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Keeping fuel pump cool

charrois

Member
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
 
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. :eek:
 
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
 
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
 
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.
 
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.
 
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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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.
 
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.
 
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.
 
A few things I?ve learned about preventing vapour lock. It?s caused by one or a combination of factors. This is nothing new for most of you but judging by some of the comments above, and for the sake of the newbies, I?ll mention what I?ve learned.

The higher the elevation, the closer you get to vapor lock conditions when sucking the fuel through a restriction upstream such as a fuel flow sender all things considered equal. In theory, if you could climb high enough, probably any non pressurised liquid will boil at the ambient temperature. Vapor lock is caused by heated fuel, low pressure (sucking rather than pushing the fuel) and sharp corners/obstructions to the flow causing turbulence inducing bubbles to form which then collect in a high point down stream until a bigger bubble is formed (or combinations of these factors and I?m m probably missing a few). The mechanical fuel pump on the back of the engine cannot pump the bubble.

Put a 2? flange on the firewall heater control valve exit for a scat tube to control the discarded hot air and route it down to the lower cowl exit area. Do not simply stop airflow from going through the heat muff. Unneeded hot air is dumped overboard to prevent overheating of the muffler/heat muff if this is what you have.

Use a heat shroud around the engine?s mechanical fuel pump supplied with a supply of cold air from the upper plenum.

Minimize the volume of fuel on the engine side of the firewall (hot zone). FWF temperatures after engine shutdown increase due to succession of air moving through the engine compartment. Devices that hold a volume of slow moving fuel such as filters and gascolators have more time to transfer heat into the fuel. If you just have to have them, insulate them and protect from sources of heat and radiant IR.

My 2cents worth.

Bevan
 
Thanks for the suggestions everyone. It's good to know there is a way of putting a fuel pump shroud on without having to remove the pump entirely. I already have a simple blast air tube directed towards the pump, but I'll look into putting a deflector plate in at the valves, because it should be quick, easy, and relatively straightforward. If that still doesn't help enough, at least the shroud route doesn't have to be as invasive as I thought.
 
cool trick

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.
Bruce - very clever! I didn't put a shroud on because I'm not sure I'll need it, and I felt bad removing the factory-installed pump with it's beautiful safety wiring. Your trick changes everything. Thanks!
 
Rather than band-aid the pump with a cooling shroud, try to determine the actual heat sources and eliminate them.

The problem can be broken down into two categories, (1) heating the fuel before it reaches the pump, and (2) heating the pump itself.

The RVs with fuel routed up the cabin centerline can transfer heat to the fuel through the floor; the source is engine cooling air outflow, and exhaust flow. Measure the floor and tunnel temperature. Insulate the belly skin externally if desired. Add tailpipe length or point it/them downward.

RVs with a bunch of firewall forward fuel plumbing upstream of the pump pick up heat from the engine compartment air. Components like filter bodies, gascolators, and fuel flow devices make it worse. Relocate the extra parts to a cool area. Perfect is a short, insulated line from the firewall bulkhead fitting to the pump, and nothing else.

Can't do much about engine block heating of the pump, but I suspect a few measurements would find that block heat isn't a big contributor. The overlooked wildcard is radiant heating from the exhaust tubes. It's not unusual to see a pair of pipes near or under the bottom of the pump. Think about it. The only parts of the pump which can significantly heat fuel are the inlet chamber and the valve disk. Both are at the bottom of the pump, and isolated from the engine block by rubber diaphragms. The thermal conductivity of rubber is more than a 1000x less than aluminum. So if there is no efficient conductive heat transfer path from the 200F engine block, might we consider the 600~800F tailpipes shining on the bottom of the pump...you know, where the fuel is located?

Pump%20Cooling.jpg
 
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Inlet fuel lines pressure tested?

Don't forget, a tiny leak on the suction side will create many of the same symptoms.

Tunnel temps? It is a 10, right?
 
Showplanes pump shroud

Incidentally I have a Showplanes shroud that I was planning to use when I lived in TX. Don't really need it now that I'm back in the frozen tundra of WI so PM me if anyone is looking for one.
 
The overlooked wildcard is radiant heating from the exhaust tubes. It's not unusual to see a pair of pipes near or under the bottom of the pump. Think about it. The only parts of the pump which can significantly heat fuel are the inlet chamber and the valve disk. Both are at the bottom of the pump, and isolated from the engine block by rubber diaphragms. The thermal conductivity of rubber is more than a 1000x less than aluminum. So if there is no efficient conductive heat transfer path from the 200F engine block, might we consider the 600~800F tailpipes shining on the bottom of the pump...you know, where the fuel is located?

This would be my primary area of concern if I was chasing your issue. I was dealing with boiling fuel in my injector lines (a unique installation problem) on my 6 and undertook several steps in an attempt to reduce absorption of heat across the fuel distribution network in the cowl. I made heat shields (easy to make from .063 6061) for the exhaust pipes near the servo, as I believed that I was picking up a lot of heat from there. It made a noticable difference. Radiant heat from the exhaust is often much more problematic than the hot air blowing around.

Larry
 
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