Correct, but....the circuit is not dead headed running against the relief, there is some flow in and out allowing (some) heat to be carried away.
Yes, that's right.
Here's EFii's animation of the flow path with the BPM boost pump.
Correct, but....the circuit is not dead headed running against the relief, there is some flow in and out allowing (some) heat to be carried away.
Our pump has the lowest suction loss (by far) of the three options available because we use a better check valve.
Most hard failures of the mechanical pump on a Bendix type FI system will cause the engine to stop.
Interesting. Lots of opinions, and not one little bit of temperature data.
Dan- I don?t think the concern is exceeding any particular temperature- that is only one condition contributing to fuel vaporization. In fact, outright failure of the pump itself is unlikely and not even much of a concern.
There is no means of extracting fuel vapor in systems without return lines, and these pumps cannot effectively pump foam or vapor. If the incoming fuel is already warm and the pump assembly is heating up due to continued use while recirculating nearly all of the fuel it is moving, you could reach a point where output pressure falls below the minimum level required to sustain operation of the metering valve, distribution block and injectors. As net flow through the FWF plumbing subsequently falls off, the situation grows even worse, and there is literally nothing you can do to remedy it. That is why you would want to operate at the highest reasonable fuel flows if you found yourself relying upon the boost pump after failure of the engine driven pump.
It might be possible to set up and test conditions required for this, but doing it safely would require very careful planning.
Engine driven pumps are actually very reliable, but their failure modes can be quite grim and, ironically, use of a boost pump during some of them will make the situation worse. I?m now assembling a fully redundant dual electric pump setup with no single-point failure modes for the Airflow Performance FI system I?ll be installing on my 7A next Winter. It will include fuel return lines and a duplex valve that switches both feed and return lines between tanks. I will be completely removing the engine driven pump, and will document all on VAF.
I will be watching and reading with interest. That sounds like an interesting set up. Thanks.
My post was only responding to the other post where it was believed that an FI engine would run without fuel pressure somehow. The backup boost pump would supply that pressure.
Yes, and I was confirming that your post was correct, but adding that engine driven pump failure modes can be complicated. You cannot simply say that the backup pump would supply pressure required to keep the engine running. Insuring that would require some very complicated valving that completely bypasses the engine driven pump.- Otis
Hi Otis, do you have more info on these failure modes? I for one have an engine driven fuel pump that has a backup boost pump (andair) and while I can imagine some mechanical pump failures that would stop the fan from spinning, I've never heard of one happening. Thanks!Yes, and I was confirming that your post was correct, but adding that engine driven pump failure modes can be complicated. You cannot simply say that the backup pump would supply pressure required to keep the engine running. Insuring that would require some very complicated valving that completely bypasses the engine driven pump.- Otis
....but just to show some numbers in English units:
I had 5000 hours on a Walbro EFI type pump in our shop car. Still quiet and working fine when the car was sold in 2016.
120k miles over 17 years on a Ford-spec Bosch frame mounted pump on my EFI-converted ancient Ford Bronco. Still good. I don?t know who makes the pump AFP sells. No markings other than a part number. All the Walbro and Bosch pumps I?ve seen had the manufacturer?s name on them.
I think AFP uses or used Airtex pumps (someone should verify that) which have a limited lifetime warranty from the manufacturer. Interestingly, they only warranty their mechanical pumps for 12 months. Vehicles are the word they use, an airplane is a vehicle technically right?
An alternative engine vendor supplied Airtex pumps with their packages and I've heard of a fair number of failures of those where they start to growl and make metal.
I've owned a bunch of pretty high mileage EFI cars and worked on many others in my repair business. Saw one Nissan pump fail at 325,000 km, one BMW at 360,000 km so those are at something over 200,000 miles. Probably over 5000 hours in both cases with usual average speeds and idling.
I've seen a bunch of ancient (early '70s) EFI D Jet Volvos in the bone yards with some really crusty pumps (externally mounted under the car). From the condition of them, they had not been changed for a very long time if ever. Two cars had 290,000 to 430,000 MILES on the odos. If the latter one was original, that's certainly over 10,000 hours. I took both pumps and used them on a test bench for years more. Still worked fine.
Bosch had some massive pre-filters on these cars which may be the secret to longevity since lots of folks never change filters.
I've sold hundreds of the Walbro pumps over about 15 years and nobody has ever reported a failure unless they were mounted incorrectly (those two failed in a few hours sucking a lot of air). Air or junk going through them will severely affect lifespan of roller vane pumps.
So you have an engine driven pump (Mechanical) and an electrical driven pump as a secondary boost pump. The only time the boost pump should be ON is during engine prime/starting and an emergency should the primary (Mechanical) pump fail. Keep in mind, anytime you compress a liquid it will get warm. However, your fuel injected system should be under 50 psi. Just add an indicator panel light to the secondary pump switch so you have a visual indication that the pump is ON. That should prevent you from accidently leaving it on for extended periods of time.
The conditions under which Lycoming recommends operation of the fuel boost pump are as follows:
1. Every takeoff.
2. Climb after takeoff unless Pilot?s Operating Handbook says it is not necessary.
3. When switching fuel selectors from one separate fuel tank to another, the fuel boost pump should be ?on? in the new tank until the operator is assured there will be no interruption of the fuel flow.
4. Every landing approach.
5. Any time the fuel pressure is fluctuating, and the engine is affected by the fluctuation.
6. Hot weather, hot engine ground operation where fuel vapor problems cause erratic engine operation.
7. Some General Aviation aircraft require the use of the fuel boost pump during high-altitude flight. This will be spelled out in the Pilot?s Operating Handbook.
8. If the engine-mounted fuel pump fails.
If the fuel boost pump is used during ground operation, don?t fail to check the condition of the engine-mounted fuel pump before takeoff by turning the boost pump off briefly, and then back ?on? for takeoff. If the engine-mounted pump has failed, it would be safer to know that on the ground rather than in the air when the fuel boost pump is turned ?off.?
When in doubt, do the safest thing and use the fuel boost pump with Lycoming engines. Don?t be ?stingy? with the boost pump. In most cases, they last the overhaul life of the engine, and are then exchanged or overhauled themselves. AS A REMINDER, the airframe Pilot?s Operating Handbook is the authority if boost pump information is spelled out in it.