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  #431  
Old 10-17-2019, 04:42 PM
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jcaplins jcaplins is offline
 
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Quote:
Originally Posted by charosenz View Post
I am continuing to migrate engine systems and components over to the airframe that were functional on the test stand.

Below is a pic of my dual walbro fuel pump system.

I am happy with the set up. For those who may not want to build this piece by piece SDS EFI has a dual pump system that is very compact unit.

I did get my prop today. Very excited to test the engine with this. Unfortunately things are in limbo until I get the fuel system functional and with another trip out of state soon, it may be awhile before I can test the prop.

Ill keep this thread updated as long as there seems to be interest. And since it past over 60k views, I think for now its worth sharing what I am doing.

Charlie
Making progress. Cool.

It looks like there are 2 fuel inputs; one with a filter (the black thing?) and one without. Are you running the return fuel straight back to the fuel pumps? if so, the prevailing wisdom is to run the fuel back to the tank. It's even suggested to run the return line in the tank far from the fuel pickup. This is primarily because the return fuel is warm/hot. I don't know how much of a difference it really makes, my return fuel dumps into the tank right above the pickup tube.

I'm also imagining having the return fuel going back to the fuel pumps would create a problem with regulating the fuel pressure. Fuel would be pumped to and sucked from the regulator, and maybe not suck from the tank.

(Just raising a concern; I don't mind being wrong.)
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RV7 N76CX
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  #432  
Old 10-17-2019, 09:31 PM
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charosenz charosenz is offline
 
Join Date: Aug 2009
Location: Longview, Wash
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Default Fuel return lines....

Jeff,

Good to hear from you.

Comments and suggestion are always welcome. I am pretty familiar with the discussion and debate on the returns lines. While I hate to crack open that debate...here are are my thoughts.

I agree that most of the concerns regarding the return line usually is tied to two thoughts. One, like you mentioned is tied to concern about the temperature of the fuel. (which leads to the second concern). High pressure systems like this sees most of the return at idle. While I don't have hundreds of hours of testing on this, I can tell you that the return lines during my testing are not perceptibly warmer than the supply lines, and those, even in the hottest time of summer. The second and actually more common concern is "vapor lock". Many people do not understand that this is a concern for the suction side of the system and virtually a non-issue for the pressurized side of a fuel system. The risks associated with what is commonly called vapor lock are prevalent on a carburetor systems and especially older systems that have mechanical pumps with limited fuel flow, low pressure and long supply lines. The problem occurs when a particular system is unable to move the fuel due to being "stalled" (sometimes called cavitation) with air that that particular system is unable to handle, for a variety of reasons.

When I do my test runs I hook up the system with an external fuel tank. (I don't have wings mounted on the fuse). As you can imagine the lines are nothing but full of air - absolute worse case of risk of "vapor lock". Right?! it takes a few seconds for the air to pass the injectors and then it runs absolutely perfect.

Some even think the return lines are to allow bubbles to return to the tank - not realizing of course that once they pass the pump, the are a non issue. (in a relative sense).

Now, should we be mindful of the value of keeping air out of our supply lines, especially in low pressure carburetor systems. You bet. Will I continue to do testings as the systems evolves? Absolutely.

Sometimes we take information that is applicable to one kind of system and then believe that is applies to all systems even though they are significantly different and that appears to be happening on this topic as well.

In short. Yes, I have thought about it. I have read every tech article I can find and I am basing my decision on credible information, backed by testing.

I appreciate you sharing your thoughts and concerns.

P.S. On a somewhat humors note, I am tempted to put a schrader valve on the suction side and inject very low pressure air in to the system just to verify how it would handle. Any one want to take a $100 bet it handles it fine? I need lunch money.

Take care.

Charlie

Last edited by charosenz : 10-17-2019 at 09:38 PM.
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  #433  
Old 10-18-2019, 06:00 AM
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rv6ejguy rv6ejguy is offline
 
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The biggest concern would be if you run a tank dry, you'll have a lot of air in the system and it will take many seconds to process that through the injectors. The engine will be too lean to run. Maybe not a big deal with lots of altitude but would be down low.

Returning to the pump inlet has caused running issues on the dyno in one instance I can recall during a test.

Any air entrained, going through the pump gears is unfriendly to them.

Returning to the tanks is the safest and most proven. Worth considering IMO.
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Ross Farnham, Calgary, Alberta
Turbo Subaru EJ22, SDS EFI, Marcotte M-300, IVO, Shorai- RV6A C-GVZX flying from CYBW since 2003- 434.4 hrs. on the Hobbs,
RV10 95% built- Sold 2016
http://www.sdsefi.com/aircraft.html
http://sdsefi.com/cpi2.htm


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  #434  
Old 10-18-2019, 09:46 AM
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charosenz charosenz is offline
 
Join Date: Aug 2009
Location: Longview, Wash
Posts: 279
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Quote:
Originally Posted by rv6ejguy View Post
The biggest concern would be if you run a tank dry, you'll have a lot of air in the system and it will take many seconds to process that through the injectors. The engine will be too lean to run. Maybe not a big deal with lots of altitude but would be down low.

Returning to the pump inlet has caused running issues on the dyno in one instance I can recall during a test.

Any air entrained, going through the pump gears is unfriendly to them.

Returning to the tanks is the safest and most proven. Worth considering IMO.
Ross,

Always I good to hear from you.

I should add that my comment about air in the system was certainly not to suggest that it is a good thing, and running a tank dry is definitely not good in any situation, IMHO (Although I have read folks who routinely do that before switching to a full tank - not my idea of good fuel management.)

I mentioned the part about my start ups with air in the line just to help people have a real world perspective and to understand in an unlikely chance an air bubble develops in a system like mine it is (IMHO) not going to cause vapor lock, like in the older low pressure systems.

My testing continues. While the system I had on the test stand was functionally similar, I have not done testing on this particular set up. I hope to get going on it very soon and I will post all my experience, good and bad.

I appreciate comments like yours and Jeff, even while they might be different from my plan they are respectful, and helpful to us all. Your knowledge and experience is something I respect.

Take care.

Charlie.

Last edited by charosenz : 10-18-2019 at 09:49 AM.
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  #435  
Old 10-18-2019, 02:47 PM
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charosenz charosenz is offline
 
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Location: Longview, Wash
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Quote:
Originally Posted by Bavafa View Post
Excited about your progress, can't wait for the time you take it up.

With best wishes
Me too! Thanks for the support. This is a very fun and challenging project. So far I am encouraged with the progress.

Charlie
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