REFLECTOR: Engine failure causes

[nmflyer1 at aol.com]

Al, 

I thought of those same questions when Airflow Performance built me the setup. I didn't design it. I specifically asked if I should run a return line to the sump in order to keep the fuel cooler. They stated that at low flows, the fuel wouldn't be getting all that hot  from running around the fuel pump setup. They also stated that in a high power setup, the amount of old fuel traveling the return path would be so diluted by the new fuel that it would not cause a problem. 

They also said that, if you have pure fuel and not gulps of air, there should be very little if any bubbles forming due to the pumps. I guess they use this fuel pump system on many things, and have never had a problem. I have a friend with an RV-6 with a similar fuel pump setup and they have not had any problems. 

Still, the hi-power test seems like a must at this point. 

Interesting point on the fuel filter. I just went outside to double check what I thought I remembered about the flow path. I was right, the flow is such: 

Out of the sump, through the flowmeter, to the filter. Then through pump 1, 2, or both. then up to the top of the spar, past the pressure sensor and to the FI. 

Again, I kind of deferred to Airflow Performance expertise on this. 

Some more pieces of the puzzle. thanks Al :) 

Kurt 











I too have injection and a return line. Airflow performance built my fuel pump setup (2 parallel electric pumps) but the return l
ine does not go back to the sump. Instead it feeds right back in before the pumps. They say this keeps the flow up but avoids fuel frothing. 

Kurt;

I think there may be two potential issues with this; one is that any bubbles (foam) that form happens at the exit of the regulator. Now you’re feeding those bubbles back into the pump, which is OK since it will collapse them again – up to a point.  Which relates to concern #2; running that same fuel around the loop heats it up, and along with incipient bubbles already coming in before the pump, aggravates potential for vapor lock.

I also have another filter just before the pumps.

This would be concern #3.  A filter before the pump is a restriction to the flow; especially when it gets some debris in it.  That’s asking for vapor lock.  We want easy gravity flow to the pump at all times; unless the pump is in the tank.  Especially since you have sediment filters in the line to the sump (which probably have screens in them?), and the sump is a big sediment trap; no need for filter before the pump.  Put a good 15-30 micron filter downstream from the pump, and the fuel pressure sensor downstream from that. Then if the filter ever gets dirty you see a bit of reduction in pressure, but you don’t get surprised by a vapor lock and engine stall.

FWIW,

Al




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