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Fuel pressure decrease in climb

Aiki_Aviator

Well Known Member
I recently have become aware of a need to watch my fuel pressure on climb. I have had some interesting readings of late and wanted to get some feedback/thoughts etc...

Engine: Thunderbolt YIO-540, Airflow Performance Fuel Injection

Today I completed a max altitude, with a new oxygen system test and kept an eye on my fuel pressure. Basically it continued to drop from initial of 26 psi down to 14 psi at 11,500 feet. I flattened out for a small time and it jumped back up to 22 psi then it dropped back to 14 psi when I recommenced my climb.

My climb rate was not excessive, 500 fpm. So I switched on my electrical pump and got it back to 27 psi.

After achieving 19,500 I flattened out and switched off my electrical pump and it settled back to 24 psi.

I am wondering if this appears normal, or if this seems a little "under done".

Eager for thoughts.

BTW FYI: OAT was 23 Celcius...
 
Do you have the fuel pump shroud with a cooling hose attached to it? And are you running the electric fuel pump until TOC?

Vic
 
I had an issue with a partially blocked vent line (not on the RV though) which I identified in flight by slow drop of my fuel pressure (from 25 to 16 psi) switching tank raised the pressure back to 25psi.
Probably not your case but did you increase your fuel flow during the climb ? Was all the flight performed on the same fuel tank ?
 
Just a little more theory.

You may already know all this, but just to lay the foundation: Fuel will have some turbulence and eddy's as it passes through sharp bends and expansions as it flows from the tank through the mechanical fuel pump. When the absolute pressure and temperature of the fuel is near the vapor pressure, it will create bubbles. These vapor bubbles do not condense back to liquid as fast a they form. If formed in the pump or when they reach the pump it will diminish the flow/pressure post pump. In reference to Vic's comment, the pressure and temperature of the fuel could be close to that point, so when the suction is increased due to an elevated pump location relative to the fuel level in the tank (nose up in climb), it is just enough to trigger this effect.

If you were low on fuel, using fuel with higher than normal vapor pressure, and clearly on a hot day, all the conditions are pushed to the limit. A direct number of flight angle could be correlated to a needed temperature reduction to eliminate the problem since in your case seems tightly defined. Its not necessary to solve the issue though.

Reduction of fuel temperature or reduction the suction should move it the right direction. Roughly, adding the cooling shroud to the fuel pump should clear it up - to Vic's question/point.

If your fuel flow transducer is located before the mechanical pump, reduced suction could be achieved by relocating it to a position after the pump.

Bastien: I think if it were a vent, it would actually help pressurize the tank with altitude increase as the air in the tank attempted to escape. If it were on descent, it could overcome the nose down suction reduction and create more suction due to the restriction. Good question about fuel flow.
 
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You may already know all this, but just to lay the foundation: Fuel will have some turbulence and eddy's as it passes through sharp bends and expansions as it flows from the tank through the mechanical fuel pump. When the absolute pressure and temperature of the fuel is near the vapor pressure, it will create bubbles. These vapor bubbles do not condense back to liquid as fast a they form. If formed in the pump or when they reach the pump it will diminish the flow/pressure post pump. In reference to Vic's comment, the pressure and temperature of the fuel could be close to that point, so when the suction is increased due to an elevated pump location relative to the fuel level in the tank (nose up in climb), it is just enough to trigger this effect.

If you were low on fuel, using fuel with higher than normal vapor pressure, and clearly on a hot day, all the conditions are pushed to the limit. A direct number of flight angle could be correlated to a needed temperature reduction to eliminate the problem since in your case seems tightly defined. Its not necessary to solve the issue though.

Reduction of fuel temperature or reduction the suction should move it the right direction. Roughly, adding the cooling shroud to the fuel pump should clear it up - to Vic's question/point.

Reducing suction could be achieved if the fuel flow transducer is located before the mechanical pump.

Bastien: I think if it were a vent, it would actually help pressurize the tank with altitude increase as the air in the tank attempted to escape. If it were on descent, it could overcome the nose down suction reduction and create more suction due to the restriction. Good question about fuel flow.


All good except I think you meant to say that reducing suction could be achieved if the fuel flow transducer is located AFTER the fuel pump.

Vic
 
I think you meant to say that reducing suction could be achieved if the fuel flow transducer is located AFTER the fuel pump.
Vic

To Bill and Vic's point...the popular Electronics International FT-60 has a relatively small internal passage, about 0.141" diameter to be precise. That is less than half the diameter of a -6 hose, and almost 6x less flow area.

EI asks that we not pull fuel through it, but rather, install it downstream of the engine driven pump, either in the line to the carb, or in the case of constant flow fuel injection, in the line to the fuel divider. Lots of us have ignored the advice without a problem, installing it in the line between the electric pump and the engine driven pump. However, stack up enough little factors and sometimes you get a big result.

Regarding fuel pump cooling shrouds, has anyone yet demonstrated that the fuel temperature is reduced by shroud use?

 
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Interestingly enough, the majority of the RV-10's out htere have the sensor installed in the tunnel, as per the plans. I had mine there for over 1000 hours in this airplane and recently moved it to the engine compartment between the fuel servo and the fuel distribution unit. I figured I had enough data and observation ow to see if it made a difference.

So, I have noted 2 observations--- the fuel flow indication does not change at all now when the electric pump is turned on or off. Second, fuel pressure seems to be about 4 PSI higher.

Vic
 
I also get low presure EFIS indication on my Superior IO360 equipped RV 8 when I am in a sustained climb. I have never noted any loss of power, so I'm still getting sufficient fuel. It occurs when I have a noticeable deck angle in a sustained climb. Stops immediately with activation of the boost pump. I don't have the red cube. My Flowscan is installed between the servo and spider. The pressure reading drops from mid 20's to mid teens psi.
 
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Interestingly enough, the majority of the RV-10's out htere have the sensor installed in the tunnel, as per the plans. I had mine there for over 1000 hours in this airplane and recently moved it to the engine compartment between the fuel servo and the fuel distribution unit. I figured I had enough data and observation ow to see if it made a difference.

So, I have noted 2 observations--- the fuel flow indication does not change at all now when the electric pump is turned on or off. Second, fuel pressure seems to be about 4 PSI higher.

Vic

Yes - on the transducer location wording. It was confusing. It is clear now.

Interesting on the 4 psi. The EI pressure drop for 12 GPH is only .090 psi. But that combined with the acceleration/decel of the fuel through that orifice clearly has a much larger effect on the "system". On initial observation it would not seem to make any difference, but all the factors of a real world test are undeniable. Excellent data.
 
Regarding fuel pump cooling shrouds, has anyone yet demonstrated that the fuel temperature is reduced by shroud use?

I have never instrumented to obtain temp delta data, but I can say for certain that adding a mechanical fuel pump cooling shroud fed with a SCAT hose sourced from the right rear baffle, to an RV-10, had a noticeable effect on fuel pressure drop during climb in hot conditions.

There can be other factors also though.....

I have changed fuel pumps that were having a hard time maintaining fuel pressure in a hot conditions climb (in both instances they also wouldn't get to nominal pressure in level flight cruise), and after the pump change, the fuel pressure reduction during hot climbs was noticeably lessened. Not meant to imply that a dip in fuel pressure indicates a failing pump (because it can happen with new pumps), just a point to watch. If it has never been normal four your temps to reduce in hot conditions but they begin too, changing the pump might be warranted.
 
Here's another datapoint on the FT60 location, I'm chasing a problem now on my 9A and I am pretty sure I've got it figured out but haven't had a chance to confirm the final fix in flight, gimme a few days.

I mounted my FT60 on the fuel line just forward of the pump shroud, immediately aft of the firewall, just below the heated air inlet to the cabin. All was well until this winter when I started flying with full heat on pretty regularly. Coincidentally with that I started seeing fuel flow variations on my EFIS, sometimes swinging by a 2-3 gph over a few minutes, that had me strongly suspecting either a fuel leak somewhere or a problem with the fuel injection servo. Datalogging and graphing the egine CHT/EGT against the fuel flow quickly indicated that I was definitely NOT burning that extra fuel - and I could find no evidence of a leak. I watched the fuel computer closely for total gallons burned versus total gallons pumped back into the tanks, and realized the fuel computer was short of fuel everytime. The extra flow is not real. I finally connected the dots and figured out that hot air was blowing directly on the FT60 where I have it mounted, and I am about 98% certain that I was heating that device sufficiently to make vapor bubbles in it, accounting for the additional flow.

I have modified the heat-directing aluminum vents just above the FT60 where it penetrates the firewall, and I anticipate that will solve my problem.

Bottomline - the FT60 is subject to vapor bubbles just like the mechanical pump - don't put it someplace where it will get too hot. Mounting it in the upper deck area above the cylinders should be just fine, that will be the coolest spot FWF in flight.
 
two points to consider in your diagnosis. The mechanical pumps are variable in flow and pressure and after a certain flow rate they are inversely proportional. However, we are using under half the max flow rate, so we don't see much pressure drop and our flow rates. However, a worn pump will potentially show these traits, as the max flow rate is reduced.

Second, the sensor is dynamic and operates on a similar principal to your ASI / pitot static system. If the static pressure vent is partially blocked or very slow to bleed air, your fuel pressure reading will decrease during rapid reductions in air pressure (i.e. climb). Once level and stable pressure, the bleed port will allow the pressure on the back side of diaphragm to eventually equalize and you'll see the measured pressure come back up. This was the problem Dynon had with the initial Kavlico's (they issued an SB). The seal was too tight and they wouldn't equalize or were slow to do so. Most of these parts are made for applications that don't see rapid pressure change.

Larry
 
I have heard of this issue on all RV's and I don't think it is a vapor or fuel pump issue.. I think it is inherent to the design of the wing and tank venting system, as it happens a lot to all versions of the plane. Don't know this as fact but if you search the archives I am sure you will see it come up several times. I supplied many engine driven pumps under warranty to many of my customers just to prove that the pumps weren't an issue. Old pump same results as the replacement every time.... Redoing fwf fuel cooling and heat protection didn't have an effect either. Happens on the low pressure engine systems as well.

Good Luck,
Mahlon
 
Diesel tech by day here, a long shot but you may want to check this off of a possible cause, I have seen in the past, a fuel pressure problem caused by air being sucked into the fuel supply line while under a high volume demand, ( full fuel ) and then the problem goes away at idle ect, the problems I have seen is where a fuel line connection would pull air in and not leak fuel out while static. very hard to find. I would check all fuel supply lines carefully for this.
 
Every now and then my fuel psi will fall for no reason on my IO 360 engine mounted pump. A few seconds with the boost pump always corrects it. It has never happened during climb. Only when Ive been cruising a while usually LOP and more often than not, shortly after refueling. The last time it happened was several months ago and I had been cruising straight and level for about an hour and made a 360 deg turn to get a second look at something. A year ago, I moved my Red Cube from the firewall between the mechanical pump and the servo to the engine mount in a more level position and replaced a 90 degree elbow entering the Red Cube and insulated it with fiberfax and aluminum tape. It helped some but it has happened a couple of times since. No leaks in the fuel lines and the air vents are clear. You can do a search on my situation and get more on the story. I do not have a shroud on my fuel pump. The fuel could be heating up in my tunnel but it has never happened during descending when the flow is very low. Its a puzzler for sure but I have gathered from reading these forums and talking to RV pilots, I'm not the first one with this situation.
 
Some of the fuel pressure problems here may be due to ground loops--sender and gauge grounded at different points, small current induced in the loop that interferes with the tiny sender values. I pulled my hair out over erratic fuel pressure indications for a long time. They were sometimes consistent but weird; sometimes just weird. Switching the grounds to the same point solved it.
 
MikeyDale:

Same exact thing is happening with my RV-8. In a strange way, it's comforting to know that I'm not the only one. I actually started wondering if my engine driven fuel pump was going bad. But a quick search of VAF posts on the subject helped me to realize that I'm just one of many.

Anyway, in my case (and perhaps yours), it seems unlikely that fuel temperature is the cause. I cruise LOP, usually between 7,500 and 9,500, fuel flow is about 7.5 gph, and CHTs are all around 340 F. TAS is 170 kts at these settings, so there's plenty of cooling air flow in an engine compartment that's relatively cool already.

The only thing I can think of is the reduced mass air flow due to higher pressure altitude, but then I wouldn't think that cruising at 9,500 ft would cause sufficient mass air flow reduction to decrease the cooling effect and allow the fuel to heat up.

This is a head scratcher indeed!

Every now and then my fuel psi will fall for no reason on my IO 360 engine mounted pump. A few seconds with the boost pump always corrects it. It has never happened during climb. Only when Ive been cruising a while usually LOP and more often than not, shortly after refueling. The last time it happened was several months ago and I had been cruising straight and level for about an hour and made a 360 deg turn to get a second look at something. A year ago, I moved my Red Cube from the firewall between the mechanical pump and the servo to the engine mount in a more level position and replaced a 90 degree elbow entering the Red Cube and insulated it with fiberfax and aluminum tape. It helped some but it has happened a couple of times since. No leaks in the fuel lines and the air vents are clear. You can do a search on my situation and get more on the story. I do not have a shroud on my fuel pump. The fuel could be heating up in my tunnel but it has never happened during descending when the flow is very low. Its a puzzler for sure but I have gathered from reading these forums and talking to RV pilots, I'm not the first one with this situation.
 
Same issue...

I have been seeing a loss of fuel pressure since the first flight. A couple of things to note, it happens on on initial start at low rpm, or during an extended climb. I have an RV 7 with a stock o-360. I have replaced the mechanical pump, insulated all fuel lines fwf, added a cooling shroud and blast tube, all to no effect. I can resolve the issue instantly by eith lowering the nose, or turning on the boost pump. It doesn't seem to matter which tank is in use or how much fuel is in the tank. I'm just not sure where to go from here! By the way, my fuel system is per Van's plans, and I had the same issue before adding a red cube. I'm sure the pressure drop is real, as I have experienced momentary rpm drops when pressure falls below 2 psi on the EFIS. Frustrating! Any additional thoughts appreciated!
Thanks, Jim
 
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