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  #1  
Old 01-15-2019, 09:56 PM
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Hartstoc Hartstoc is offline
 
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Default Anatomy of a High-Pressure Electric Fuel Pump.

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.


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:

(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/an...p/?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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Last edited by Hartstoc : 01-20-2019 at 03:35 PM.
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  #2  
Old 01-15-2019, 10:34 PM
rv7charlie rv7charlie is offline
 
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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
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Old 01-15-2019, 10:57 PM
SHIPCHIEF SHIPCHIEF is offline
 
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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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Last edited by SHIPCHIEF : 01-15-2019 at 11:03 PM.
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  #4  
Old 01-15-2019, 11:02 PM
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rocketbob rocketbob is offline
 
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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.
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  #5  
Old 01-16-2019, 06:32 AM
BillL BillL is offline
 
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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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Last edited by BillL : 01-16-2019 at 06:35 AM.
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  #6  
Old 01-16-2019, 07:07 AM
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rv6ejguy rv6ejguy is offline
 
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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.
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  #7  
Old 01-16-2019, 08:36 AM
rv7charlie rv7charlie is offline
 
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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...-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
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  #8  
Old 01-16-2019, 08:55 AM
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rocketbob rocketbob is offline
 
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Quote:
Originally Posted by rv7charlie View Post
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.
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Old 01-16-2019, 10:00 AM
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airguy airguy is offline
 
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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.
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  #10  
Old 01-16-2019, 11:10 AM
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Toobuilder Toobuilder is offline
 
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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.
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WARNING! Incorrect design and/or fabrication of aircraft and/or components may result in injury or death. Information presented in this post is based on my own experience - Reader has sole responsibility for determining accuracy or suitability for use.

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