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I'm having a problem with fuel delivery. My system should use the mechanical pump on my Corvair engine and an electrical "Facet" pump available at the click of a switch.
Here's a picture of the system:
I'm not sure why this is happening, but if I put a "L" in place of the "T" after the check valves the Facet pump works and I get a fuel pressure reading of about 2 or 3 psi. I'm use to electric fuel pumps from my experience with MGs and on these cars the pump would only pump when there was a need. Those would slow down and stop pumping when the float bowl was full and only pump once in while when there was a need for more fuel. This Facet pump continues to sound like it's pumping (making lots of noise) even when the system is full of gas all the way to the float bowl and the bowl is full. Is that normal?
Why should there be any difference between having a check valve right above the "T" and bypassing the mechanical pump with an L. Here's a sketch of the firewall forward part of the system:
Dan, one thing that makes me curious about your system is the use of an in-line fuel filter. If you have finger screens in your fuel tanks and a gascolator, adding an inline fuel filter is both redundant and potentially dangerous. Depending on how fine the filtration material is in the in-line fuel filter it is possible to block fuel flow to the carb if a fine sediment is suspended in your fuel. Given that the finger screens are a coarse filter and the gascolator has a fairly fine mesh in the top of it, plus a sediment bowl to remove any crud during the walk-around, supplemental filtration should not be needed unless you are going with electronic fuel injection. Generally, a fine suspended sediment would pass through the carbs commonly used on Corvair conversions without any problems but could potentially plug a fine in-line filter.
Of course it might just be my "old school" aircraft thinking but I am always nervous about in-line fuel filters. None of this gives you any advice about your curious fuel flow issue but I figured I'd better add my two cents. Glad to see your getting close with your build.
CH-701, Northwest Ontario, Canada
For the record, we use the finger screens, then 200 hour change in line filters (for fuel flow monitoring since they are transparent!) - one per tank - then we hit the gasco at the low point, with a fine teflon coated filter (water separation at its best from Andair) at about 80microns. That is where it stops for the 912UL and 912ULS, but for the 912iS. from there we go to a large10micron fuel filter... before hitting the engine. We monitor fuel pressure in all installations and have had no issues with Rotax mechanical, or electrical fuel pumps. We have a good few thousand hours operations, in our conditions with our fuel.
The little block facets are cute and effective... you are right, they keep that pump tacky tacky tacky tacity tacky tactititty tack noise up - but you should find that the pressure remains the same. The float system in the bowl is intended to stop fuel going in further and flooding the bowls - if all is set up right. IF you are using the pierburg pumps you will need a bypass line, but we generally find no need for one with the facet on the carb Rotax installations that we have in our particular installations. As always, this is not advice, just comments, and you are recommended to seek professional advice - especially if you live in California.
Thanks for the feedback Doug, it's food for thought. I guess one thing I was thinking was that I'd be catching a lot of the stuff that might have remained in the fuel lines, bits and pieces during the plumbing stage. Especially the making of the flex lines provided opportunities for tiny stainless steel particals to remain after blowing them clean with air. This is all the stuff that a finger screen just isn't going to catch.
I put the filter as close to the carburetor as possible to eleminate stuff from all the possible lines, and the fuel pressure gauge after, so I'd no if I was loosing pressure for any reason. This was one of the first things I checked when I wasn't making pressure. There was no noticable resistance.
The filter itself is rated at 130 gallons per hour and is used on engines up to 1000 HP.
I might just finish all the ground testing and eleminate the filter when I'm closer to flying time. Again thanks for the feedback.
Could be a bad check valve. Do the check valves have springs to keep them closed or are they the flapper style? The ones with flaps have the word top written on them to show orientation. If the top isn't on top they won't close right.
Thanks Donnie for the input but they aren't the flap type.
I believe Facet recommends using a filter in front of their pump. I note that yours is after the pump. But I don't know what this has to do with the problem you are seeing: probably nothing. I've heard people question the filter before. The only reason I can see for Facet requiring it is that it's an electronic pump, and the technology may be unusually sensitive to small particles that wouldn't usually matter. That might have a long term impact on the pump.
Now to the issue at hand. When I look at your diagram, the thing that would appear to be eliminated with an "L" installed is the return fuel path. That would suggest to me that the check valve in the return fuel path is suspect.
Thanks for the reply. I've been coming to the same conclusion. With the engine off, the mechanical pump isn't pumping and I think fuel would back flow through it if the check valve weren't working, and prevent the facet pump from making any pressure. Fuel would just be going around in a circle. Maybe there's a bit of dirt in the checkvalve or a manufacturing defect.
Most of the Corvair community has given up using the mechanical pump and instead are using 2 facet pumps inline. This is a far simpler system that eliminates the check valves.
The thing I dislike about that system is that in the case of an alternator failure you have to supply both spark and drive the pump with the battery until you can find a safe place to land. I'd prefer it were just the spark being supplied in such a failure mode.
But I am starting to second guess that decision. If these check valves aren't reliable then I'm just adding different failure modes that I had not thought of.
The other reason William got away from the engine driven mechanical fuel pump was pump leakage. The new production pumps (and I doubt you would use a 50 year old pump from the core engine) do have a tendency to seep/leak. The fuel pump is on top of the engine, so a leak could give critical components a bath if the leak is external. William actually built a drain bucket/cage thing around the fuel pump on his flight engine when it had the mechanical pump. If the leak is internal, the fuel ends up in the oil sump where it dilutes your oil and also creates a potentially explosive situation in your crankcase due to vapors.
My 1962 Corvair had an internal leak about 1968 so even the old pumps could and did leak. I realized there was a problem when the oil level started to rise instead of the usual consumption rate. Eventually I figured out the oil smelled like gasoline and traced the problem to the fuel pump. The car ran fine the whole time, there was no indication of a problem other than the rising oil level. If I had not found it, the oil would have failed after a while (once it became mostly gasoline) or possible the fumes in the crankcase might have ignited.
I plan to go with dual Facet pumps per William's recommendations. I also plan to have a reserve battery of some kind that can only feed the ignition and fuel pump systems. Once I run down the ship's battery and have to switch over to the reserve battery, I will know it is time to land RIGHT NOW. The reserve battery may be as simple as a 12 volt dry cell lantern battery or it may be a lead acid battery isolated from the ship's electrical system with diodes, but there will be a "last ditch" reserve battery of some kind.
This has been a really good discussion of the pros and cons of various fuel delivery methods. I'm leaning toward ditching the mechanical pump in favor of going with 2 inline electrical pumps.
Dan, these pumps work very well with the fuel plumbed through them inline, with the pump switched on or not.
Your system has to be one of the most complicated I have seen, aircraft or other device. You can safely remove all that pipework and simply have a single line.....intake to electric pump, to gascolater, to mechanical pump onto carburator.
Just my opinion......
Thanks for input Darryl. I kind of like the idea. If the electric pump were down stream of the mechanical pump it wouldn't be pressurizing it (which I think would be a bad thing to do).
I'm using a Continental, not a Corvair, but I also have two Facet pumps. Like any choice, it has pros and cons. You point out one of the cons: on alternator failure, there is just that much more load.
I do have check vales on both pumps. I didn't want the chance of a pump failure, and having fuel go around in a circle in the way you describe. I'm not sure what the probability of that may be, but neither do I want to find out the hard way.
So far, the fuel system has been one of the nastier problems in the airplane.