Fuel Return Line to Bottom of Tank? - LotusTalk - The Lotus Cars Community
 
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post #1 of 6 (permalink) Old 10-08-2018, 07:39 AM Thread Starter
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Fuel Return Line to Bottom of Tank?

Winter is quickly approaching Denver and I'm starting to plan maintenance and upgrades. I plan to change the fuel lines again as I've seen some cleaner installs than mine, while I haven't had an issue with fuel I think I can clean it up some. I currently have the BOE surge tank with the fuel return line going into the fuel filler neck. I'm not positive this is the best place to return the fuel to the tank (easy yes). I've read that returning the fuel above the fuel level can add air or bubbles into the fuel. I'm not sure how much this affects the BOE surge tank though. While I'm already working on the fuel system I'm wondering if there is a better way to run the fuel return line to the tank. Has anyone done something other than returning the fuel to the filler neck? Thoughts on how else to plumb this? I can drop the tank and plumb in a return somewhere else, provided there is clearance, and have a tube on the inside return the fuel to the bottom of the tank.
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post #2 of 6 (permalink) Old 10-09-2018, 11:07 AM Thread Starter
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Has no one plumbed their return line to the bottom of the tank? I think this video shows why that is a better method. Trying to see if there are any pitfalls to watch out for before I tackle this project.

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post #3 of 6 (permalink) Old 10-09-2018, 11:52 AM
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@LostPawn - There are several things in the demo video that don’t replicate a typical return via the filler neck:

1) The person conducting the demonstration holds the return line a few inches above the surface of the fuel (mineral spirits). As a consequence, the high velocity and high volume stream entrains air as it enters the surface of the fuel. This is the exact opposite of what happens when fuel returns via the filler neck. In that configuration, the return fuel stream is a significant distance from the surface of the fuel in the tank, it is entering via a very large opening (the fuel filler) and it is at low velocity. As a consequence, very little air - if any - will be entrained in the return stream.

2) When the person conducting the demonstration aims the return fuel stream with entrained air directly at the pump pickup, the pump rpm changes. Of course it does, he’s intentionally feeding it air under conditions that don’t exist in your fuel tank. The fuel return and fuel pickup in your Lotus are not 3” apart as in the demo.

3) The highest fuel return rate will be at idle, where entrained air is not a big deal (if it existed). At high throttle settings when you want maximum power, fuel return and the potential for aeration is minimized.

My guess is that any aeration would be largely attributable to fuel sloshing in the tank as a result of vehicle motion, not fuel return.

My Porsche returns fuel via the filler neck and my VW dumps it at the top of the tank . . . I’ve never experienced any fuel-injection problems, including at idle where the inability of the fuel system to deliver fuel at a constant volume and pressure would result in an erratic idle (especially in the VW, which has a very crude second-gen fuel injection system). One other related data point . . . I have designed and built lots of fuel systems for 1/4 scale RC aircraft that use gasoline and pump systems. The engines are extremely sensitive to pressure or volume changes. Despite that, I’ve never had a problem returning fuel to the top of the tank.

So unless you are actually experiencing problems related to foaming fuel, I doubt you need to “fix” this. A header tank to ensure fuel delivery during prolonged turns, of course, is a totally different problem and solution. But if there was any aeration, the header tank would likely remove almost all of it.

Glen

2011 Lotus Elise SC

Last edited by Glen; 10-09-2018 at 06:47 PM. Reason: add air after entrained at 2
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post #4 of 6 (permalink) Old 10-09-2018, 12:23 PM Thread Starter
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Thanks @Glen - I'm not experiencing problems with fuel delivery or foaming. I'm planning to clean up the various fuel lines and replacing some of the rubber hose sections with hard lines. The return hose is fairly long and if I can make the system better at the same time that would be worth it. I'm not sure how much better running the hose to a fitting on tank would be vs. returning it to the filler neck. Part of me sees the fitting on the filler neck as a potential place for leaks where the adaptor adds two additional hose to metal fittings with clamps vs. a screw on -AN fitting.
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post #5 of 6 (permalink) Old 10-09-2018, 04:07 PM
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If you really wanted to clean things up and dont mind dropping the tank, just weld an AN fitting to anywhere on the top of the tank.

There is a radium adapter to the fuel sending unit to -6AN FYI so you can run good lines from the tank to the sending unit.

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post #6 of 6 (permalink) Old 10-09-2018, 05:51 PM Thread Starter
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Quote:
Originally Posted by kfennell View Post
If you really wanted to clean things up and dont mind dropping the tank, just weld an AN fitting to anywhere on the top of the tank.

There is a radium adapter to the fuel sending unit to -6AN FYI so you can run good lines from the tank to the sending unit.
I have the Radium adapter and had BOE add the -AN fitting to a fuel rail so I can run all new lines. As long as Iím going to be in there I was looking at options for the return line. Thought I would see if anyone had done something different.
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