Fuel Flow while on Aux Pump
 

Hello -

On my fuel injected 540, I have noticed that my fuel flow in level flight increases significantly if I turn on the aux pump.

For instance, a few days ago, in stabilized level flight as 7500 feet burning 12 GPH. I wanted to see what the fuel pressure did if I turned on the aux pump.

When I turned on the aux, the pressure came up a couple of PSI as I would expect, but the fuel flow went from 12-ish to 15-16-ish.

Why is this ? I am assuming this is not making it to the engine for combustion (nothing about the engine measurements change).

Is there some sort of over-flow return line path in this case ? Meaning, the excess is returning to the tank but still getting measured as net flow by the transducer ?

My carbureted -9A did the same when the Facet pump was on, and I did not have any sort of return line. I assumed it had something to do with turbulent pulses of pressure from the pump... but I too am curious about the true cause...

Let's see now...you have a red cube transducer in the line between the electric pump and the engine-driven pump, right?

If so, what you're seeing is universal. Nobody has been able tell us the exact physical reason. We're pretty sure the rotor is collecting more counts; why is the question.

It won't do it if the cube is installed in the line between the fuel control and the flow divider.

I now have a dead red cube on my bench, soon to be dismantled in pursuit of knowledge ;)

FWIW My Red Cube is connected after the fuel selector but before the auxiliary pump/mechanical pump and I have the same issue. Lycoming O360-A4M. Curious minds want to know.
:confused:

Just a data point to back up what Dan said about cube location. My cube is mounted after the mechanical fuel pump and FF is unaffected by the electric pump's operating status.

I am pretty sure the manufacturer of the Red Cube indicates poor performance if installed on the suction side of the pump.

Larry

It's a good reminder to turn your aux pump off...when fuel flow reads 11 gph when it should be 8, or whatever.

Just a guess here, but I think you'll find the answer to be pressure differential or pressure related. In a pressure application, both sides of the cube are constant. In a suction applications, the input side is going to see fluctuation based upon slop/movement and fuel level height in the tank. Because the vacuum/negative pressure is so low it is easy for slop to get introduced. I would expect that the cube is designed/engineered for pressure and a negative pressure environment would require a different design. They infer as much in their documentation and require installation on the pressure side of the pump. If the orifices are small, it could create a higher vacuum on the output side due to that orifice that wouldn't exist on the input side.
Larry

I guess I feel better about it, seeing everyone else's responses...meaning, it seems that 1) everyone with the same transducer location experiences this and 2) its a measurement error.

Would love to get a good answer as to why...

If the red cube measures fuel with a wheel turning in the flowing fuel, and some device to count the paddles of the wheel as they go by, and if this red cube is located in the fuel line between the two pumps, it would seem to me that the increase in readings would be caused by a hydraulic interaction between the two pumps. One would think that the boost pump is a more constant flow/pressure than the diaphragm type mounted to the engine. This would cause the fuel at the measuring device to have a component of back and forth movement as well as pure forward movement. This back and forth movement would be picked up by the counter as and extra "count" at certain points in its rotation hence the larger reading.

My thoughts only.

Bevan

Following the above theory Matronics sells a damper (vertical tube with trapped air at the top) which they claim eliminates the error. Has anyone tried it? I see the same thing, although not as large as reported above.

I solved this and another problem by relocating the flow sender (Floscan) to the line between the fuel servo and the spider. Now my readings don't change with application of aux pump.

A pressure damper may work but where do you put it? before or after the measuring device or both?

This would add more fittings, weight and possibly more volume of fuel in the engine area which could be subject to absorbing heat depending on location.

Bevan

That is precisely the explanation I've heard numerous times since 1999 (RV-6, Facet pump) when this question arises.

Had the same problem on the 10. Moved it to between the servo and the spider. As rightrudder mentioned, now I forget to turn off my fuel pump :)

One negative of having the flow sender between the two pumps is the slightly inaccurate fuel totalizer numbers. Not a big deal though unless you use the aux pump for longer periods. Even then you're still on the safe side cause you'll have more fuel in the tank than the totalizer reports.

The sender I had in the tunnel was a FloScan and it was jumping around too much for it to be accurate enough to collect data for LOP injector balancing. The redcube is much smoother, probably because of its new location.

Lenny

Hi guys,

Maybe I can offer a bit of insight. When the FT-60 is mounted between the boost pump and the engine driven pump and the boost pump is turned on, it creates the hammer effect against the engine driven pump similar to what is observed in residential plumbing. We've all heard the knocking pipes. This is what happens in a the fuel system, to a much lesser degree of course. However, when this does occur, it causes the impeller of the FT-60 to rotate forward then backward quickly. Those additional rotations are counted and displayed as higher fuel flow. That is the primary reason why we recommend the transducer be mounted after any and all fuel pumps.

My experience too, having the sender b/w the tank selector and boost pump. Since I do a lot of pattern work when it's not cross-country season, having the boost pump on all the time makes my fuel totalizer pretty much useless. One of my next projects is to place the sender between the engine pump and carb.

Thank you David. That makes perfect sense. Understanding it means I won't worry about it. I plan to leave it as is and treat it as a reminder to turn off the pump !

Fuel flow indication
 

My red cube is between the electric and mechanical pumps, and it used to read acurately before I swapped my Champion mechanical pump for a Tempest pump. Now fuel flow reads high with boost turned on. It certainly caused me concern until I followed up the related threads.

There is/was nothing wrong with either of my mechanical pumps; I damaged the Champion while attempting a pump shroud installation. The replacement Tempest unit was verified by my engine shop after I noticed the erroneous flow readings.

I will replace all my hoses at 5 years, and relocate the red cube near the servo at that time. I would be curious to see if anyone else has correlated the erroneous flow readings with a specific brand of mechanical pump

Jay

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