressure than early EFI vehicles partially to help reduce the amount of vapor in the fuel but mostly to increase the efficiency of atomizing the fuel in the cylinder. But then again my diesel pickup runs at 4500-5000psi and it still has issues with vapor in the fuel.

Whee, I think both suppliers want fuel return to the mains to be at least 4" from the feed, as the returned fuel causes aeration at the surface of the fuel tank as it returns. Literally, bubbles. Looking at aircraft header tanks that I have seen, they like to return fuel to the bottom, or near the bottom of the header tank so that the head pressure keeps the fuel from aerating. The header tanks all seemed to use a vent line, whereas returning fuel directly to the mains, the return line is acting as a bit of a vent for the system.

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?