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  #1  
Old 01-31-2019, 07:42 PM
charrois charrois is offline
 
Join Date: Jan 2019
Location: Legal, AB
Posts: 8
Default Keeping fuel pump cool

Hi everyone.

I've been having ongoing issues with fuel pressure fluctuations at higher altitudes and am trying to get them resolved once and for all. I've done everything I can think of to keep the tunnel cool (insulated firewall and floor of the tunnel, pipe wrap insulation around the scat tubes there, etc.), but that hasn't helped. I'm now in the process of moving my fuel flow transducer out of the tunnel and instead between the engine pump and servo as the "red cube" manufacturers suggest to avoid the possibility of the transducer being a source of vapour lock. I've also changed my fuel pressure sensor in case it was faulty in some way. I'm still working on these changes so haven't done a flight yet to see if the situation improves, but while the plane is down am trying to improve the performance of the fuel system as much as possible.

But something which still strikes me as "poor design" is that when cabin heat isn't required, heat from the exhaust muffs is directed to the area between the firewall and engine, right where the engine fuel pump is. I have the stainless cabin heat selector vents as well, which help to prevent heat from leaking into the cabin when it's not wanted, but does nothing to keep the area aft of the engine cool.

The way the heat system is designed is that hot air is always being produced around the exhaust muffs, and all you can do is choose whether to bring it into the cabin or the area aft of the engine. Is there any reason for this? Wouldn't it make more sense to just block the air flowing over the muffs in the first place if cabin heat isn't wanted?

I'm wondering if a "better" method would be instead to have butterfly valves right at the entrance to the heating scat tubes in the engine compartment, and routing them directly through the firewall instead of using the current heat selector vents. Operation with cabin heat on would be the same as now, but I can see two advantages when cabin heat isn't required. First of all, it wouldn't be dumping that heat into the area where the engine pump is. Secondly, there would be more air available for cooling the engine and oil since none would be consumed by the scat tubes. And of course, when cabin heat isn't required, that's when engine cooling is more important.

Not that I'm about to go and make these changes at this point (perhaps it is too much effort for too little gain), but I'm wondering why it isn't done this way in the first place. Or is there some unseen "gotcha" I'm not aware of, like the exhaust muffs getting too hot if there isn't air flowing over them all the time?

Another option I know people sometimes use is a shroud over the fuel pump. But that seems like a "hack" to circumvent poor design in the first place (like running an air conditioner at the same time as central heating on full blast instead of just turning both off). Besides, it looks like to get the shroud on, I'd have to remove and reinstall the fuel pump, and I've heard that can be tricky to do.

Any input?

Thanks! Dan
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  #2  
Old 01-31-2019, 07:47 PM
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9GT 9GT is offline
 
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Location: Southern Michigan
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Quote:
Originally Posted by charrois View Post
Another option I know people sometimes use is a shroud over the fuel pump. But that seems like a "hack" to circumvent poor design in the first place (like running an air conditioner at the same time as central heating on full blast instead of just turning both off). Besides, it looks like to get the shroud on, I'd have to remove and reinstall the fuel pump, and I've heard that can be tricky to do.

Any input?

Thanks! Dan
Its not a fun job to do after the engine is hung but it is doable. Ask me how I know.
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  #3  
Old 01-31-2019, 08:20 PM
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Carl Froehlich Carl Froehlich is offline
 
Join Date: Dec 2007
Location: Dogwood Airpark (VA42)
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Building my RV-10 I too looked at the cabin heat boxes, specifically the issue you point out. On a hot day they will be dumping hot air out of the heatmuffs right at the mechanical fuel pump.

This is what I did:
- I used a piece of KoolMat from ACS in between the cabin box and the firewall, hole cut out for the heat box into the cabin flange. The Koolmat then extends over the top of the cabin boxes then over the front of the boxes. This results in dumped hot air being directed down to the cowl exhaust instead of at the engine.
- On the two, 2 SCAT hose flanges on the rear baffles I inserted a 3/4 orifice (a piece of aluminum with a hole, the orifice inserted on the flange. As there is way too much heat for the RV-10 this orifice still passed more than enough air to the heat muffs. In summer this translates to less dumped heated air, and more air for engine cooling. On the coldest day I still just had the rear heat cracked open.

Carl
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Old 01-31-2019, 08:33 PM
rv7charlie rv7charlie is online now
 
Join Date: May 2006
Location: Pocahontas MS
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Dan,

I understand the concern about heating your fuel, but the reason for the airflow through the muff *not* being blocked is that if you restrict flow through the muff, it'll create a hot spot (I know; funny) at the muff. That can either damage the muff, or the exhaust itself.

Charlie
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Old 01-31-2019, 08:45 PM
charrois charrois is offline
 
Join Date: Jan 2019
Location: Legal, AB
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Quote:
Originally Posted by rv7charlie View Post
Dan,

I understand the concern about heating your fuel, but the reason for the airflow through the muff *not* being blocked is that if you restrict flow through the muff, it'll create a hot spot (I know; funny) at the muff. That can either damage the muff, or the exhaust itself.

Charlie
Hmm.. I was worried that something like that might be the case. So if the muff needs to always have cooling air flowing over it, I guess the alternative is to redirect the heat coming from the heat selector vents as Carl suggested. Next time I'm up at the hangar, I'll see if I can do something similar in line with what I can easily access.
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  #6  
Old 01-31-2019, 09:17 PM
Scott Hersha Scott Hersha is offline
 
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Location: Cincinnati, OH
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You could also just take the heat muff off the exhaust pipe. Remove the scat tube from the baffle and tape over the opening. Put it back on in cold weather.
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  #7  
Old 01-31-2019, 09:58 PM
lr172 lr172 is offline
 
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Location: Schaumburg, IL
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If you have not confirmed your fuel pressure with a second sendor, it is inappropriate to assume your fluctuations are actual pressure fluctuations and not fluctuations in the the senders output.

The higher you go, the cooler it gets and the less likely you are to experience vapor lock or other heat based anamolies in your fuel system. I don't see how you could be fine pumping out 100% power at 50 AGL with no issues and all of a sudden have vapor lock at 75% power and 8000' (in theory with 10-20* cooler ambients).

Many of us have seen similar behavior and most see it at altitude and almost all see no evidence that the pressure drops either exist or exist dramatically enough to result in engine performance change. I personally believe it is sender inability to balance the changes in atmospheric pressure on the backside of the diaphragm. THink about what happens to your airspeed indication if your static port becomes clogged. The fuel pressure acting upon the sensor's diaphragm has to be balanced against atmospheric pressure on the backside of the diaphragm and I sense that these units do not effectively equalize pressure in the chamber on the back side of the diaphragm.

Just a guess on my part, but lots of data to suggest these low readings at altitude do not truly reflect the actual fuel pressure. I have seen it a lot on my plane, but NEVER below 5000'. To me that rules out heat as an issue. Not saying the fluctuations are not real (though I speculate they are not), just saying they are not caused by heat.

Larry
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Last edited by lr172 : 01-31-2019 at 10:11 PM.
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  #8  
Old 02-01-2019, 08:45 AM
bpattonsoa bpattonsoa is offline
 
Join Date: Apr 2005
Location: Indepenence, Oregon
Posts: 295
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I put a cooling shroud on my fuel pump without removing it from the engine.

Cut the shroud in half vertically and rivet pieces of hinge to each half. Then you can put it on by removing and replacing the bolts one at a time.
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RV-10 996S tail, quick build wing and slow build fues., - dual Skyviews with complete system, two radio and not much else. Interior completely finished with Zolatone. CF plenum, interior complete. 1624 lbs, FLYING after a 21.5 month build.
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  #9  
Old 02-03-2019, 02:40 PM
CDN CDN is offline
 
Join Date: Nov 2011
Location: Calgary, AB
Posts: 90
Default Cooling shroud and deflector

I had the cooling shroud on my engine before I installed it but I still worried about the heat being dumped directly onto the back of the engine. I had similar conccerns when I noticed fuel pressure fluctuations that seemed to happen every time I climbed though 7-8k. I took an 8"x5" (ish, I dont remember the exact dimensions) piece of .030 aluminum sheet, bent it on a 45 angle about an inch in along the long edge, attached it to the engine mount with a couple of adel clamps to deflect the air down and away from the fuel pump. I haven't noticed those issues since so it seems to work for me. Sorry I don't have any pictures on my phone.
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  #10  
Old 02-03-2019, 06:03 PM
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Vlad Vlad is offline
 
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I routed a simple blast tube to fuel pump from the plenum. Safety wired the end of the cooling conduit at pump safety wires. RTV at the top I am sure it helped pump to get to 3,000 hours.
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