Fuel/Vapor Return Line

Fuel/Vapor Return Line

Postby lpaaruule » Sat Feb 06, 2016 12:39 pm

For those that have added a recirculating fuel system, I have some questions.

For Jabiru owners, are you using the mechanical fuel pump? If so, are you putting the T-fitting right after the mechanical fuel pump, which would be at the highest point, or still as close to the regulator/carb as possible.

If you're only using the electrical fuel pump, do you only use it during takeoff and langings?

Do any of you use a fuel flow meter? I have one, but unless I install it right next to the carb, and put the fuel return line T-fitting right before it, I'll be measuring both the fuel burn, and the fuel return flows.

I had started installing my fuel system, and had planned on using the mechanical fuel pump since it's there, but my fuel system is starting to seem cumbersome. I'm considering two options.

1) forget about adding a fuel return line, and just use the electrical fuel pump to pressurize the system during takeoff and landings.
2) still use a return line, and electrical fuel pump, but don't used the mechanical fuel pump.

For option 2 The fuel flow meter would still be behind the firewall, but have very inaccurate high readings during takeoff and landing. Not ideal, but everything seems to have a compromise.

I realize I could also buy a second fuel flow meter to measure and subtract the return line flow, but I really don't want to go there.
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Re: Fuel/Vapor Return Line

Postby SonexN76ET » Sat Feb 06, 2016 3:12 pm

I have a fuel return line and a Facet fuel pump on my Aerovee with a Rotec TBI. I would like to make a couple of observations I have from this set up.

With these items in place, if you put the aircraft in a climb attitude without the fuel pump turned on the fuel pressure at the TBI goes way down. In my case in a couple of touch and goes it was enough to have the engine cut out. In a level attitude I measured 12 GPH fuel flow, in a tail low configuration it dropped to 9 GPH and it seemed like I had a momentary greater reduction as the tail was lowered. Sonex has recommended against "burp tubes" with the AeroInjector because they can cause low fuel pressure in a nose high attitude. Also, the Facet fuel pump (when off) does cause some restriction in the fuel flow versus just a straight fuel line.

The concern this causes for me is that with the fuel pump off you can not safely take off, do a touch and go, do steep climbs, go arounds, or slow flight. Operationally, I could keep the fuel pump on when doing these maneuvers but what if the pump fails or I have an electrical failure? I had originally just put the fuel pump on as a precaution against vapor lock. Now it is a critical component and a single point of failure.

I have considered putting a bypass fuel line around the fuel pump with a check valve in an effort to have more fuel pressure to the TBI when the pump is off, but that will complicate the fuel system further. Also, I don't know if that will improve the fuel pressure enough to overcome this issue.

In all honesty I am not exactly sure of the solution or what exactly is causing my troubles. It could be my TBI's fuel pressure regulator being faulty or it could be the combination of fuel pump in line but off and the fuel return line. Rotec says if you do a fuel return line it should have a restricted fitting. Mine goes from 6AN to 4AN. Perhaps that is not enough of a restriction.

So, I do not have any solid answers for you, but I want to point out these important considerations.

Thanks,

Jake
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Re: Fuel/Vapor Return Line

Postby mike20sm » Sat Feb 06, 2016 4:56 pm

In regards to the one fuel flow sensor, I believe the solution is to have two meters one measuring the outflow and the other the inflow. Then the efis calculates the difference and you get an actual fuel flow consumed value. I learned that when looking into the UL power fuel injection system.
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Re: Fuel/Vapor Return Line

Postby fastj22 » Sat Feb 06, 2016 8:12 pm

When I installed my Rotec TBI, I decided to try it without a burp tube or fuel pump. So far, it works great with just gravity feed. But I haven't flown when its hot out yet. Without a burp tube, the Aerocarb burped all summer. Even with a burp tube, really hot days would get your attention.
If I decide to put a fuel pump in-line, I'd put the fuel flow sender after the tee to the return back to the tank. Probably directly to the TBI. I'd then only turn the pump on during takeoff and while taxiing to eliminate any burps. The return will be an AN-4 line to increase pressure to the TBI which is AN-6. I have all the parts, just haven't had a reason to use them yet.

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Re: Fuel/Vapor Return Line

Postby lpaaruule » Sat Feb 06, 2016 8:41 pm

Thanks for the replies so far.

It seems like the using the mechanical fuel pump may not be a bad idea. Although I don't want to do it, having a return fuel flow sensor is probably the right way to do it. That way, both fuel flow sensors can remain behind the firewall.

As far as restricting the fuel flow on the return line, I planned on filling a fitting with soldier, and drilling a 1/16" hole in the soldier. I think this is the size hole that Rotec said a restriction fitting should have. I got the idea of drilling the soldier from the sonex plans. They say to do it if you're using the fuel level sight glass.

I was thinking about not using the mechanical fuel pump, and just using the electrical pump, but Jake's post has me concerned about doing that due to fuel line restriction of the fuel flow sensor, and the electrical pump when it's off.
Paul LaRue
Sonex N454EE Plans# 1509
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First Flight 12/21/2017
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Re: Fuel/Vapor Return Line

Postby WaiexN143NM » Sun Feb 07, 2016 3:51 am

Hi all,
We are using the rotec tbi with our jab 3300. No fuel pumps. Just gravity feed. We do have a red cube fuel flow cockpit side inline, back side of firewall. We installed a facet pump and ran flow tests, too much restriction in flow, took it back off. We fly in the Tucson heat, no burbs, or vapor lock.
We want to wrap our exhaust soon, I think that will keep inside the cowl cooler too.

Off topic: been crazy this week in SF. Enjoy the 50th anniversary. Super Bowl!!

WaiexN143NM
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Re: Fuel/Vapor Return Line

Postby lpaaruule » Sun Feb 07, 2016 11:10 am

Thanks, this is more good info.

It sounds like no more than a fuel flow meter can be put inline with a gravity fed system...at least with a Rotec carb. I noticed on the Rotec regulator (I have version 1) there are sharp 90 degree turns inside the input and output ports -- not too good for flow.

If I go forward with a fuel return line the mechanical pump will probably need to be used, unless Jake's idea of putting a parallel check-valve next to the electric fuel pump works. But since the Jabiru has a mechanical fuel pump, that's probably the solution I'd use. The drawback is that it's use requires more fuel line, more exposure to heat, and more fuel agitation.

So either
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Sonex N454EE Plans# 1509
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First Flight 12/21/2017
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