Fuel Flow while on Aux Pump

[istrumit]

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 ?

[Lionclaw]

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...

[DanH]

Originally Posted by istrumit

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

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

[GalinHdz]

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.

[Auburntsts]

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.

[lr172]

Originally Posted by GalinHdz

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.

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

Larry

[rightrudder]

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

[lr172]

Originally Posted by DanH

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

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

[istrumit]

Originally Posted by rightrudder

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

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...

[Bevan]

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