Fuel flow varying/wondering during cruise

[FireMedic_2009]

I have G3X with a full engine monitor on my RV3. My fuel flow varies/wonders by 0.6gph at cruise over a 5 sec interval. I know the fuel line has to be straight 6” before and after the fuel cube however it a little difficult with a narrow firewall so one of the lines may be 4”. And I believe the fuel cube should be at or below the height of the carb. So I’m wondering if this is normal for carbureted engines since the float starts and stops fuel flow constantly thus causing the fuel flow to wonder around

Thanks
Daren

[Steve Barnes]

I'm wondering if you mean wandering?

[PaulvS]

Fuel flow readings on my Dynon fluctuate around 2-4%, at the same throttle setting (Rotax), it hasn't indicated any problems or been any cause for concern.

[TS Flightlines]

FT60 red cube, or Flo Scan?

Tom

[FireMedic_2009]

It’s a flo scan. When I had my RV6A the fuel flow was rock solid on the IO-320.

So is it normal for a carbureted, not fuel injection, to fluctuate???

[lr172]

Originally Posted by FireMedic_2009

So I’m wondering if this is normal for carbureted engines since the float starts and stops fuel flow constantly thus causing the fuel flow to wonder around

Thanks
Daren

The float doesn't work that way. It does not really start and stop flow and will definitely NOT cause fuel flow to wander, especially at higher flow rates. Having the sensor upstream of the pump can cause issues like this, though not universally.

If the sensor is downstream of the pump, I would speculate that the issue is in the floscan unit. An erratic fuel pump could cause these symptoms. Have you tried turning on the boost pump and observe if the problems remains or disappears?

Height of the floscan shouldn't matter.

Larry

[Ironflight]

The short answer is that this is not uncommon, in my experience - I have a FloScan on the carbed RV-8, located per Van’s plans (from 20+ years ago) down by the left foot, (downstream of the boost pump, upstream of the firewall), and see fluctuations of probably +/- 0.2 (or 0.3) in cruise. That’s with my GRT EFIS, so I don’t think the EFIS has much to do with it. The good news is that it wanders around the mean flow, so the effect on the totalizer quantity is effectively zero. That’s 2080 hours of experience.

The answer is to “why” is also short - “I don’t know”……

(Similar setup on our -6, but with a G3X Touch system, and we see the same thing….)

Paul

[lr172]

Originally Posted by Ironflight

have a FloScan on the carbed RV-8, located per Van’s plans (from 20+ years ago) down by the left foot, (downstream of the boost pump, upstream of the firewall), and see fluctuations of probably +/- 0.2 (or 0.3) in cruise.
Paul

From my research, the mechanical fuel pumps do not produce pulsing on the pressure side, BUT DO produce pulsing on the suction side. Input check valve is open on the pump upstroke and closed on the down stroke, creating the starting and stopping of flow with the slightest amount of backflow as the check valve closes. I believe this why the flow sensors are recommended to be installed downstream of any pumps. Haven't looked at floscan, but EI definitely makes this clear for the red cube in their documentation. The constant starting and stopping of flow on the suction side makes the sensor go wonky. It is counting the vanes moving by and the starting / stopping makes this very difficult to smooth out, as the vane can reverse a bit when flow stops and can be counted twice if the vane stopped right at the sensor and reversed a bit. The sensor is not directional, it just counts vanes moving past and therefore, reversals are not accounted for. The red cube is optical, but don't know if the floscan is optical or something else. Optical is VERY binary and the slightest fraction of movement in reverse can result in double counting a vane IF the van stopped very close to the sensor.

I suspect in your case, the fluctuations stop when the boost pump is on (iow, sensor now on the pressure side).

Both of my planes have the cube downstream and flow is solid as a rock in cruise.

Larry

[Ironflight]

Originally Posted by lr172

From my research, the mechanical fuel pumps do not produce pulsing on the pressure side, BUT DO produce pulsing on the suction side. Input check valve is open on the pump upstroke and closed on the down stroke, creating the starting and stopping of flow with the slightest amount of backflow as the check valve closes. I believe this why the flow sensors are recommended to be installed downstream of any pumps. Haven't looked at floscan, but EI definitely makes this clear for the red cube in their documentation. The constant starting and stopping of flow on the suction side makes the sensor go wonky. It is counting the vanes moving by and the starting / stopping makes this very difficult to smooth out, as the vane can reverse a bit when flow stops and can be counted twice if the vane stopped right at the sensor and reversed a bit. The sensor is not directional, it just counts vanes moving past and therefore, reversals are not accounted for. The red cube is optical, but don't know if the floscan is optical or something else. Optical is VERY binary and the slightest fraction of movement in reverse can result in double counting a vane IF the van stopped very close to the sensor.

I suspect in your case, the fluctuations stop when the boost pump is on (iow, sensor now on the pressure side).

Both of my planes have the cube downstream and flow is solid as a rock in cruise.

Larry

Good theory, and I agree with most of it, but alas - the fluctuations do not stop with the boost pump on or off…but the flow does become greater by about 1.5. Fortunately, we don’t fly with the boost pump on for very long, so again - it makes little difference to the totalizer fuel quantity.

[Carl Froehlich]

How the fuel flow sender is mounted makes a difference. The rules I stick to:
- Do not mount on the engine. This goes against how Van’s tells you to mount it on the RV-14 - which I had first hand experience with huge fuel flow measurement issues. After moving the sender off the engine the problem was resolved.
- Mount it down stream of the mechanical fuel pump. On the first RV-10 I mounted it in the tunnel like Van’s says to do (between the boost and mechanical fuel pumps). It was accurate unless the boost pump was on - then it shot up 1-2 GPH.
- Mount it so the hose run from the sender to either the carb or fuel manager is up hill. Need not be much, but up hill.
- No 90 degree fittings - other than the nice smooth tube kind.

Of late I mount the sender on the engine mount using Adel clamps.

Carl