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  #1  
Old 04-14-2021, 10:08 AM
Larry DeCamp's Avatar
Larry DeCamp Larry DeCamp is offline
 
Join Date: Apr 2010
Location: Clinton, Indiana
Posts: 1,069
Default EFI fuel return question ?

The point has been expressed that returning the bypass fuel to tank will heat the fuel. How much is too much ? My dilemma is, if I dump the fuel in tank bay nearest the pickup ( simple bulkhead fitting in the access plate), How is that different than piping the return to the bay nearest the wing tip with only 5 gallon left in the whole tank ?

Would appreciate any real experience from fuel returns in the tank at the root rib.
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Larry DeCamp
RV-3B flying w/7:1 0320 / carb / Pmags / Catto 3b / digital steam
RV-4 fastback w/ Superior roller 360/AFP/G3X/CPI/Catto3b
Clinton, IN
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  #2  
Old 04-14-2021, 10:49 AM
Ralph Inkster Ralph Inkster is offline
 
Join Date: Feb 2008
Location: Calgary, Alberta
Posts: 1,249
Default

Quote:
Originally Posted by Larry DeCamp View Post

Would appreciate any real experience from fuel returns in the tank at the root rib.
Real experience - NOT a concern.
If anything, just attach a short length of tube to the bulkhead fitting bent to direct returning fuel away from the pickup.



Practical experiment - Build a simple box that a space heater would fit in & can be heated up to 150 - 200 degF. Build a temporary fuel circuit (5 gal bucket / delivery / electric fuel pump / coil of alum tube about 10' that is coilded above the heater / press regulator / return back to bucket) Use water (or fuel, your choice) Take liquid temp & turn it on for about 5 or 10 min. Liquid would be circulated at about 30 gpm as it would be in your plane. Taked a resulting liquid temp and report back to us... Note that an airplane's tank is cooled by the airflow during flight too.
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  #3  
Old 04-14-2021, 11:06 AM
Freemasm Freemasm is online now
 
Join Date: Nov 2017
Location: Orlando
Posts: 322
Default Check with your OEM

What are your concerns about heated fuel?

Making some assumptions here: guessing your FI system will utilize redundant vane pumps and be back pressure-regulated. Fuel velocity is calculated:

v=(GPMx0.408)/(tubeID)^2

If your system is back pressure regulated, the system flow > the highest engine flow demand. Using a conservative like 25GPM out of the pump (it should be higher) yields:

133ft/sec in 3/8 tube
63ft/sec in 1/2

Why all the minutiae? Trying to say the fuel that returns doesn't spend much time forward of the firewall. Heating won't be that great. I'd be more concerned about aeration by having the return to close to the pick-up. So, give us some more info on your system set-up and characteristics. We'll see what comes to mind. Once your concerns are defined, reach out to Ross Farnham at SDS. He is wickedly knowledgeable of such systems (though he probably doesn't like me very much).

Edit below. Added from one OEM website/Instruction Manual

Return line installation in wing tanks – If you are installing fuel return fittings into wing tanks,
make sure the return fuel dumps into the tank at least 4 inches from the fuel pickup tube in the
tank. This will allow any bubbles that are in the return fuel to percolate out without being sucked
into the pickup tube.

Last edited by Freemasm : 04-14-2021 at 01:04 PM.
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  #4  
Old 04-14-2021, 11:11 AM
Larry DeCamp's Avatar
Larry DeCamp Larry DeCamp is offline
 
Join Date: Apr 2010
Location: Clinton, Indiana
Posts: 1,069
Default Thanks much Ralph

I believe you. Bulkhead fitting in the access plate it will be.

BTW, I am only doing this as a courtesy for future owners. My 4 is AFP injection, but I feel the Bosch style will eventually prevail in the market.
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RV-3B flying w/7:1 0320 / carb / Pmags / Catto 3b / digital steam
RV-4 fastback w/ Superior roller 360/AFP/G3X/CPI/Catto3b
Clinton, IN
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  #5  
Old 04-14-2021, 01:18 PM
lr172 lr172 is offline
 
Join Date: Oct 2013
Location: Schaumburg, IL
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Quote:
Originally Posted by Larry DeCamp View Post
The point has been expressed that returning the bypass fuel to tank will heat the fuel. How much is too much ? My dilemma is, if I dump the fuel in tank bay nearest the pickup ( simple bulkhead fitting in the access plate), How is that different than piping the return to the bay nearest the wing tip with only 5 gallon left in the whole tank ?

Would appreciate any real experience from fuel returns in the tank at the root rib.
Pretty much every car produced in the last 30 years has a fuel return setup similar to what you describe. Clearly there are no issues with this type of arrangement, include heat related problems. Can't see any reason that the location of the reutrn has ANY impact on fuel heating. Heat will RAPIDLY transfer in the fuel.
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  #6  
Old 04-14-2021, 01:48 PM
Larry DeCamp's Avatar
Larry DeCamp Larry DeCamp is offline
 
Join Date: Apr 2010
Location: Clinton, Indiana
Posts: 1,069
Default Thanks to all, good info.

The fuel pump flow vs temperature analysis is relevant and reassuring.

The 4" spacing for bubbles sounds like the defining issue.

Potentially a LONG delay on the ground (high return rate /low consumption) plus under cowl heating could raise the fuel temp more than flying. This still might not be an issue at 30+psi fuel pressure.
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Larry DeCamp
RV-3B flying w/7:1 0320 / carb / Pmags / Catto 3b / digital steam
RV-4 fastback w/ Superior roller 360/AFP/G3X/CPI/Catto3b
Clinton, IN
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  #7  
Old 04-14-2021, 03:37 PM
Freemasm Freemasm is online now
 
Join Date: Nov 2017
Location: Orlando
Posts: 322
Default

Quote:
Originally Posted by Larry DeCamp View Post
The fuel pump flow vs temperature analysis is relevant and reassuring.

The 4" spacing for bubbles sounds like the defining issue.

Potentially a LONG delay on the ground (high return rate /low consumption) plus under cowl heating could raise the fuel temp more than flying. This still might not be an issue at 30+psi fuel pressure.
Any issue would occur on the pump(s) suction side in the negative (gage) pressure zone.

Probably not needed but it's one more reason to keep those heat sinks optimized (fuel tanks full). Your peace of mind and fuel tank sealant will thank you; something you already knew.
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  #8  
Old 04-14-2021, 03:52 PM
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M McGraw M McGraw is offline
 
Join Date: Dec 2012
Location: Greenback, TN
Posts: 560
Default

Quote:
Originally Posted by Larry DeCamp View Post
The fuel pump flow vs temperature analysis is relevant and reassuring.

The 4" spacing for bubbles sounds like the defining issue.

Potentially a LONG delay on the ground (high return rate /low consumption) plus under cowl heating could raise the fuel temp more than flying. This still might not be an issue at 30+psi fuel pressure.
Larry,
Much like Ralph I have been running an SDS for 600 hours. I fly in all kinds of heat conditions. Heated fuel has never been an issue. Turn on the pump and press start, nothing more. I’ve even ran a tank down to 2 gallons with no heat issue.
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