Poll: Engine Driven Fuel Pump Cooling

If you have an opinion on whether a cooling shroud and blast tube is necessary on the mechanical, engine driven fuel pump on a Lyc or Lyc clone, please answer the poll and post here if you like. Thanks!

I had to put other

For the simple reason that a mechanical fuel pump in the first place is a hydraulically poor solution.

If I HAD to live with a mechanical pump then forced cooling is an absolute essential in my opinion.

Frank

I checked "other".....

A pump duct and shroud is a cooling drag producer (increased mass flow) and probably reduces cooling elsewhere (reduced pressure difference across the cylinders and oil cooler; it's a leak of sorts). Perhaps it is worth considering why it gets hot.

Ya'll check me. I see only three sources. There's conduction from the engine case and engine oil, hot air downstream of the cylinders, and radiated heat from the exhaust system.

Ballpark, I'd guess the case and oil would be 200-250F. Cylinder air would would ballpark at 150F in cruise and surely be less than 200 the rest of the time. Radiated heat from the exhaust? There's a wildcard. I suspect it could be significant given two pipes in close proximity. Anybody here familiar with the physics of energy radiated from hot bodies?

A few notes.

Case/oil temp would be higher than what we see on a panel gauge, since that measurement point is after the oil cooler.

The final temperature of an object is a matter of heat input vs heat output.
Cylinder air is cooler than the case/oil temperature and thus probably cooler than the pump temp. Cylinder air is probably a cooling media, not a heat source.

I recall James (Flyeyes) posting the only comment I've ever seen about actually measuring temperatures in the accessory case area. You out there buddy? Got some numbers?

fuel type

I'm with the "Flying - no probs - 100LL" group.

Steve

I marked "other". The cooling issue is when the aircraft are on the ground and the fuel system heat soaks leading to vapor lock. Unfortunately, a blast tube isn't going to help in that situation because it ain't blasting until you get the engine started, and at that point you don't need it...

DanH said:

I checked "other".....

Ya'll check me. I see only three sources. There's conduction from the engine case and engine oil, hot air downstream of the cylinders, and radiated heat from the exhaust system.

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I'd add one more, forced convection from the oil cooler, which, if baffle mounted, puts the fuel pump directly between the oil cooler exit and the cowl exit. I would suspect that this is one *might* be the worst offender?

Kyle Boatright said:

I marked "other". The cooling issue is when the aircraft are on the ground and the fuel system heat soaks leading to vapor lock. Unfortunately, a blast tube isn't going to help in that situation because it ain't blasting until you get the engine started, and at that point you don't need it...

Click to expand...

So with FI systems which use a second, high pressure pump upstream which is ostensibly immune from vapor lock, this problem goes away, no?

I did not have a shroud on the fuel pump, but I did aim the exhaust from my vac pump at it for cooling, per Van's instructions. Only works if you have steam gauges with a vacuum pump, though. No problems noted in 4 years.

Roberta

I may change plans

I had planned to have a blast tube without the shroud on my fuel pump, along with alt and mag, but now I confused if that is worth it without the shroud and I should just leave it off the fuel pump for now.

The shroud is alot of money and I thought to just direct a blast tube towards the pump.

I have just the blast tube but if the cooling shroud was available before I hung my engine, I would have added it then. Without the shroud, it's hard to get the blast tube positioned to cool the pump.

Also, with regards to the statement:
"The cooling issue is when the aircraft are on the ground and the fuel system heat soaks leading to vapor lock. Unfortunately, a blast tube isn't going to help in that situation because it ain't blasting until you get the engine started " , I agree totally. But I had the good fortune of getting the suggestion to plumb a purge valve off the carb while I was building.

Basically, just used a port off the carb to a firewall mounted electric valve (I used a primer solenoid) and routed the output back to one tank. I used an off-on-mom switch for the fuel pump. In the "mom" position, the pump is on and the valve is open. So electric pump pumps the hot fuel back to the tank. It's makes a very noticible difference in the fuel pressure when the line is purged before starting the engine.

I believe there are two different issues that folks have. The first is a true vapor lock where it is nearly impossible to get the engine started. The second is an inconsistent fuel flow to the fuel injection nozzles causing a very rough running engine. My Superior IO-360 exhibits the second during very hot ambient conditions with a fuel stop of around 20 minutes. It starts normally but is very rough at low power settings. There was a post about this in the recent past that discussed the problem of a combination of fuel being heated by the mechanical fuel pump and very low pressure (approx 1/2 psi) at idle in the fuel lines downstream of the fuel servo. I purchased a shroud for the mechanical fuel pump and intend on installing it with a blast tube and additionally insulating the lines from the spider to the individual nozzles to alleviate some of the heat soaking. My problem disappears when power is applied for takeoff and the fuel pressure from the servo is increased. BTW I have gotten some interesting looks when I tell people the engine will run fine when we takeoff.

Rockyjs said:

I believe there are two different issues that folks have. The first is a true vapor lock where it is nearly impossible to get the engine started. The second is an inconsistent fuel flow to the fuel injection nozzles causing a very rough running engine. My Superior IO-360 exhibits the second during very hot ambient conditions with a fuel stop of around 20 minutes. It starts normally but is very rough at low power settings. There was a post about this in the recent past that discussed the problem of a combination of fuel being heated by the mechanical fuel pump and very low pressure (approx 1/2 psi) at idle in the fuel lines downstream of the fuel servo. I purchased a shroud for the mechanical fuel pump and intend on installing it with a blast tube and additionally insulating the lines from the spider to the individual nozzles to alleviate some of the heat soaking. My problem disappears when power is applied for takeoff and the fuel pressure from the servo is increased. BTW I have gotten some interesting looks when I tell people the engine will run fine when we takeoff.

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I submit that your "rough engine when hot" problem goes away when you power up for takeoff because there is a LOT more fuel moving through the fuel system. At idle, your fuel system is only moving a gallon or two of fuel an hour. At takeoff power, you're probably moving 15 GPH. A heat soaked fuel system that causes a 30F increase in fuel temperature at 2 GPH only causes a 4F increase at 15 GPH.

Noah said:

I'd add one more, forced convection from the oil cooler, which, if baffle mounted, puts the fuel pump directly between the oil cooler exit and the cowl exit. I would suspect that this is one *might* be the worst offender?

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Falls in the "hot air from the cylinders" category, although oil cooler air should be lower temperature than cylinder air. Better heat exchanger efficiency, but roughly half the delta-T.

I'm guessing the worst offender to be radiant heat from the pipes. I suspect a heat shield below the pump might be as effective as forced air cooling and without cooling penalty. The "problem" doesn't seem to be a hot pump in normal flight, but rather, heating during ground operation.

Besides, it is possible to kill two birds with one stone, maybe lowering cooling drag even further

frankh said:

For the simple reason that a mechanical fuel pump in the first place is a hydraulically poor solution.

If I HAD to live with a mechanical pump then forced cooling is an absolute essential in my opinion.

Frank

Click to expand...

A hydraulically poor solution in your opinion it might be, but 6,000+ flying RV's tell a different story....

Just trying to present both sides of the equation when one might be considering reivention of the wheel here.

Cheers,
Stein

Beautiful solution, Dan..

DanH said:

Falls in the "hot air from the cylinders" category, although oil cooler air should be lower temperature than cylinder air. Better heat exchanger efficiency, but roughly half the delta-T.

I'm guessing the worst offender to be radiant heat from the pipes. I suspect a heat shield below the pump might be as effective as forced air cooling and without cooling penalty. The "problem" doesn't seem to be a hot pump in normal flight, but rather, heating during ground operation.

Besides, it is possible to kill two birds with one stone, maybe lowering cooling drag even further

Click to expand...

Sweet. Is it fabbed from stainless, Dan? I wished the -6/7/9 came with a radiused firewall bottom.

Regards,

Pierre,
Just leftover 2024 removed from the original turtledeck. The underbelly ramp aft of the firewall is stainless.

My primary intent wasn't pump shielding, and I'm not claiming any improvement in that regard just yet. We'll see.

From Formula 1 racers...

...at Reno, also the big Bearcats I helped work on years ago. Their firewalls are radiused all the way around and clean airflow resulted.

To do this on a -7 would entail some work but I think there is a lot of speed still available there.

Thanks,

I used it "cause"

I got my engine used and it came with a lot of stuff. It was an imported engine, from England, originally installed in a Bulldog.

It had a complete set of baffles, shrouds, scat tubes, oil and fuel hoses, gascolator, inverted oil system, generator, starter, bendix mags, woodward prop governor and was mounted in a big yellow military shipping crate. Everything except an oil cooler.

It had mag cooling tubes and a fuel pump shroud. I figured that since the Bulldog was used as a fighter trainer, those military guys had all those tubes and shrouds on there for a reason, so I transplanted them all to my aircraft.

Of course, I'm sure those guys worked that engine a LOT harder than I ever intend to.

I added the tube....

But do not have a shroud. I place a temp probe at different location around the fuel pump and didn't get much of a diff in reading.

Kent

Kent,
What temperatures did you see?

Dan, as I was typing...

DanH said:

Kent,
What temperatures did you see?

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I just knew that someone was going to ask that question. Of course I don't remember. I'll look around my computer tonight and see if I can find the data.

Kent

Dan,
Here are some temps from another posting. See # 24

http://www.vansairforce.com/community/showthread.php?t=28871&highlight=fuel+pump+temperature&page=3

No shroud for me!

Well, as a result of this poll, I guess I'll hold off on installing that new shroud I bought from Vans. I already have an Airflow Performance purge valve installed, as well as an Airflow Performance high-pressure FI pump, so pre-soak during quick-turn fueling ops in the summertime shouldn't be a problem with the option to completely flush the fuel system (including the mechanical pump) with cool, high pressure fuel. The only potential issue I can see is lean ops / stumble during taxi as described by somebody here but no safety of flight issues. So if I have to go back later to put it in so be it. I already have the pump installed and safety wired so its not like it would be super easy to put it on now, anyway. Thanks for all the responses, very helpful!

no shroud, but. . .

I usually have to prime a little for starts after hot shutdowns with a short holdover time (ie fuel stops). either prime a little before cranking, or hold the primer & start button at same time. a touch of vapor lock, I guess. O360, 1 mag, 1 EI.

dual muffler exhaust exacerbating the problem?

I'm wondering if my dual muffler Vetterman exhaust is contributing to my rough idle when hot issues. I have used the cabin heater, which is off of just one of the muffler heat shrouds and it puts out very hot air. Now multiply that by two, and having the cabin heater off dumps all of that hot air down behind the motor, near where the fuel lines and mechanical pump are located. I have the AirFlow Performance purge valve, so getting cool fuel back into the systems for hot starting isn't an issue, but it does get your attention when the engine sputters to a halt taxiing off the runway after landing. I wouldn't want that to happen on final approach!

I also picked "other".
Last summer I flew my T-18 to the Arlington WA Airshow. It was a hot day by our standards.
The fuel was 91 Octane ethanol free Mogas. I had not removed the air restrictor plate in front of the oil cooler, based on log book entry advice from the previous owner. This engine has the old style cooling system with out the Vernatherm. On the way I noticed the oil temp approaching 200f.
N18TE sat on the sunny flight line with the other T-18s and a Mustang II, near the cheerful pack or RVs. This is the heat soak period.
I experienced a slight stumble during departure from the pattern, which seemed to correct with mixture control (I told myself). It got worse over the next half hour, to the point I was more concerned than I had been flying behind my Mazda Rotary powered RV-8. My wife was downright upset.
We had just passed Crest airpark heading for Kiona Airpark when my tolerance for abuse ran out. The oil temp was above 200f. I turned back & landed @ Crest (my RV-8 is hangared there). My son was working on his Pitts S1E. We decided to switch to 100LL, so I decanted out enough mogas to allow a 50% - 50% mix. I should have remembered to remove the air flow restrictor to the oil cooler as well. None-the-less, the engine ran fine thereafter.
Later I removed the plate and lowered the oil temp, so I agree with Dan H about high oil temp soaking into the mechanical fuel pump. Heat soaking the fuel tank in the sun is also a factor. Another factor could be the age of the fuel, also, it was probably winter blend mogas. The worst of all possible combinations.
I replaced the mechanical fuel pump during the condition inspection as a precautionary measure (no way to know how old the pump was) and was rewarded with higher fuel pressure. I tried to install a pump shroud, but the one I have won't fit the old style laminated arm pump, even after mods to the shroud.
An Exhaust heat shield seems a good idea, I'll take that advice.
One other thing, CHT's have gone up about 20F since I added one Pmag, from about 330f to about 350f. Perhaps that factoid should be in a Pmag thread, but it would effect oil temp, thus invalidating the log entry about seasonal use of the oil cooler plate.

Posted Flying no shroud no problems.
300 hrs.
Caveat is that max ambient has rarely been over 95F.

Don't remember any conversations specifically about FI vs Carb installations regarding heat induced fuel vaporization in the lines.

My assumption is that the higher fuel pressures forward of the firewall on FI systems would be more resistant to fuel boiling than the lower pressure carb systems.