Returnless FI?

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.
 
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.
 
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.
 
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
 
Mark Goldberg said:
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
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This is true. Folks definitely need to understand what they're doing.

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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.
 
rodsmith said:
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.
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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.
 
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.
 
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.
 
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.
 
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.
 
Here is a picture of AFP's boost pump. It is rated at 32 gph, at 30psi. In their manual it states it is rated for continuous duty. At the pump outlet, there is a non-adjustable fuel pressure regulator. Fuel is fed, and pressure is maintained at 30 psi, by returning unused fuel to the pump inlet, as I believe Dave wants to do. EFII's boost pump looks pretty similar. Andair's is quite a bit different. It is British, after all.

AFP's docs also state that if the fuel pickup ever becomes un-ported, or a tank is run dry, neither the boost pump or engine driven pump would be able to create enough pressure to open the valve at the fuel divider. I guess that is engineer/lawyer speak for "your engine will cease functioning as a propulsion device."
 

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Dave, I am glad you started this thread. I think I have almost talked myself into doing a return-less system, even though there seems to be consensus amongst fuel system experts that if you CAN return, it is better.

The BH fuel system has many thousands of hours gravity feeding the inlet side of a pump in a return-less fuel system, feeding a Lycoming engine. The parts downstream of that, shouldn't matter to the aircrafts' fuel system.

Fuel heating ins't supposed to be an issue with SDS's fuel system, and they run at a much higher pressure than the Bendix, which helps with vapors.
 
I see AFP put gascolators and pumps fwf in schematic #2, huh. I don't know much about fuel dividers, but why wouldn't a pump be able to re prime the system?
 
And I almost agree with them too, but then if a system is unnecessary it's just added failure points, cost, and weight. To reiterate your earlier point doing a header or pump return is sticking with a stock BH fuel system as far as flow is concerned, if I were to do a full return I'd want better flow of 1/2" lines like whee did. The high pressures pretty significantly preclude vapor lock, and that was with high-RVP winter E10 data from the EPA. My goal is to run E10 (auto), regular too, but then have the option to switch to other fuels on the road :D
 
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I have been trying to understand the Bendix style stuff, and am surprised how complicated it is, even the flow divider. It is more complex than just a manifold. I found a link to TCM's version of FI, and their engine driven pump looks a little more robust as far as removing air. There is a chamber in the pump to eliminate vapors.

This also kind of got me thinking of just creating a little manifold, right before the inlet of the electric pump, to separate air, and vent it back to the tanks. It wouldn't have to be much. Half a Pint. Tall and skinny with a 1/4 inch or smaller vent at the top. An upside down gascolator might do it, if you could replace the drain filter with a AN4 male fitting.

Return fuel from wherever you want to to just upstream of this little manifold. If you returned it from the fuel block, the whole thing would be self bleeding on first start up. Or just from the outlet from the pump. But, then I am adding complexity back in.
 
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I'm uncomfortable. This thread is starting to scare me, but I don't know enough to give input. At work I've been trained to ask "Does everything look right and feel right?" to my colleagues when we've been rushed or weird stuff happens and we get out of our routine.

Things are starting to feel "Not quite right" to me. I think its lack of data, abundance of unanswered questions, mixed in with a misunderstanding I'm seeing here and there.
 
You're not alone. Been having the same feeling since this thread initiated. I could say more, but I will just leave it at that...
 
I've noticed a lot of misunderstanding myself actually too, and you're right it is unnerving...

Not sure what feelings have to do with designing aircraft systems, I'll stick to engineering and testing with that.

I'd say if you don't know enough to give input, maybe ask questions first before criticizing?
 
I have read enough stuff in the last few days that I am actually getting quite comfortable, much more than before. There is probably nothing that hasn't been tried. Folks with Bendix FI injection have created a return line, some have dumped the engine driven pump for 2 electric ones. Folks with EFI have used an engine driven pump in the belief that these are more reliable. Some folks return to the tank, some to the inlet side of the fuel pump. Some have used a T, with a tiny orifice, to return a tiny amount of fuel to get rid of fuel vapor. Some plumbed them to the fuel tank vents.

Below is cut and pasted. It is a great description.

"What most folks refer to as "Vapour lock" is not vapour lock at all. VL is when the fuel pump sucks on the fuel (that has been warmed and/or is at a low enough pressure) causing it to boil at the inlet of the pump. This is relatively deadly...I.e it can kill you! This is most likely to happen whenever the fuel is hot, at low pressure (high altitude) and is being sucked upon. Autofuel is even worse because it has a higher vapour pressure which means it is more prone to boiling.

Without getting too involved in this, think about the mechanical fuel pump location.....In a hot cowl. and sucking from the tank...Not good!..Thats why my fuel pumps are in the wingroots which is much cooler than the cowl and its not sucking a long way from the tanks either. Mind you with a 3/8ths line you could suck it a afair distance without causing an issue. Being an engineer that designs pumping systems for high VP liquids I wanted them in the hydraulically correct place.

Anyway, lots of folks call localised boiling in the injector lines as vapour lock....This is not correct. The only reason it happens is when you shut down the fuel lines get heat soaked and because the pump is not making pressure (hot fuel won't vapourise when its under significant pressure) the fuel boils...When you got to start the engine has to be cranked to pump all the hoses free of vapour and get wet fuel in there. Hence the classic hot start...Crank crank crank..."

By my reading this, the worst possible combination is an engine driven fuel pump, suction feeding. The scenario that sounded the worst was flying along, pull the power back, rpm drops, fuel pressure drops from the engine pump, and you get vapor lock that stops the engine.

Almost all newer cars run return-less. Most Lycoming using Bendix style FI run return-less. AFP includes a purge valve to return for 15-20 seconds for hot starting. After start, it runs return-less. TCM FI, by the diagram I saw, returns fuel from the servo (not the divider) to the pump, and the pump has a vapor return to the tanks (or somewhere downstream). After the servo, it looks return-less to the divider and injectors.

AFP, EFII, and Andair boost pumps all bypass and return fuel from the pump outlet, to the pump inlet. At least AFP rates theirs for continuous operation. Not sure about the other 2.

EFI has nothing between the pump outlet and the fuel injectors. 15 minutes after a hot shutdown, you might get some vapors on top of the engine, like any engine. Turn the electric pump on, and the 45 psi slams the vapors right back into suspension. No more fuel vapors. The pumps are mounted aft of the firewall. On startup they get cool fuel, gravity fed, at the pump inlet. No vapor.

The hot start problem is a mechanical FI problem. The fuel pump and servo are mounted on the engine, and get hot. The fuel in them gets hot after shutdown, and vapors form.

If I was going to use Bendix style FI, I think I would use all electric pumps as well. No hot start issues. No boost pump needed. It is cheaper than a boost pump plus an engine driven pump. Electric pumps might last longer, and are far cheaper to replace. It would be easier to start, easier to operate, and cheaper to maintain.

Everything that we can think of has already been tried.
 
Yeah if anything this has shown I'm quite unoriginal, this has already been done before. We're just making sure it doesn't over tax the BH system.
 
The only reason there is a mechanical pump is because it's far more redundant to have a mechanical and electric than two electric. From a pump performance and cost standpoint electric is superior, but it's not redundant in the same way different drive mechanisms is. From an aviation engineering perspective, this is how things are normally designed to be safe. Not just 2 pumps(whether it's fuel, pneumatic, hydraulics) but also different ways of powering them.

Now the hot start thing. I have quite a few thousand hours behind Bendix injection. Mostly with turbos under the cowls as well for extra heat. If you understand the system and how to correctly start it, you'll never have a hot start. It's not a thing(It is a thing, but you shouldn't experience it). It's always operator error. I'll even go as far as to say Bendix injection will always start, down to temperatures you should be way past pre-heating, where carbs won't even start anymore.
That said, I don't personally see the benefit of removing redundancy from a system(2 electric pumps in lieu of 1 mech and 1 electric) for a flaw that is purely in the operator, not the equipment.
 
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Bendix is a proven and tested system. No arguments. On a light aircraft, with limited electrical redundancy, I would say that 1 engine driven and 1 electrical pump offers more redundancy in some ways, mostly while airborne.

Almost everything I have flown has 2 or 3 of everything, if not more. The 2 or 3 were all identical, but the electrical redundancy/separation was far greater.

I agree with the operator error part. But deep into 5 digits in flight time, operator error is unfortunately part of my DNA. I would rather just turn the key, and have it start. Less for me to screw up.
 
I was hoping there would be a better discussion of the technical reasoning behind using a header tank. But it's apparent that knowledge isn't here.

I'm doing CFD analysis on gascolators and fittings and reading EPA tank design guides for equations on extrapolating TVP from RVP. So "I don't know what you're talking about, but neither do you" is an especially rediculous response from my perspective.

Turns out that a header tank isn't necessary with the SDS system configured as I intend, backed by the fact other systems are configured the same. If you have technical justification for why this isn't the case by all means let's discuss it. Else all these opinions are just that, and mostly uninformed it seems...
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Archer39J said:
I was hoping there would be a better discussion of the technical reasoning behind using a header tank.
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The header tank is a choice some designs use, but not all. It may simplify a fuel system on a FI aircraft that requires engine return fuel. On carbureted engines, its used to prevent fuel starvation if a wing tank is un-ported for a period of time. Many taildraggers with little or no wing dihedral and a wing fuel tank with only one fuel pick were manufactured. Even a crowned runway with 1/4 tank can un-port the fuel pick up and starve the engine so the header tank was installed.

Fuel flows into a header tank via gravity at the rate that an engine consumes fuel. It is vented, and so it always remains full until the main tanks are empty. The fuel port out of the header tank to the engine is alway is covered with fuel. The header tank is a good place to plumb excess engine return fuel to.

Cessna 310R, C-185, and Beech Bonanza's use Continental's Fi system which returns excess fuel from the engine back to a tank. The 310 always returns the fuel back to the respective main tank, even when the Aux tank is selected. So the C-310 main tanks fill back up when burning off the Aux tank. The Bonanza returns fuel back to the selected tank via a duplex valve. The C-185 has a header tank that is labeled a "Fuel Accumulator Tank".

The Continental FI system design requires a fuel return because that system pumps excess fuel, like SDS. Having a header tank simplifies the fuel system.
-The header tank is a destination for the engine return fuel and the tank supply fuel,
-The Header tank is always full even if one tank is run dry,
-A simple ON-OFF fuel valve may be used to control engine supply to the engine.
-It allows for a L-BOTH-R fuel selector.
-Multiple return fuel lines do not have to be plumbed back to the wings all the way back to the wings,
-A duplex fuel valve is not required.

The original Super Cub fuel system is gravity fed with a header tank located aft of the baggage area. Each wing tank in a Cub had one fuel line supplying fuel to the header tank. (Bob's design has two, one near the front, one near the rear to allow fuel supply with near empty tanks in a nose low or nose high attitude)

The header tank allowed take-off with 1/4 tanks without worry of unsporting a tank. The C-140 did not have a header tank, but had a placard "Do not take off with less than 1/4 tanks." (I un-ported a C-140 tank on takeoff once and momentarily loss power even though we honored the placard.) A Super Cub STC does away with the header tank if the system is modified to what the Bob Barrows original design is.

So, the technical reason for having a header tank is to maintain a positive supply of fuel to the engine in all normal aircraft states, to simplify fuel system plumbing, and in some cases to have a place for fuel to cool after traveling through an engine compartment.
 
Bcone1381 said:
So, the technical reason for having a header tank is to maintain a positive supply of fuel to the engine in all normal aircraft states, to simplify fuel system plumbing, and in some cases to have a place for fuel to cool after traveling through an engine compartment.
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Except in the BH fuel system, for the reasons I've explained multiple times, these goals can be achieved without a reservoir of fuel. As proven by analysis and real world examples.

You forgot about serving as a place for gasses to collect (but I won't use that as a basis for claiming you don't know what you're talking about), which is really the only concern left going returnless, but again this is how other systems do it.

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I appreciate that. It's a decent review of header tank fundamentals for those who don't already know or don't do any research beyond this forum.
 
Bcone1381 said:
I'm uncomfortable. This thread is starting to scare me, but I don't know enough to give input.
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I know the feeling. Years ago my wife and I were relaxing in a natural hot spring after enjoying a day of winter whitewater kayaking. Two attractive females appeared out of the darkness and asked if they could join us. Of course we said yes. They disrobed and slide into the pool next to me. There I was sitting in a hot pool with 3 attractive and naked women... My engineering professors didn't teach me how to handle such a situation so I leaned over to me wife and said "I'm uncomfortable."
 
A thread on another forum lead me to read this article: https://generalaviationnews.com/2011/08/14/autogas-experts-explain-why-pump-gas-is-mogas/

I found the following quote interesting:
Todd Petersen, owner of Petersen Aviation, commented recently on Lycoming’s assertion that autogas airworthiness from the FAA is a matter of luck, not by design:

“I must take issue with Michael Kraft’s statement that using pump gas is achieving airworthiness not by design but by luck. The STC’s were not developed through luck. Detonation and endurance testing was required in order to develop the STC’s in the first place, as well as flight testing for airframe STC’s which make the engine STC’s worthwhile. In certificated airplanes engine approval by itself is, without airframe approval, largely irrelevant.

Auto fuel specifications are indeed designed for ground vehicles, which is why testing was done to approve airplanes to use it. Yes these specifications change frequently. If they changed to the point of creating adverse circumstances then Mr. Kraft’s point would be well taken, but with one exception (ethanol) they have changed only for the better, making autogas (without ethanol) more like avgas than it ever was.

Mr. Kraft also stated the following: “93 AKI is “Super Premium” fuel. That AKI rated octane is what is needed to achieve the exact same power performance on the approved engines as 100LL avgas. 93AKI is produced worldwide, but not necessarily distributed worldwide.”

This statement implies that an engine burning 91 octane doesn’t give the same amount of power as 93 octane. In fact, octane has to do with improving the detonation margins. Power is more a function of BTU’s per gallon. You do not need 93 AKI to achieve the same amount of “power” as on 100LL. Our detonation testing demonstrated that you get about 3% more power on auto fuel than on 100LL because there are more BTU’s per gallon. If it were less, the FAA could never have issued the STC’s.

While limiting volatility to class A-4 may reduce the chances of vapor lock it will not eliminate it, and it will not make a Bendix injection equipped Lycoming safe to operate on mogas or pump gas. I have been personally been involved with hot fuel flight testing of airplanes using Bendix injection systems. Regardless of the type of airplane, it failed. I’ve spoken to owners of Bendix equipped airplane who have it happen frequently on avgas.

Lycoming erred when they approved Bendix injection. I’m sure they feel that it’s an airframe issue and therefore not their problem, and indeed that may be the case — in certificated airplanes. Owners of Experimentals though are using Lycoming engines without any sort of testing of the airframe fuel system against vapor lock. Those guys are test pilots whether they know it or not. Let’s hope that Lycoming makes a change to the injection system which will make their proposed I0-233 safe against vapor lock. If they fail to do so, then it’s Airworthiness by Luck.

I welcome Lycoming’s revised stance on the use of autogas in general. It’s especially refreshing given the repeated attempts by Lycoming throughout the 1980’s to discredit auto fuel STC’s.
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Here is another interesting article that talking about the testing necessary to prove a fuel system is safe with E0 mogas:https://generalaviationnews.com/2011/04/11/autogas-for-everyone-part-i/ None of us are going to so thoroughly test our fuel systems so as Mr Petersen said in the quote above, we are truly test pilots when we start changing fuels or fuel systems. Makes since why fuel issues are the #1 cause of EAB wrecks.

Archer39J said:
Except in the BH fuel system, for the reasons I've explained multiple times, these goals can be achieved without a reservoir of fuel. As proven by analysis and real world examples.
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There are also real world examples proving the opposite and no doubt someone else could prove by analysis the same. As with any technical analysis, the data we use and assumptions we make affect the result. I'm not saying your wrong but I do think you have your mind made up and are dismissing valid points. However, I am all for others doing exactly what they want to do. I just hope some of them don't fly over my house.

Literally everyone told me I couldn't run my Continental IO360 on mogas with I asked on some forums about it. Everything seemed to check out when I looked into it. It was clear that the detonation margin was going to be close and that vapor formation at the intake of the electric boost pump could be an issue but there was not glaring issue telling me it wouldn't be worth the effort. I called Mr. Petersen, discussed it with him. I called another outfit that currently handles the big bore fuel injected Continental STCs and discussed it with them. I determined that I may have to install pumps in the wings as is required on Cessna 210, 185, 180s when running the FI engines on mogas but I could perform some low risk tests to determine if the pumps were necessary on my installation. So far I've flown 50hrs on mogas. Detonation margins are indeed quite minimal but it is manageable, vapor formation doesn't seem to be an issue but it is something I'm still watching, and unexpectedly I've discovered that my engine runs like crap LOP on mogas.
 
What valid points am I dismissing? Those who insist the BH system may not provide adequate flow despite the fleet history and actual system design? The ones who insist vapor lock can occur despite apparently not having any understand of fluid vapor pressures or bothering to do any analysis themselves? The people who continue to hold up examples of systems that heat soak the engine side fuel lines as reasons why this doesn't work?
 
High wing aircraft have the luxury of gravity keeping the pump intake at a slight, positive pressure. Suction feed is much worse, and much more prone to vapor problems, especially with Mogas.
 
Dave, I think you are on to something. AFP has a good writeup of what happens on Bendix FI when you shutdown and try to start up a hot engine. It is a selling point for their purge valve, which is nothing but a switchable return line used for 15-20 seconds. Experimental aircraft only.

I can think of one or two minor issues, but the EFI fuel system has none of the engine heated components of the Bendix. We have the dual benefits of a gravity fed fuel pump inlet, and double the operating pressure of Bendix, after the pump. I can't think of a reason not to do it. A return line is theoretically better. Like we used to say in the military, "If the minimum wasn't good enough, it wouldn't be the minimum." Bendix FI return-less seems to be working perfectly with the BH fuel system. Ours should as well.

Possible issues is the fuel pump gets too hot, and it will be harder to prime the system on either a first start up, and if you ever run the tanks dry. I think the hot pump might only be an issue on the ground, at idle. Once you are burning 7-15 gph, there should be enough cool fuel going through the pump to keep the pump cool.
 
The later 172's (R model and later) had IO-360's. Their fuel systems also used a return line to a header tank. The diagram wasn't great, but it looked like the return came from somewhere on the servo. If that is correct then from the servo to divider and injectors is still return-less.

I never flew one, just found it online. I think I have an hour or 2 in a 172 in 1981.
 
It sure looks like it from what I can determine, testing will of course be the proof. A purge valve... now there's a brute force solution lol, but maybe the only one for that application.

Plenty of RVs have trouble with vapor on the ground with stock Bendix, a couple guys had to take out their engine pumps to run E10 and are still plagued with vapor issues. Glad I'm going this route.
 
svyolo said:
Dave, I think you are on to something. AFP has a good writeup of what happens on Bendix FI when you shutdown and try to start up a hot engine. It is a selling point for their purge valve, which is nothing but a switchable return line used for 15-20 seconds. Experimental aircraft only.

I can think of one or two minor issues, but the EFI fuel system has none of the engine heated components of the Bendix. We have the dual benefits of a gravity fed fuel pump inlet, and double the operating pressure of Bendix, after the pump. I can't think of a reason not to do it. A return line is theoretically better. Like we used to say in the military, "If the minimum wasn't good enough, it wouldn't be the minimum." Bendix FI return-less seems to be working perfectly with the BH fuel system. Ours should as well.

Possible issues is the fuel pump gets too hot, and it will be harder to prime the system on either a first start up, and if you ever run the tanks dry. I think the hot pump might only be an issue on the ground, at idle. Once you are burning 7-15 gph, there should be enough cool fuel going through the pump to keep the pump cool.
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There is a lot of comparison between the bendix system system, the SDS system, a returnless system on a car, and the system proposed, but they are not the same, and saying that it works for bendix so it should for us is dangerous. Here is why:

1. The bendix system is a true returnless system. The fuel isn't bled off somewhere and returned to another part of the system. Thus, the entire fuel system past the pump is pressurized. What is being proposed here is not a returnless system, it's a return system that bleeds back to before the pump. The concern is that fuel will get really hot being cycled around, then that hot fuel will be fed to a very low-pressure point right before the pump which could cause cavitation.

2. The SDS system is a true return system that returns to the tank. Given the volume of the tank, it won't have hot fuel at the inlet of the pump.

3. The returnless system in a car is a true returnless system where fuel isn't fed back to a pump. Instead, the pump is driven by a pump controller that cycles or throttles the pump based on a fuel pressure sensor. Lots of people in the high-performance spaces convert to a return fuel system because a bleed off regulator is generally more consistent than the feedback loop of a pump sensor and controller which will always have some lag.

If you both do build this, then please:

1. Test to see how hot fuel gets running just the pump on the ground for a few hours.

2. Test to see how hot the fuel can get before the pump cavitates.

3. Test to see if lost pressure results in boiling fuel, and if so, how long does it take to hot restart. If it's not instant then consider what could happen in air.

4. First flights on 100LL.

5. Introducing junk fuel (that's pretty much the only reason to do this) at altitude only and from a dedicated tank for testing. IE, run 100LL in the left tank and mogas in the right, and only a few mistakes high until you absolutely know what's going to happen.

I can think of more tests, but you you get the idea. You need to totally understand what is going to happen and be ready for the engine to quit.

schu
 
Let’s try not to misrepresent what’s been discussed, that’s happened too much already. We understand they’re not the same, obviously, the comparisons only relate to the aspects of the systems that are the same and how ours is an improvement from a heat/vaporization standpoint. You can see where we’ve been discussing the drawbacks of having a return, like recirculating trapped/sucked gasses, which I’ve always understood wasn’t on the Bendix systems.

Tests will be performed commensurate with an understanding of the pressures and properties of the fluids involved. “Junk fuel” means nothing from a design standpoint.
 
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I'm not misrepresenting what is being discussed.

Svyolo writes:

We have the dual benefits of a gravity fed fuel pump inlet, and double the operating pressure of Bendix, after the pump. I can't think of a reason not to do it.
Click to expand...

This is a comparison of the benefits of the bendix, with the "after the pump" modifier, with the conclusion that there is no reason not to, but that "after the pump stuff" makes them completely different systems.

At any rate, I think I've "helped" enough... back to building my own airplane.
 
Context matters. If you saw that as a statement the systems are identical so we don't need to worry then you're not paying attention. A common theme in this thread...
 
I don't see the high wing thing as a material advantage, yes it's better than suction but only the pressure difference is not material compared to the massive pressure your proposed recirc-SDS system needs to operate - I understand they need about 40 to 50 psi?

Some relevant facts:
1. Bendix FI runs continuously at just 3 or 4 psi.
2. Gravity feed in a Bearhawk will not run a Bendix, not even close, thus gravity head post losses is much less than 3 psi.
 
Yeah it’s only material on the pump inlet side with the concern of “heated” recirc fuel being subjected to negative relative pressures. ”Heated” because with proper layout heat from the engine isn’t allowed back, hence the focus on how much the pump will heat fuel itself, which does need to be quantified. SDS specs 45-50 psi on an adjustable regulator. Minimum static head pressure in a 4-Place was like 1.15 psi back of the napkin.
 
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.
 
Battson said:
I don't see the high wing thing as a material advantage, yes it's better than suction but only the pressure difference is not material compared to the massive pressure your proposed recirc-SDS system needs to operate - I understand they need about 40 to 50 psi?

Some relevant facts:
1. Bendix FI runs continuously at just 3 or 4 psi.
2. Gravity feed in a Bearhawk will not run a Bendix, not even close, thus gravity head post losses is much less than 3 psi.
Click to expand...

I believe EFII's and AFP's boost pumps put out 30 psi, non adjustable (no MAP reference, ATMO only) I thought they both claimed that is about 5 psi greater the the engine pump. Does the servo drop the pressure to 3 or 4? I was trying to figure out why the divider was so complicated. I never found an explanation, but it looks like the valve with the spring and diagram might also act to smooth out pressure variations. Just a guess.

The 1 psi of gravity pressure won't run the system, but as long as it can gravity feed faster than the pump is feeding, it should keep a slight positive pressure at the inlet. That is much better than several PSI of suction at the face of the pump. Much less chance of vapors coming out of suspension. I didn't realize why SDS and EFI don't recommend gascolators. I believe it is because they have a small pressure drop, and create some turbulence internally, both of which can cause vapors to form. But that is on a suction time system, like an RV. With a gravity fed system, it shouldn't be an issue, or at least much less of one. If I don't use my 2 liter header tank(with a drain at the low point), I will use the kit gascolator.

Gravity isn't much pressure, but it is a lot better than a bunch of suction.

90's fuel injection it was common to run the system at 30-35 psi. That is what EFII uses on their system now, as they use a fuel rail with a full return to the mains or header tank. SDS went away from that and use a small manifold that sits where the Bendix divider sits, on top of the engine. It is a manifold only, no moving parts. Fuel is returned from there, so from that manifold to the injectors, is return-less. So the place where the fuel is hottest on shutdown, will be at the inlet to the injectors. Because of this, SDS recommends their system be run at 45-50 psi. They said the extra pressure is because they already run return-less to the injectors. Helps with hot starts. A lot of newer cars run even higher pressure for the same reason.



As far as testing, the easiest test and worse case, is just turn a fuel pump on with the engine off. 100% of the fuel will be returned, continuously. Keep monitoring the temp of the pump and fuel. At cruise rpm and fuel flow, 50-80% of the fuel is still returned, so some of the same fuel will go through the pump 2-5 times. To me this is a cause for concern, but AFP rates their boost pump, which does the same exact thing, for continuous operation. My pumps will be mounted on an aluminum plate, maybe that will help to act as a heat sink/radiator. We will see.
 
I still have a Bob rebuilt carb that I bought, pickled and wrapped in plastic. I bought it in case I couldn't figure out a fuel system layout for the FI that I liked. It has to work the the stock BH feed.
 
I read about half of AFP's Intallation Manual from their website. Under Documents. The first half of it is much less about their system, but aircraft fuel systems, Bendix FI, and the different ways to run and plumb FI. Things that work, and things to avoid. It is a wealth of knowledge. There is even a decent section on plumbing your air intake, and pitfalls to avoid there. It was definitely worth a look for me.
 
After looking at the gascolator I absolutely wouldn't use one with a full return. Pressure loss increases exponentially with flow, so 45GPH through that would be a non starter to me. For the reasons you state SDS says, if you have to, put a gascolator on the pressure side, still not ideal imo
 
svyolo said:
...
1 psi of gravity pressure won't run the system, but as long as it can gravity feed faster than the pump is feeding, it should keep a slight positive pressure at the inlet. That is much better than several PSI of suction at the face of the pump
....
Gravity isn't much pressure, but it is a lot better than a bunch of suction.
Click to expand...

If we assume a metre of head generates +0.15psi in the fuel system, then one might assume -300mm of head in a low wing would relate to -0.05psi [suction] assuming a linear relationship and all other things being equal.

That was my thinking when I said the difference is probably immaterial compared to 45psi required by the system.

Yes it does help, however it might be an extra ~0.5%. A lot better might be overstating it.
 
In a static system, I agree. But when you turn the pump on, with the low wing you are sucking fuel. I will look for the calculation how much suction to pump 32 gph through a 3/8 line. I don't know but I would think a few psi. High wing, if you can gravity feed faster than the pump is taking in fuel, you shouldn't have that suction. Unless I am missing something.
 
But the both systems have the same dynamic pressure loss through 3/8 lines, so the different between dynamic and static is common to both assuming both systems are the same up to the point of fuel supply... In reality, the more complex piping in the Bearhawk could add to the pressure loss, or not if you chose a low-wing with complex plumbing. You would need to test it to know for sure. But it's surely an insignificant difference anyway, so it's a moot point I guess.
 
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Yeah I think you are right. The BH lines are a little longer than the RV. But the BH has an extra T over the RV's, but I am guessing at the RV fuel system. I couldn't find an equation, but found a really detailed on line calculator. I had to estimate the losses from a gascolator and fuel valve and I estimated them as having the same loss as T's. I am not sure how accurate that is, but for comparison purposes it doesn't matter because the RV and BH both have them.

Feeding 25 gph through 3/8" versatile (.305" ID) was around 2 psi loss. Bumping up the flow to 45 gph (SDS with full return) it jumps to over 5 psi. 1/2" lines drop that down to way under 1 psi, but that assumes that the gascolator and fuel valve are a size up as well.
 
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That calculator had a lot of assumptions. Based on the BH fuel flow tests I kind of think the real measured loss is a bit under 1 psi. Not a huge amount if you are running Avgas all the time. but I never really thought about the vapor pressures of fuel and how that related to atmospheric, and pumping. I have been out of this side of aviating for a long time, and didn't do much of it, when I did it.
Avgas is almost twice the price of Mogas here. The vapor margins for running Mogas are thin. Flying high or operating from high airports in the summer you might not have any margin, or worse. I guess if you are that close, you shouldn't run Mogas.
 
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