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Anatomy of a High-Pressure Electric Fuel Pump.

Hartstoc

Well Known Member
I’m in the process of installing this fully redundant dual fuel pump assembly with independent external check and relief valves that I’m assembling using exquisite parts obtained from Don Rivera at Airflow Performance.
2v2EQpSVGxBELK5.jpg


Return lines not shown here will carry bypass fuel from the relief valves back to the tank of origin through a two-stage selector valve, and power will be provided by a redundant electrical system including two powerful EarthX batteries. The engine-driven fuel pump will be eliminated completely. I’ll describe this design and the reasoning behind it in a future thread, but here I want to share my exploration of the electric pump’s innards.

I will be depending upon these sealed, non-serviceable pumps full-time. Even though they are rated for a 100% duty-cycle, I thought It would be a good idea to look inside one. There are many variant automotive applications for these pumps with assorted inlet/outlet configurations and applications, and I was able to obtain a nearly identical new one on eBay at a good price for the purpose of an “autopsy”. This photo shows the innards of that pump spread out , and a detailed photo album with captions appended to each photo can be viewed using the link below it:
2v2EQpV4NxBELK5.jpg

(Edit note:the dismantled pump is an airtex E8228. It is externally very similar to those supplied by AP. Don Rivera indicated to me that they no longer use Airtex pumps, but that the Airtex equivalent to the ones they do use would be P/N E-2315. The pumps in the top photo are Delphi FD0011’s, rated at 106PSI. I wanted to keep the thread generic because, from what I’ve seen in various parts diagrams, most are pretty similar and the same concerns would apply to all roller-vane pumps.)

https://public.fotki.com/Hartstoc/anatomy-of-a-fuel-pump/?view=roll#1

The experience of dismantling this pump was both reassuring and a bit frightening. Quality, materials and construction of all components is quite impressive and supports the 100% duty-cycle rating, but the tortured pathway taken by fuel as it passes through this pump really got my attention. The pump is completely flooded internally with, and lubricated by, the fuel passing through it. The motor’s brushes, contacts and rotating armature(which beats the living bejesus out of the passing fuel) are all fully submerged. Most of these pumps were initially designed to also be entirely submerged full time inside gas tanks, and have been variously adapted to external installations. When submerged inside a tank, much of the pump’s heat can migrate out directly through the sidewalls, but when mounted externally nearly all heat must be carried away by fuel passing through the pump itself. The heat involved here is substantial, as nearly all energy from the 4-8 Amps(at 12V) consumed continuously ends up as heat imparted directly into the passing fuel.

All of these sealed roller-vane type pumps are constant-displacement, meaning that they must be allowed to pump a fixed flow of 30-50GPH or so, depending on their size.

Based upon what I saw, I’ve drawn two personal conclusions. These are my own personal opinions and all are welcome to offer theirs in reply. I know the second one will be controversial, so I leave it to each reader to decide if what I have to say rings true:

1- Running a tank dry while operating on one of these pumps could be a really bad idea for a couple of reasons. They are not very good at self- priming and cannot pump vapor effectively. A fair amount of pressure may be required get fuel flowing through system components like the flow divider and to the injectors. I’m planning on installing a manual bypass of the relief valve circuit to allow free flow back through the return lines in the event that the pump needs to purge air and prime itself should a tank be run dry inadvertently. Another possible concern is internal arcing and detonation. Though impossible with the pump completely full of fuel(no oxygen), it is conceivable that this could happen at least briefly within a pump that has been run dry and becomes filled with an explosive air-fuel mixture.

2- I’ve concluded that these pumps should never be installed in aircraft unless full-sized fuel return lines are provided to route bypass flow, which is always a far greater volume than throughput flow to the engine, back to the tank from which the fuel is drawn so as to allow it to cool and to harmlessly dissipate any vapor that may have formed. Of particular danger, wether used as a backup boost pump or as the sole source of fuel pressure, are pumps of this type equipped with short-loop recirculation of bypass fuel back into the pump inlet, be it internal or external.

Consider that this recirculated fuel is being severely agitated, heated, pressurized, and then passed through a relief valve where the pressure drops to near zero each and every time it makes a circuit around this little loop. If the pump is flowing 35GPH and the fuel burn is 7GPH, then 80% of the total flow is essentially locked into the recirculation loop.

If the fuel supply is cold, the throughput of 7GPH may well be capable of stabilizing full system temperature at a level below the fuel vapor point at the relief valve exit. On the other hand, if the incoming fuel is warm, and perhaps the flow has been reduced for a descent, the temperature of the recirculating fuel will rise, and a tipping point will eventually be reached at which the hot fuel will generate a lot of vapor when exiting the relief valve. This vapor is not easy to re-liquify, and these pumps do not pump it effectively, so in short order this positive feedback loop will impede flow to the engine, and the pump will enter into an ever-hotter, destructive, vapor-locked condition that can only be remedied by shutting it down and waiting a very long time for it to cool down.

Some may wonder why, if I’m right, the combination of high pressure engine driven pumps with short-loop recirculating electric pumps as backup/boost pumps has not caused big problems in the hundreds, maybe thousands, of aircraft so equipped. I think it is partly due to the very low failure rate of the engine driven pumps, and typically very limited use of the boost pumps, mostly at very high throughput flows during takeoff and climb. This cannot be taken as proof that such installations are inherently safe, only that few pilots ever venture into their danger zones.

As an aside, all of this also suggests that if you are flying with an electric backup/boost pump having short-loop recirculation, and you find yourself using it as backup in an emergency scenario, you should not “baby” the plane, but rather fly full-throttle, full rich to the nearest safe landing area to maximize throughput to the engine and insure that the pump stays cool.- Otis
(Edit note: As often happens tapping into VAF “group wisdom”, comments later in this thread have led me to a much better pump than the one pictured here- details can be found below)
 
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I think you raise legitimate questions, and you're not the only one who's raised them. But have you tested to see if the data backs up your concerns? If not, have you asked Don at AFP whether they have done the testing? If not, that might be a good place to start. I asked him very similar questions several years ago, and he was very accommodating in answering my questions.

Charlie
 
Hmm;
That's a lot to digest.
I ran my RV-8 with EFI (Tracy Crook EC-2T) on a Mazda Turbo Rotary.
When I converted to Lycoming IO-360, I used Airflow Performance mechanical Bendix style injection but retained the dual automotive high pressure fuel pumps and the original Mazda fuel return regulator. No engine driven fuel pump.
I fully endorse the use of full size (-6) fuel return plumbing, all the way to the tank, which is the greatest available heat sink. It can remove heat from the fuel that passed through the fuel pump & engine compartment.
My automotive fuel pumps are rated for frame mount, not inside tank mount. The system runs about 38 PSI or a little more. It frequently alarms my Dynon EMS for high pressure as the alarm point can't be set higher?
I have worn out one pump in about 100 hours. It was a NAPA part, no longer available. I had to source a substitute. Priming the new pump, or either pump after draining the tanks for W&B has never presented a problem.
I have not yet run a fuel tank dry in flight. I don't know how to simulate that on the ground. Windmilling..switching tanks...waiting...
I very much like what Don Rivera is doing, but I feel the fuel discharge line should pass very close to the Injector Servo before returning thru the turn down regulator and then to a fuel tank. That's probably just me. I have not heard of any incidents involving his dual fuel pump setup.
I have considered removing one electric fuel pump and replacing it with a Romec pump driven off the unused vacuum pump pad. I would still want to retain the turn down regulator system, I think it's the best.
I think the manual bypass purge valve idea has merit, but in my experience, any air in the fuel system is quickly pushed through the fuel servo, especially when the mixture and throttle are full forward.
 
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These are very common automotive pumps. One of the manufacturers of these pumps is just a few miles from me. AFP used to use Carter pumps, these look like the Delphi version. They are very reliable pumps.

You can eliminate the selector by having one set of pumps for each tank, in series (primary and backup total of four pumps). You would still just use the two check valves and relief valves. This is what I have on my Rocket, but low pressure Facet pumps with built-in check valves that don't require relief. Both left/right pumps on always, which means not having to mess with left/right balancing. You have the option to do that by switching off pumps.
 
Excellent write up Otis, those little pumps contain a lot of detailed engineering.

It's good to think about how things work and look at them, and take them apart. Then there is experimental testing to find real limits that are either closer than we thought or non-existent.

Dr. Charles Kettering wrote a book in 1949 "Get off Route 25 Young Man" He had 185 patents, and pretty sure he did not get them as a patent troll.

Basically, he is saying that one must test the limits to really know the limits. Any assumption short of that denies [society of] advancement. He did not invent that concept, it was the basis for the success of Wilbur and Orville as well.
 
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Good report. I've been dealing with these types of pumps for about 35 years as they come from the automotive EFI world.

I believe the pump in the photo is Airtex, a brand where I've heard of and seen a number of failures at pretty low time, even when used for their intended purpose (EFI) with high return flow back to the tank. Not a big fan of these.

I've also got a lot of experience with Bosch which are very reliable as long as they never run dry and are well filtered. However these also don't prime very well like the Airtex and are expensive. I have seen crusty, 30 year old ones on D-Jet Volvos in the junkyard with 400,000+ miles on them- call that around 10,000 hours. Running them dry for 30 seconds or feeding them a lot of aerated fuel is enough to send them more rapidly to an early grave.

For low cost and reliability, I've found the Walbro GSL series are the best. They prime well even when well above the fuel level and are very reliable. I had one on our shop car for 19 years and roughly 5000 hours and we've sold hundreds of them over the last 15 years. Only seen one failure recently and I haven't had time to dissect it yet. These have a run dry spec of 90 seconds I believe but that can't be good for any roller vane design.

As discussed, feeding aerated or hot return fuel back to the inlet of the pump is a bad idea. They were never designed for this and you can expect a large reduction in lifespan.
 
Scott makes a good point about the size of the return line. And since actual data is a good thing, a friend who's a retired research scientist (he tests *everything*), measured the pressure in a ~5 foot length of -4 line fed by a Walbro GSL393 and open on the other end. IIRC, it was well above 5 psi. Since far more fuel is returned than consumed, a restriction in the return line is going to affect actual flow to the engine, varying with consumption (likely getting richer as you reduce power). As an aside, that data point stopped me from using Walbros as transfer pumps. If I were to overfill the destination tank and fuel started flowing in the vent, it would destroy the tank.

To Bob,
I don't know if you mis-typed, but these pumps are nothing like the design of the Facet cube pumps. In my opinion, putting a pair of these positive displacement pumps in series would be a very bad idea. I've got an Airtex (likely the identical pump shown in the teardown) and a couple of Walbro GSL393s in front of me as I type this. The Airtex (roller-vane), I can barely suck any air through with my lungs, and can barely blow through in the flow direction. The Walbro (which according to Walbro is a gerotor (think oil pump) design), might as well be a closed valve. I have serious doubts that you could push fuel through one if it's off, and trying to suck through either design would be instant vapor lock.

Ross,
The pump that Real World solutions sold for many years was the Walbro GSL393 (155-160 lph). When I clicked on a few of my saved web links to Walbro info, I discovered that they'd completely restructured their website (not unusual), but the GSL393 has disappeared from their product catalog, with the smallest gerotor listed as the 190 lph.
https://aftermarket.tiautomotive.com/universal-pumps/#inline
But you can still find them for sale from various sources. However, I did see something I've never noticed before, the '7.00228.51 inline screw pump'. Any idea about the nature of that animal?

Charlie
 
To Bob,
I don't know if you mis-typed, but these pumps are nothing like the design of the Facet cube pumps. In my opinion, putting a pair of these positive displacement pumps in series would be a very bad idea.
Ross,

You are correct I had forgotten that these pumps are not in the flow path when off. Not the case with Facet pumps. One could easily work around this with an additional check valve.
 
Lots of good data here, and I'm running a system almost exactly as Otis describes, with good results. I recently upgraded pumps from the dual AFP pumps with some home-grown plumbing to the SDS dual-pump module with Andair duplex valve. This gives me two pumps in parallel for backup, with returns direct to the source tank (yes full size #6 - excellent point on that) and the SDS pressure regulator to hold 40 psig on the Bendix RSA5 and return all excess fuel not needed back to the tank. I've got an AFP purge valve on the injector divider to purge the system with cool fuel for hot-starts, and that works great.

Additional changes that I've done specific to running 91 automotive premium with ethanol blend - I have double-insulated all fuel lines FWF with firesleeve and replaced the injector orifices with .022" restrictors. I've got over 300 hours trouble-free with the setup now and really enjoy paying $2 to $2.25 for my fuel.
 
One thing not mentioned with this style pump is the threads on the inlets/outlets. Its a metric thread similar in size to the familiar 1/8 NPT. Seems to be fine for well supported fuel lines and banjo fittings, but when I see a whole assembly of check valves, filters and other hardware hanging off one end it gives me pause. I started to build a dual pump module and I just couldn't reconcile the problem of mounting the assembly in the airplane without introducing stresses on the fittings. Thats when I gave up and bought the SDS pump module- the pumps float on O rings within the module chassis and are isolated from tension, compression and bending loads. It makes mounting a "no brainer", and is an example of the thoughtful design work typical of the SDS brand.

With the pump module shown in the OP I'd be VERY careful about mounting. LOTS of opportunity to introduce a lot of bending load at some very small, threaded fittings.
 
Good report. I've been dealing with these types of pumps for about 35 years as they come from the automotive EFI world.

I believe the pump in the photo is Airtex, a brand where I've heard of and seen a number of failures at pretty low time, even when used for their intended purpose (EFI) with high return flow back to the tank. Not a big fan of these.

I've also got a lot of experience with Bosch which are very reliable as long as they never run dry and are well filtered. However these also don't prime very well like the Airtex and are expensive. I have seen crusty, 30 year old ones on D-Jet Volvos in the junkyard with 400,000+ miles on them- call that around 10,000 hours. Running them dry for 30 seconds or feeding them a lot of aerated fuel is enough to send them more rapidly to an early grave.

For low cost and reliability, I've found the Walbro GSL series are the best. They prime well even when well above the fuel level and are very reliable. I had one on our shop car for 19 years and roughly 5000 hours and we've sold hundreds of them over the last 15 years. Only seen one failure recently and I haven't had time to dissect it yet. These have a run dry spec of 90 seconds I believe but that can't be good for any roller vane design.

As discussed, feeding aerated or hot return fuel back to the inlet of the pump is a bad idea. They were never designed for this and you can expect a large reduction in lifespan.

You are correct, the dismantled pump is an airtex E8228. It is externally very similar to those supplied by AP. Don Rivera indicated to me that they no longer use Airtex pumps, but that the Airtex equivalent to the ones they do use would be P/N E-2315. I wanted to keep the thread generic because, from what I’ve seen in various parts diagrams, most are pretty similar and the same concerns would apply to all rolling vane pumps. (I’ve edited this into the original post)

My concern about run dry goes beyond possible damage to the pump itself. It is probably unlikely but is based uopn this very “devil’s advocate” Scenario: : I was imagining a perfect storm, in which tank A is run dry and switch made to tank B, which maybe even gravity feeds to the pump inlet. Fuel pushes air in line from valve into the inlet of operating pump, then reaches the impellor, which is at the inlet end- air/fuel within the empty pump and manifold as far as the relief valves is compressed to 25psi before relief valve opens, and sparks are arcing off the motor brushes continuously, and these are located at the outlet end of the pump. The detonation ruptures fuel line ahead of relief valve and pump continues to feed a nasty fire in the cabin. This might well be impossible, but I’m just going to make it a policy to avoid running tanks dry, and also plan to be sure to switch tanks and flip on on the second pump immediately any time the engine sputters.- Otis
 
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The detonation ruptures fuel line ahead of relief valve and pump continues to feed a nasty fire in the cabin. This might well be impossible, but I?m just going to make it a policy to avoid running tanks dry,

You do realize the unwashed run their cars dry from time to time?
 
My concern about run dry goes beyond possible damage to the pump itself. It is probably unlikely but is based uopn this very “devil’s advocate” Scenario: : I was imagining a perfect storm, in which tank A is run dry and switch made to tank B, which maybe even gravity feeds to the pump inlet. Fuel pushes air in line from valve into the inlet of operating pump, then reaches the impellor, which is at the inlet end- air/fuel within the empty pump and manifold as far as the relief valves is compressed to 25psi before relief valve opens, and sparks are arcing off the motor brushes continuously, and these are located at the outlet end of the pump. The detonation ruptures fuel line ahead of relief valve and pump continues to feed a nasty fire in the cabin. This might well be impossible, but I’m just going to make it a policy to avoid running tanks dry, and also plan to be sure to switch tanks and flip on on the second pump immediately any time the engine sputters.- Otis

The sparks are all generated before the inlet to the geo rotor chamber and therefore occur in an area of no pressure or a slight vacuum. This obviously reduces ignition potential. If you view this NASA study https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20000053468.pdf, you'll see that they don't even bother to test energy at voltage levels below 100 volts. We learned over 100 years ago that you need a lot of volts to produce a spark with enough energy to ignite fuel vapor. Most 4 cycle engines use ignition voltages measured in 10's of 1,000's of volts and that is to ignite a vapor that is highly compressed (ignitability increases with pressure, up to point of self-combustion). The primary concerns are from static electricity, which is usually 1000's or 10's of 1000's of volts. All the stories you hear about fuel vapor ignition are typically related to static electicity, like that created from fluid's friction with plastics.

While I have done no research, I struggle to see how sparks from a 12 volt motor could ignite fuel vapor, even if that vapor ended up being near an optimal fuel air ratio. The fact that 10's of millions of these style pumps have been installed in autos and assuming that more than a few people have run their tanks dry without starting on fire, would seem to backup that assumption. As you mentioned, any time air is present, there is no meaningful pressure production and therefore no fuel is recirculating in the AFP style pumps. Therefore, the use case in autos is the same when in this state.

Your logic here is interesting, but not sure I concur with the threat concern. I post a counter argument mostly so that other pilots don't develop a fear of using their boost pump if they run a tank dry after reading your post.

Larry
 
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I think you raise legitimate questions, and you're not the only one who's raised them. But have you tested to see if the data backs up your concerns? If not, have you asked Don at AFP whether they have done the testing? If not, that might be a good place to start. I asked him very similar questions several years ago, and he was very accommodating in answering my questions.

Charlie

Charlie- I could not agree more about the importance of scientifically valid testing- this was the essence of much of the work of the CAFE Foundation during my tenure there. At the same time, I?ve pretty much convinced myself that short-loop fuel recirculation back to the pump inlet is a bad idea, so elected to pursure the return line option. I?d be interested in any testing done by others, but it is pretty obvious that, with any given set of operational perameters, there will be a critical minimum fuel throughput beyond which fuel vapor will be generated within the loop.

Btw- I?ve come up with a way to install quality return fittings into existing tanks without removal of tanks or wings, and that prevents the introduction of aluminum shavings into the tank. I?ll be documenting that here in due course.- Otis
 
One thing not mentioned with this style pump is the threads on the inlets/outlets. Its a metric thread similar in size to the familiar 1/8 NPT. Seems to be fine for well supported fuel lines and banjo fittings, but when I see a whole assembly of check valves, filters and other hardware hanging off one end it gives me pause. I started to build a dual pump module and I just couldn't reconcile the problem of mounting the assembly in the airplane without introducing stresses on the fittings. Thats when I gave up and bought the SDS pump module- the pumps float on O rings within the module chassis and are isolated from tension, compression and bending loads. It makes mounting a "no brainer", and is an example of the thoughtful design work typical of the SDS brand.

With the pump module shown in the OP I'd be VERY careful about mounting. LOTS of opportunity to introduce a lot of bending load at some very small, threaded fittings.

Your points are well taken- the pump photo is just a tabletop put-together to illustrate the concept, and the actual installation(which I?ll document here later) will be an ultra-stable installation. - Otis
 
The sparks are all generated before the inlet to the geo rotor chamber and therefore occur in an area of no pressure or a slight vacuum. This obviously reduces ignition potential. If you view this NASA study https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20000053468.pdf, you'll see that they don't even bother to test energy at voltage levels below 100 volts. We learned over 100 years ago that you need a lot of volts to produce a spark with enough energy to ignite fuel vapor. Most 4 cycle engines use ignition voltages measured in 10's of 1,000's of volts and that is to ignite a vapor that is highly compressed (ignitability increases with pressure, up to point of self-combustion). The primary concerns are from static electricity, which is usually 1000's or 10's of 1000's of volts. All the stories you hear about fuel vapor ignition are typically related to static electicity, like that created from fluid's friction with plastics.

While I have done no research, I struggle to see how sparks from a 12 volt motor could ignite fuel vapor, even if that vapor ended up being near an optimal fuel air ratio. The fact that 10's of millions of these style pumps have been installed in autos and assuming that more than a few people have run their tanks dry without starting on fire, would seem to backup that assumption. As you mentioned, any time air is present, there is no meaningful pressure production and therefore no fuel is recirculating in the AFP style pumps. Therefore, the use case in autos is the same when in this state.

Your logic here is interesting, but not sure I concur with the threat concern. I post a counter argument mostly so that other pilots don't develop a fear of using their boost pump if they run a tank dry after reading your post.

Larry

Thanks for this, Larry- no argument! I did suggest that the scenario I described might be impossible- my purpose here was to toss out every devil?s advocate possibility I could imagine and I hope that was clear.I won?t be losing any sleep about this particular issue.- Otis
 
Otis, if you want to redesign for an all-electric aircraft fuel system, make the jump to the 21st century. Investigate brushless fuel pumps. No brush wear, less heating, higher efficiency, and your explosion concern (valid or not) goes to zero.

While there, look at PWM control. No re-circulation, reduced power demand, and if the set points are adjustable, a dual controller setup would be auto-switching...one pump running at a time.
 
Excellent write up Otis, those little pumps contain a lot of detailed engineering.

It's good to think about how things work and look at them, and take them apart. Then there is experimental testing to find real limits that are either closer than we thought or non-existent.

Dr. Charles Kettering wrote a book in 1949 "Get off Route 25 Young Man" He had 185 patents, and pretty sure he did not get them as a patent troll.

Basically, he is saying that one must test the limits to really know the limits. Any assumption short of that denies [society of] advancement. He did not invent that concept, it was the basis for the success of Wilbur and Orville as well.

Bill- Thank you-I could not agree more! My initial post cosists entirely of questions generated intuitively after examining the innards of this pump. Thourough testing of the limits of any real-world installation are certainly called for.- Otis
 
Otis, if you want to redesign for an all-electric aircraft fuel system, make the jump to the 21st century. Investigate brushless fuel pumps. No brush wear, less heating, higher efficiency, and your explosion concern (valid or not) goes to zero.

While there, look at PWM control. No re-circulation, reduced power demand, and if the set points are adjustable, a dual controller setup would be auto-switching...one pump running at a time.

If cost is a factor one can just control brushed pumps with PWM. At least the relief valves could be eliminated, and power draw would be minimal. Using something like an Arduino a liquid sensor could be integrated to protect the pump when dry.
 
Otis, if you want to redesign for an all-electric aircraft fuel system, make the jump to the 21st century. Investigate brushless fuel pumps. No brush wear, less heating, higher efficiency, and your explosion concern (valid or not) goes to zero.

While there, look at PWM control. No re-circulation, reduced power demand, and if the set points are adjustable, a dual controller setup would be auto-switching...one pump running at a time.

Dan, are you thinking returnless system? I remember seeing them on Ford Explorers starting around 1998 or so (there were/are probably lots of others, those are just the ones I came across) when I was scavenging Ford EFI parts for ground-bound projects. Presumably they were controlled by the ECU with pressure or current feedback, but I never investigated. Why not something like that for aircraft? Sure would simplify the plumbing and cost compared to adding a return line. Though in exchange for added complexity due to the need for a controller instead of a dumb switch and pressure regulator.

As an aside, an engineer at the California Air Resources Board told me that one of the big reasons for developing returnless systems was to decrease heating of the fuel, which thus reduced evaporative emissions.

I just looked at some BDC rotary fuel pumps. Not cheap...
 
A few more things indicated by my research (I always say to go ask for yourself):

Thread/fitting security: the inlet end of the Airtex is plastic, though it is pretty large. The Walbro gerotor pumps are all-metal, and it's difficult to believe that you couldn't hang the entire aluminum fuel network off one and risk compromising the pump itself.

Short loop return flow: Again, rather than trust 'instinct', why not either test, or ask the guys who actually did do some testing? I asked Don at AFP, and he gave me an answer that satisfied me, along with pointing out that certified Weldon boost pumps have a shorter return path than the AFP (or any of the other experimental offerings) and have been used safely in certified a/c for decades.

'Air-locking' a pump: I could see it happening with the Airtex; the one I have bleeds a fair amount of air with enough pressure on it. But the Walbro gerotor pumps I have are truly positive displacement pumps. It's hard for me to imagine them failing to self prime, especially if run with Bendix style dribble injectors. I *could* imagine one airlocked if it's pumping against a closed regulator & the return loop is completely filled with air. *But I have not tested either situation.*

Running one dry: It's happened on occasion to some of the rotary guys (electronic injection, Walbro pumps), and it does seem to have a significant effect on shortening pump life with the Walbros. Not that they commit instant suicide; just shortens life a lot.

PWM: Again, go to the source and ask! I wanted to do it with Walbro gerotor pumps, so I called their tech line and asked. The answer I got was, best not done with the gerotor pumps. And they gave me the reason: Raw PWM is a series of voltage spikes, and can cause the gears to be effectively hammered to eventual failure. We did *not* discuss using filtered PWM which should result in a varying DC voltage driving the pump. I don't have the time/energy to test that; easier for me to go with what's known to work well. They *did* say PWM is perfectly fine for their turbine pumps (which no one's mentioned yet in this thread, and are used in all the automotive PWM fuel pump systems). BUT: turbine pumps will NOT self prime, and are intended to be immersed in fuel (in-tank). So PWM drives us to turbine pumps (which are a lot cheaper), which drives us to finish the modern auto-style fuel injection system for our a/c (in-tank pumps). :) Again (yet again), go to the source and ask.

Most of what I just wrote comes from what I got 'from the source', but they are my words and my interpretation of what I heard/read. I tried to identify the parts that were just my gut talking, instead of what I heard from a respected authority or real world experience. I do wish we would do more of that, because we seem to discover more and more 'old pilot lore' (meaning what we heard from someone who heard from someone) that is simply wrong. TBO, Lean of Peak operation...etc etc

I'm just another guy you heard something from; go ask the sources. :)

Charlie
 
Otis, if you want to redesign for an all-electric aircraft fuel system, make the jump to the 21st century. Investigate brushless fuel pumps. No brush wear, less heating, higher efficiency, and your explosion concern (valid or not) goes to zero.

While there, look at PWM control. No re-circulation, reduced power demand, and if the set points are adjustable, a dual controller setup would be auto-switching...one pump running at a time.

Thank you, Dan, I will definitely look into this option.-Otis
 
I should probably have emphasizedthat my concern is mainly with roller-vane pumps operated continuously, as mine will be (unless suggestions here lead me down another path).

I certainly would not suggest that anyone flying a typical engine driven pump + roller-vane electric backup should necessarily alter anything, even with short-loop recirculation, only that they consider special management strategies to insure ample throughput when it is operating. Problems are unlikely if the electric pump is used briefly as as booster during high-power operations because of the high throughput. I?m not sure it would be a good idea operate it during extended aerobatics, though. Return lines would be a good idea for that.

Using an electric pump as emergency backup is inherently problematic anyway if you consider the potential failure modes of a high pressure engine driven pump. A rupture of the lower diaphram could simply cause the electric backup to dump fuel overboard(or into the engine compartment if not properly plumbed with a drainline). If the upper diapham had an undetected rupture when the lower one failed, add pumping fuel into the crankcase to that bad day. Failure of the pump drive mechanism of some sort is really the only situation where the electric backup is likely to save the day.

This, and the fact that I?ll have a robust electrical system to support dual lightspeeds, is really what led me to the decision to delete the engine driven pump in the first place.- Otis
 
Just to throw out another option, this is the one I use, the Weldon 8120-G series (certified on a wide range of Piper's) and has been in use for 60+ years according to the Weldon folks.
Rated for continuous duty, built in pressure relief and bypass valves.

923a4a_16d89181bc834c8fb6fd9acc0da43dd4~mv2.jpg
 
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Walt, is this a boost pump or is it capable of delivering enough pressure for fuel injection directly without an engine driven pump?

If F/I capable, mechanical or electronic ----- which I believe requires a lot more pressure than a mechanical F/I.
 
Just to throw out another option, this is the one I use, the Weldon 8120-G series (certified on a wide range of Piper's) and has been in use for 60+ years according to the Weldon folks.
Rated for continuous duty, built in pressure relief and bypass valves.

923a4a_16d89181bc834c8fb6fd9acc0da43dd4~mv2.jpg

That is the pump that I referenced in my earlier post. Note that its regulator, and the bypass flow looped back to its input, are contained in what you see (and most of what you see is the motor). The design has been in use with certified FI engines for decades.

Now if you want to use a pair, you might need to find yourself a 2nd mortgage...
https://www.aircraftspruce.com/catalog/eppages/8120gweldonpump.php

edit: Mike, here are some specs:
https://www.weldonpumps.com/weldon-j-pumps
 
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Walt, is this a boost pump or is it capable of delivering enough pressure for fuel injection directly without an engine driven pump?

If F/I capable, mechanical or electronic ----- which I believe requires a lot more pressure than a mechanical F/I.

I'm not sure what pressure the electronic injection guys run, this will run any of the mechanical FI systems.

From the Weldon website:

Current Applications
•Emergency Fuel Boost
•Priming Injected Engines
•Lubrication (I.E. Pressure Lube Bearings)
•Turbocharger Lubrication Pump

Performance
•Rated Flow: 25 to 50 G.P.H.
•Rated Pressure: 15 to 50 P.S.I.
•Temperature Range: -65°F to +185°F

Current Fluids​​
•Gasoline
•Jet Fuels
•Diesel Fuel
•Oils

Special Features​​
•Positive Displacement
•Cammed Vane
•Self-Priming
•High Suction Lift
•Integral Pressure Relief Valve
•2.5 LBS with Permanent Magnet
•12 or 28 Volt DC Motor
 
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Real data point, albeit not instrumented

As a data point, I have the Airflow Performance pump (as in the OP's post) with the 1998 vintage pressure relief bypass short recirculation loop. This assembly is mounted on the forward side of the spar, just below my right knee. During a prolonged ground op at about 80F OAT of 1.5 hours, I had the boost pump running continuously. From time to time put my hand on the recirculation line to see how warm it got. The line never felt particularly warm. So, whatever heat was generated by the motor windings and the shearing of the fuel was essentially balanced by the heat transfer back out through the pump wall and short lengths of tubing involved, and of course the heat removed by the fuel leaving the circuit (or perhaps most heat was removed from the circuit by the idle burn rate of fuel leaving the circuit).

Keep in mind that for a incompressible fluid there is not any change in the temperature of the fuel as it goes through the throttle (pressure relief valve). Conversely, fluids do not heat up as do gases when pressurized.

It would be interesting to measure the pump electrical current vs flow rates. One would expect it to somewhat track flow rate leaving the circuit.

I think I'll bring my Fluke thermocouple reader up this summer (when the weather isn't 0F...) to measure the temperature rise as idle. I did take measurements this way before by placing a tiny wire thermocouple directly on the fuel line and wrapping in a bit of insulation, but not on this bypass line.
 
Just to throw out another option, this is the one I use, the Weldon 8120-G series (certified on a wide range of Piper's) and has been in use for 60+ years according to the Weldon folks.
Rated for continuous duty, built in pressure relief and bypass valves.

923a4a_16d89181bc834c8fb6fd9acc0da43dd4~mv2.jpg

These Weldon pumps are big, heavy, noisy, expensive and very reliable. A couple folks have used them for EFI pumps in Reno race planes with good results.
 
I think I'll bring my Fluke thermocouple reader up this summer (when the weather isn't 0F...) to measure the temperature rise as idle. I did take measurements this way before by placing a tiny wire thermocouple directly on the fuel line and wrapping in a bit of insulation, but not on this bypass line.

I am interested in the fuel temperature rise from tank to servo discharge, so I am installing some thermocouples in the next few days. 1) at the tube coming into the cabin 2) along the floor tube just forward of the fuel pump, 3) an inch from the bulkhead connector but inside the cabin and 4) a final one inserted under the fire sleeve just before the servo. I got a 4 channel data logger and will be recording each second for a flight. The objective is to see where and how much temperature rise there is in the fuel flow path. I don't think winter temps are much of a factor for that goal. Absolute temperatures can be gathered later in the summer. The TC's are not heavy or bulky and can be left in place.

I can easily leave the pump on at idle and it should yield temperature rise plotted against FF from the G3X. It should make a nice plot over a few minutes if it warms up significantly. Get back to me in in a month and we can have a data party.
 
I am interested in the fuel temperature rise from tank to servo discharge, so I am installing some thermocouples in the next few days. 1) at the tube coming into the cabin 2) along the floor tube just forward of the fuel pump, 3) an inch from the bulkhead connector but inside the cabin and 4) a final one inserted under the fire sleeve just before the servo. I got a 4 channel data logger and will be recording each second for a flight. The objective is to see where and how much temperature rise there is in the fuel flow path. I don't think winter temps are much of a factor for that goal. Absolute temperatures can be gathered later in the summer. The TC's are not heavy or bulky and can be left in place.

I can easily leave the pump on at idle and it should yield temperature rise plotted against FF from the G3X. It should make a nice plot over a few minutes if it warms up significantly. Get back to me in in a month and we can have a data party.

Bill - great stuff! I did something similar, albeit casually and without a recorder. Cabin heat flowing near and around those fuel lines makes a noticeable increase in fuel temps! I only measured a few places in the cockpit fuel lines (RV6A). Looking forward to seeing the real data!
 
Fuel Temperature rise

I am interested in the fuel temperature rise from tank to servo discharge, so I am installing some thermocouples in the next few days. 1) at the tube coming into the cabin 2) along the floor tube just forward of the fuel pump, 3) an inch from the bulkhead connector but inside the cabin and 4) a final one inserted under the fire sleeve just before the servo. I got a 4 channel data logger and will be recording each second for a flight. The objective is to see where and how much temperature rise there is in the fuel flow path. I don't think winter temps are much of a factor for that goal. Absolute temperatures can be gathered later in the summer. The TC's are not heavy or bulky and can be left in place.

I can easily leave the pump on at idle and it should yield temperature rise plotted against FF from the G3X. It should make a nice plot over a few minutes if it warms up significantly. Get back to me in in a month and we can have a data party.

OK, its late, I'm tired, and I'm a little rusty on thermodynamics, but:

In a short-loop recirculation pump setup, the cooling capacity (or heat absorption capacity) is a function of the amount of fuel flow through the pump and on to the engine (ignoring the recirculating fuel).

My airflow performance pump draws ~ 6 amps @ 14V = 84 watts or 84 J/s)

Assume a low flow case of only 7 gal/hr of fuel = 42 lbs/hr = 5.29191 g/s.

One source I found has the specific heat (Cp) of gasoline = 2.22 J/g-K.

dT = 84/(5.29191*2.22) = 7.15 K or 12.87 degrees F temperature rise.

At 14 gal/hr the temperature rise would be half that or 6.43 degrees.

The recirculating fuel is continually cooled by the through flowing fuel. The temperature rise doesn't alarm me all that much. This of course assumes 100% of the pumps power consumption heats the fuel, which of course it does not because some (but not much) of that power is being converted into potential and kinetic energy by pumping the fuel itself. Regardless, the pump isn't going to get all that warm with 7 gal/hr or so flowing through it.

Skylor
 
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Excellent point, Skylor, and most of us with the standard FI system, don't run the boost pump until it is ready for TO and at the descent point for landing. Those will be interesting flight segments for the boost pump rise and total temp rise from tank to servo. I think for the purposes of my endeavor two points are of special interest, at the mechanical pump inlet and after the red cube. As my question is how close the fuel is to vapor pressure at those points. Pressure notwithstanding.
 
Good report. I've been dealing with these types of pumps for about 35 years as they come from the automotive EFI world.

I believe the pump in the photo is Airtex, a brand where I've heard of and seen a number of failures at pretty low time, even when used for their intended purpose (EFI) with high return flow back to the tank. Not a big fan of these.
...
For low cost and reliability, I've found the Walbro GSL series are the best. They prime well even when well above the fuel level and are very reliable. I had one on our shop car for 19 years and roughly 5000 hours and we've sold hundreds of them over the last 15 years. Only seen one failure recently and I haven't had time to dissect it yet. These have a run dry spec of 90 seconds I believe but that can't be good for any roller vane design.

As discussed, feeding aerated or hot return fuel back to the inlet of the pump is a bad idea. They were never designed for this and you can expect a large reduction in lifespan.

A big thank you to Ross- after reading Ross’ post, I found this on the WalbroGSL395
2v2EQTKhhxBELK5.jpg


It is a much better match to a180HP Lyc than the pumps shown in my first entry above, and draws 3A instead of 4.5A, so less heat to worry over, a much lower bypass ratio, and 50% greater range on battery power.

I’m holding my installation to a high standard of redundancy, so really intent upon the two pumps having separate electrical, switching, relief valves , and check valves external to the pump. I’m looking at keeping all of that hardware and adapting this pump to it instead.

Ya’ gotta love VAF!- Otis
 
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Hi Alex,

I've never researched the Airtex numbers, but the link I posted earlier for Walbro has current/pressure/flow numbers in their catalog. Here's a direct link to the catalog; look about 2/3 of the way down.
https://21h1yp1945ct18r0ws184hb2-wpengine.netdna-ssl.com/wp-content/uploads/2017/05/2017-High-Performance-Fuel-Systems-Catalog.pdf

Charlie

Thanks for posting the link. I've wondered what became of Walbro, but being easily distracted, every time I've gone looking I failed to figure it out. I see now they were bought up. FWIW GSL393 pumps are still fairly easy to find, or at least they were last year. I've bought a couple of them.
 
I am interested in the fuel temperature rise from tank to servo discharge, so I am installing some thermocouples in the next few days. 1) at the tube coming into the cabin 2) along the floor tube just forward of the fuel pump, 3) an inch from the bulkhead connector but inside the cabin and 4) a final one inserted under the fire sleeve just before the servo. I got a 4 channel data logger and will be recording each second for a flight. The objective is to see where and how much temperature rise there is in the fuel flow path. I don't think winter temps are much of a factor for that goal. Absolute temperatures can be gathered later in the summer. The TC's are not heavy or bulky and can be left in place.

I can easily leave the pump on at idle and it should yield temperature rise plotted against FF from the G3X. It should make a nice plot over a few minutes if it warms up significantly. Get back to me in in a month and we can have a data party.

Excellent plan, Bill- I?ll look forward to seeing your results. It is noteworty that results will be very installation-specific, and important to include as many extreme condition scenarios as possible, such as hot day, extended operation at very low throughput values, etc.

The biggest installation specific factor will be recirculation ratio, as the more fuel is being recirculated, the more heat will be imparted to the loop.

I?m delighted to have found a pair of 130LPH Walbro?s, which will reduce the recirculation ratio in typical cruise to .5 from .8 , essentially a direct measure of improved system efficiency over the 225LpH pumps I have now. I still plan on installing return lines, though.- Otis
 
Otis- with all that said- have you looked at the SDS pump module? Since these pumps already have built in check valves, all you would need to do is add a single relief valve downstream of the module. Such a solution will likely be less expensive, lighter and more reliable than the scheme you have in your OP.
 
Excellent plan, Bill- I’ll look forward to seeing your results. It is noteworty that results will be very installation-specific, and important to include as many extreme condition scenarios as possible, such as hot day, extended operation at very low throughput values, etc.

The biggest installation specific factor will be recirculation ratio, as the more fuel is being recirculated, the more heat will be imparted to the loop.

I’m delighted to have found a pair of 130LPH Walbro’s, which will reduce the recirculation ratio in typical cruise to .5 from .8 , essentially a direct measure of improved system efficiency over the 225LpH pumps I have now. I still plan on installing return lines, though.- Otis

If concerned about running on battery power (no alternator), I would study that chart a bit more. At 12 volts (assume you are using standard FI) the flow falls off really fast at 25 PSI. Your pattery will be going down from 11.8 to 11 volts during the bulk of it's draw off and I assume the fall off is even worse as the voltage drops. If your pump is 10% off their test data, you may not be able to deliver 10 GPH at 25 PSI at 12 volts. Given that you can't control the relief valve, it will keep trying to make that pressure, even down to 0 flow.

The specs at 13.5 volts are still not pretty. You are only delivering 10 GPH at 25 PSI. That is not a larger margin of safety. What if the relief sticks a bit and it demands 30 PSI? Your flow rate is now well below cruise level requirements.

Larry
 
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Flow & Pressure

If concerned about running on battery power (no alternator), I would study that chart a bit more. At 12 volts (assume you are using standard FI) the flow falls off really fast at 25 PSI. Your pattery will be going down from 11.8 to 11 volts during the bulk of it's draw off and I assume the fall off is even worse as the voltage drops. If your pump is 10% off their test data, you may not be able to deliver 10 GPH at 25 PSI at 12 volts. Given that you can't control the relief valve, it will keep trying to make that pressure, even down to 0 flow.

The specs at 13.5 volts are still not pretty. You are only delivering 10 GPH at 25 PSI. That is not a larger margin of safety. What if the relief sticks a bit and it demands 30 PSI? Your flow rate is now well below cruise level requirements.

Larry

The flip side to that is that the RSA-5 only requires 15 psi and the pumps above can deliver 30 gpm at 15 psi, even at 12 volts.
 
If concerned about running on battery power (no alternator), I would study that chart a bit more. At 12 volts (assume you are using standard FI) the flow falls off really fast at 25 PSI. Your pattery will be going down from 11.8 to 11 volts during the bulk of it's draw off and I assume the fall off is even worse as the voltage drops. If your pump is 10% off their test data, you may not be able to deliver 10 GPH at 25 PSI at 12 volts. Given that you can't control the relief valve, it will keep trying to make that pressure, even down to 0 flow.

The specs at 13.5 volts are still not pretty. You are only delivering 10 GPH at 25 PSI. That is not a larger margin of safety.

Larry

Larry- Your concern is well taken, but my “backup” battery will be a fully charged EarthX EXT900VNT rated at 16AH., and these maintain pretty flat Voltage at 13.3 V or so right to the end. Its twin is also likely to have a substantial charge as well if I notice the flashing light and low-voltage indications from my G3X touch before it discharges too much. The engine will cruise fine at 7GPM delivered as low as 20PSI(Airflow Performance FI). The essential loads will be one fuel pump and one lightspeed.

When everything is working, I will have the option of running both fuel pumps with impunity, and one will be adequate for all cruise conditions with my 180 lyc. so it is advantagous to have the individual fuel pumps be as small as possible for the very reason that it permits sustained battery power operation for the longest duration. Any surplus capacity would simply result in more flow through the relief valve, reducing range. I will routinely take off and climb with both pumps operating. Admitedly, if the engine were even increased to 200HP, I’d have no choice but to go up one size on the pump.

ALL of this is made possible by the vented EarthX, and the pair of them together weigh 5lb less than the one PC680 I have removed, but pack more than twice the capacity and four times the CCA of the 680.. The two batteries are absolutely symmetrical, and either, but never both, can serve as primary or backup at any given moment. Note that each pump has its own external relief valve, and presumably I’ could switch to the other if a sticky valve indicated high pressure..- Otis
 
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A big thank you to Ross- after reading Ross? post, I found this on the WalbroGSL395
2v2EQTKhhxBELK5.jpg


It is a much better match to a180HP Lyc than the pumps shown in my first entry above, and draws 3A instead of 4.5A, so less heat to worry over, a much lower bypass ratio, and 50% greater range on battery power.

I?m holding my installation to a high standard of redundancy, so really intent upon the two pumps having separate electrical, switching, relief valves , and check valves external to the pump. I?m looking at keeping all of that hardware and adapting this pump to it instead.

Ya? gotta love VAF!- Otis

This pump shows only about 22psi at 16 gph - why does that work for an O-360?

Also can someone explain why the 12 vs 13.5 volt graphs cross over in the 20 gph range? Either I'm missing something or that data is wrong.
 
This pump shows only about 22psi at 16 gph - why does that work for an O-360?

Also can someone explain why the 12 vs 13.5 volt graphs cross over in the 20 gph range? Either I'm missing something or that data is wrong.

Alex- A couple of points about why this setup should work for my particular installation. First, note that these pumps are not expensive and there are higher- output Walbros that could easily be swapped out if experience proves me wrong. I?ll be setting up a good stoddard-solvent bench test for it and will do that swap-out before flying if the results are not satisfactory. I?ll also be in a position to answer your second question after this bench-testing(I?ve been wondering about that too! It also seems like 12V Amps should be higher than 13.5V Amps for a given output.).

The objective here is to end up with the most efficient single-pump operation in cruise so as to maximize range after an alternator failure, and to have an abundance of surplus when both pumps are operating. Efficiency is maximized partly by minimizing the rate of flow through the relief valve and back to the tank, as all energy used for that purpose is wasted.

My requirements in cruise range between 5.5 and 9 GPH at a target 24PSI, depending upon altitude, and my AP Bendix-type IO 360 is ok with as little as 20PSI. 7.5GPH at lower altitudes would be a lean, throttled-back economy cruise but still pretty fast and very doable. At higher altitudes I?m a big fan of LOP operation. In short, either of these pumps working alone should easily cover all of these reqirements, and significant but minimal fuel will still be flowing back to the tank.

Thanks to the fuel return lines I will be installing, either or both pumps can be operated with complete impunity any time the charging system is operating normally, and as needed to get out of a pinch even after an alternator failure. I?m pretty sure I have one of the most reliable alternator installations possible, but I should have 16-30AH of reserve battery capacity to work with if/when that happens. My minimum current draw with one pump and one Plasma III lightspeed operating will be under 6A, translating to a duration of between 2.7 and 5 hours with the G5 on integral backup minus a bit for occasional use of the remote com2 on the G3X. That will open up a big circle of real estate to choose from.

Clearly, this pump would not cut it as a good choice for this engine as a stand-alone, nor would it satisfy the needs of a higher pressure EFI system, but I?m betting it will be just right for my installation. I?ll be doing all TO/climb ops with both pumps operating, and the unlikely failure of one pump during such ops would be an event similar to the loss of one engine for a twin, but much easier to manage. Runnup will include veryfying fuel pressure at high power for each pump.

I?ll report back after I?ve done some bench testing on one, and if I reject it I?ll cut it up and show you the innards of that one!- Otis
 
max fuel flow

For what it's worth, I routinely see takeoff fuel flow numbers above 16 gph departing sea level airports (Sacramento Valley) even in hot weather. In cooler weather, above 17 gph. As measured with an EI red cube. My fuel flow numbers are unaffected by electric fuel pump operation. This has been consistent since the engine was new, currently at 740 hours. 180HP IO-360, P/A Silverhawk mechanical injection, constant speed prop.
 
This pump shows only about 22psi at 16 gph - why does that work for an O-360?

Also can someone explain why the 12 vs 13.5 volt graphs cross over in the 20 gph range? Either I'm missing something or that data is wrong.

The way these work is that they strive to achieve the the Pressure commanded by the regulator. You read the chart by looking at the PSI set by the regulator
then observe the flow rate at that PSI (e.g. x GPH @ y PSI). The pump will keep trying to deliver the PSI set by the regulator, all the way down to a 0 flow.

Larry
 
Mike,

The only two things the stock pump has going for it are no-electrics-needed, and self-regulation. Downsides are multiple leak modes (some of which can effectively take out the boost pump with them) and vapor lock issues due to sucking on long supply lines in a hot environment.

Otis,

If you switch to the Walbro, most of the stuff in your original pic could be removed. No need for check valves; the pumps will not pass fuel in either direction when off. I don't see the need for more than one regulator, either, but if it makes you more comfortable, go for it.

And since you're already so far outside the box, a transfer pump & feeding the engine from one tank would mean you could eliminate the selector valve, too (again, the Walbro won't pass fuel unless it's running).

Ain't dominoes fun?

Charlie
 
Methinks you just described an engine driven mechanical pump.

Well, however reliable the engine driven pump may be, I think getting rid of it is a noble goal, and my abundance of battery power, initially triggered by the dual lightspeeds, makes that possible. In addition to the nasty set of potential failure modes described a few posts back, the engine driven pump adds a lot of flammable plumbing and fittings ahead of the firewall, and it could equally well be described as a brialliantly designed fuel heater, especially after shutdown on a hot day when all fuel within it and the nearby lines can vaporrize completely.

Consider the simplicity my firewall forward package. There will be one short firesleeved flexible #6 teflon lined hose connecting a stainless steel bulkhead fitting to the AP fuel metering valve, One longer teflon lined firesleeved #4 hose routed along a low-heat pathway to the flow divider, and the four solid stainless spider lines to the injectors. Four flair fittings total. Vaporization of fuel in these two flexible lines after will probably never happen, and their total internal volume is small, so hot starts should be very begnign. The pressure transducer, the fuel flow transducer, and the pumps themelves will all reside in the cool, low-vibration environment along the cabin floor? Sweet!!- Otis
 
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Mike,

The only two things the stock pump has going for it are no-electrics-needed, and self-regulation. Downsides are multiple leak modes (some of which can effectively take out the boost pump with them) and vapor lock issues due to sucking on long supply lines in a hot environment.

Otis,

If you switch to the Walbro, most of the stuff in your original pic could be removed. No need for check valves; the pumps will not pass fuel in either direction when off. I don't see the need for more than one regulator, either, but if it makes you more comfortable, go for it.

And since you're already so far outside the box, a transfer pump & feeding the engine from one tank would mean you could eliminate the selector valve, too (again, the Walbro won't pass fuel unless it's running).

Ain't dominoes fun?

Charlie

Charlie, what you say is true, but in order to meet my very high standard of redundancy, I will use most of the hardware shown. I think it is too likely that the tiny checkvalves integral to these pumps could become lodged open by debris resulting from a pump failure, allowing reverse flow when pump #2 is engaged, so I’ll be using the big industrial duty external ones shown, which also serve as debris-dams from a failed pump. I do not need a regulator as such because the Bendix type metering valve is happy anywhere between 20 and 90 PSI, so each pump will have its own ground-adjustible relief valve dialed to 24PSI. A stuck relief valve, as someone brought up here, would by overridden by simply switching to the other pump. Btw there is a lot of built-in resistance to flow in these pumps, but some fuel can be forced through them in the absence of a check valve, possibly a lot depending upon the nature of the failure.

As you say, I’m going out of the traditional box, but I’m also quite conservative in my choices. For example, I’m big into technology, but I’ve got a personal rule against relying upon software and firmware for essential systems. So, I forego the advantages of EFI, programmable ignition systems, and software based control of essential electrical supply. Thus the lightspeeds, the Bendix type FI, and the use of old fashioned breakers and switches for essential loads.

I’m really loving all of the input and discussion here- thank you all! The advent of good, reliable lithium batteries has opened up a new world of possibilities for us, and it is a pleasure to be in on the ground floor of finding ways to discover and take advantage of them.- Otis
 
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The way these work is that they strive to achieve the the Pressure commanded by the regulator. You read the chart by looking at the PSI set by the regulator
then observe the flow rate at that PSI (e.g. x GPH @ y PSI). The pump will keep trying to deliver the PSI set by the regulator, all the way down to a 0 flow.

Larry

These are positive displacement pumps and they put out the same volume of fuel all the time at a constant voltage. The regulator only modulates return flow to maintain the set fuel pressure which in most EFI systems is determined by MAP vs spring pressure and the volume of fuel the engine is consuming.

The pump isn't striving to do anything. It just runs. When return flow reaches zero, the regulator can do no more to maintain fuel pressure as the engine is burning 100% of the fuel the pump can deliver. Any more flow demand from the engine will result in lower fuel pressure and the AFR will start leaning out.
 
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