Fuel Flow and Fuel Pressure variations

[redbaron1982]

Hi guys, are these fluctuations normal in FF and FP? As you can see FF and FP correlates, as in everytime there is a drop in FP there is a drop in FF. I don't think this is a sensor issue.

[skykrawler]

Look at the specs for the fuel flow on the indication system.   Digital instrumentation is analog at some point and there is a minimum resolution and then there is analog noise.

An analog to digital convertor has a limited number of bits to work with....the resolution of least significant bit determines the best case accuracy of the system.   The fuel flow spread in the plot is ~.5 gph.   The instrument display may be (probably) filtered for aesthetic reasons.  The recorded data may not be filtered.

 

 

[N201MKTurbo]

It probably is somewhat real. If you add a snubber it will reduce the effect. The mechanical gauge will somewhat filter the pulsations from the fuel pump.

The rate of the pulses on the plot is probably an alias between the sampling rate of the data acquisition system and the actual pressure pulses.

[Fly Boomer]

2 minutes ago, N201MKTurbo said:

The rate of the pulses on the plot is probably an alias between the sampling rate of the data acquisition system and the actual pressure pulses.

Never considered this before -- kind of a beat frequency between sampling frequency and pressure pulse frequency.

[N201MKTurbo]

I don’t think the FF sensor can respond that fast. Your pressure plot and FF plot are in lock step. You may have an electrical noise issue.

[N201MKTurbo]

6 minutes ago, Fly Boomer said:

Never considered this before -- kind of a beat frequency between sampling frequency and pressure pulse frequency.

For your reading pleasure.

https://en.wikipedia.org/wiki/Nyquist–Shannon_sampling_theorem

This one is a little more understandable.

https://en.wikipedia.org/wiki/Aliasing

 

 

[Fly Boomer]

9 minutes ago, N201MKTurbo said:

This one is a little more understandable.

https://en.wikipedia.org/wiki/Aliasing

Thank you!  That really cleared it up  .  I appreciate the reference, but my poor brain cells are too far gone to bring that stuff back to life.

[N201MKTurbo]

12 minutes ago, Fly Boomer said:

Thank you!  That really cleared it up  .  I appreciate the reference, but my poor brain cells are too far gone to bring that stuff back to life.

It could just be me trying to show you how intelligent my rear end is.

[redbaron1982]

A little background, I'm chasing a random stumble the engine has while in cruise. I asked Savvy multiple times and they can't figure it out. The most I got was that some they said I was running "too lean" in some occasions.

The thing is that this 20s oscillation of .6 or .7 gph makes adjusting the mixture difficult, like I can finish setting the mixture at 8.4gph and 20 seconds later I look and it is at 7.7gph. Or maybe it is at 9.1gph.

So my main concern is whether my servo or fuel pump might need overhaul or not.

[MikeOH]

3 hours ago, N201MKTurbo said:

For your reading pleasure.

https://en.wikipedia.org/wiki/Nyquist–Shannon_sampling_theorem

This one is a little more understandable.

https://en.wikipedia.org/wiki/Aliasing

 

 

Ah, Claude Shannon!

This guy pretty much developed the theories behind all digital communication that our modern world depends upon. He doesn't really get any credit.

He's a lot like Richard Feynman in that his explanations are reasonably understandable by anyone willing to think a bit, not arcane, at all.  Even his papers are quite cogent.  I've always believed the ability to explain complicated concepts to others is a sign of real genius.

https://en.wikipedia.org/wiki/Claude_Shannon

 

[MikeOH]

10 minutes ago, redbaron1982 said:

A little background, I'm chasing a random stumble the engine has while in cruise. I asked Savvy multiple times and they can't figure it out. The most I got was that some they said I was running "too lean" in some occasions.

The thing is that this 20s oscillation of .6 or .7 gph makes adjusting the mixture difficult, like I can finish setting the mixture at 8.4gph and 20 seconds later I look and it is at 7.7gph. Or maybe it is at 9.1gph.

So my main concern is whether my servo or fuel pump might need overhaul or not.

Are you certain this 'stumble' is actually being caused by the fuel flow/pressure variations you are seeing?

I've had a a problem with my fuel flow gauge where it is intermittent but no engine issues.  My fuel pressure is still measured with the factory mechanical gauge and there's no variation there.  My issue ended up being a loose connection to to the fuel flow instrument (FP5L).

I would T-off a mechanical fuel pressure gauge and see if it follows the electronic fuel pressure indications before doing anything else.

[N201MKTurbo]

19 minutes ago, redbaron1982 said:

A little background, I'm chasing a random stumble the engine has while in cruise. I asked Savvy multiple times and they can't figure it out. The most I got was that some they said I was running "too lean" in some occasions.

The thing is that this 20s oscillation of .6 or .7 gph makes adjusting the mixture difficult, like I can finish setting the mixture at 8.4gph and 20 seconds later I look and it is at 7.7gph. Or maybe it is at 9.1gph.

So my main concern is whether my servo or fuel pump might need overhaul or not.

I have found that random stumbles during cruise are always ( well, most of the time) mag related. How long does it stay in stumble mode? Do you have time to do a mag check? That is the first thing you should do.

Do you see an EGT change when it stumbles?

[redbaron1982]

1 minute ago, N201MKTurbo said:

I have found that random stumbles during cruise are always ( well, most of the time) mag related. How long does it stay in stumble mode? Do you have time to do a mag check? That is the first thing you should do.

Mag check on cruise LOP always checked good. The stumble is just a second. I can't even see anything RPM or MAP wise in the engine data. 

[N201MKTurbo]

Just now, redbaron1982 said:

Mag check on cruise LOP always checked good. The stumble is just a second. I can't even see anything RPM or MAP wise in the engine data. 

You need to look at EGT, not RPM or MP. If it only does it for a second, it may not be long enough for the EGT to change.  How often does it do it?

It is unlikely it is the fuel servo, or fuel pump. your fuel pressure isn't changing enough to matter. At cruise power, the fuel pressure has to drop below the red line before it will affect the mixture.

[gabez]

you need to plot it again the EGT which will lag behind it. if the EGT varies then it's a real FF oscillation. I had to deal with this so much for over a year that I feel your pain. Does it go away with the boost pump on? 

[redbaron1982]

This is the complete flag log: https://apps.savvyaviation.com/flights/10422228

I don't see anything on the EGTs.

The stumbles happened right when I start a descent from 10.5k to 9.5k fee, at around 51:33. They were two, a few seconds apart and lasting just an instant. At around 52:23 I turn on the boost pump to see if anything changed, but I turned it back of 5 seconds later.

[Ragsf15e]

17 minutes ago, redbaron1982 said:

This is the complete flag log: https://apps.savvyaviation.com/flights/10422228

I don't see anything on the EGTs.

The stumbles happened right when I start a descent from 10.5k to 9.5k fee, at around 51:33. They were two, a few seconds apart and lasting just an instant. At around 52:23 I turn on the boost pump to see if anything changed, but I turned it back of 5 seconds later.

Can you get it to fluctuate on the ground by running it up some?

 I once had fp fluctuations and really wanted to solve it.  I finally convinced my mechanic to T in a long fuel line with an old school analog pressure gauge which we ran into the storm window so I could see it during a ground run.  Before running with the extra line, my edm900 was fluctuating.  I ran the airplane with the extra line and it didn’t fluctuate at all.  So either the longer line dampened it, or it wasn’t really fluctuating. So then I decided to put a snubber on the fuel line. Apparently I didn’t tighten it Good enough because that caused a fuel leak which made a big blue mess. I removed this snubber which was a pain. After all of that I went back to the starting configuration and just left it alone.

we do need to make sure that your engine isn’t actually surging or stumbling from the Fuel pressure. To test the mags more than the ground test does, savvy has a lean of peak mag test, which will give you a better idea at a higher power.

[redbaron1982]

22 minutes ago, Ragsf15e said:

To test the mags more than the ground test does, savvy has a lean of peak mag test, which will give you a better idea at a higher power.

Yeah, I do those almost every flight before starting my descent. It always checks out good.

[N201MKTurbo]

If it does this every time you descend, don’t do a quick mag check, that won’t help with this issue. Just turn it to the left, or the right and leave it there while you start your descent. If it doesn’t stumble, that is a clue, you would be running on the good mag. If the engine quits instead of stumbling, you are running on the bad mag. Don’t worry, just switch to the good mag and be happy you found your issue.

[PT20J]

It is common for an IO-360 to show fuel pressure fluctuations due to heating in the fuel pump introducing vapor to the fuel. It doesn't show on the factory gauges due to the damping of the internal gauge but it clearly shows up on my Garmin EIS with both the Kavlico pressure sensor and the GPT sensor. It's not just Mooneys -- there are threads about this on vansairforce.net.

It is not common, in my experience, to see rapid fluctuations in the fuel flow. I would suspect an electrical connection issue -- especially a ground since the fuel flow and pressure seem to vary in lockstep.

 

[Bartman]

18 hours ago, N201MKTurbo said:

I have found that random stumbles during cruise are always ( well, most of the time) mag related. How long does it stay in stumble mode? Do you have time to do a mag check? That is the first thing you should do.

Do you see an EGT change when it stumbles?

Mine used to stumble about once or twice per hour at cruise altitude around 7k-9k.  Rebuilt mag at 500hr did not change anything. Changing the plugs when it was time did not change anything. Installing a new set of wires when I discovered this was the cause of my radio interference fixed the radio reception, and the stumbles. 

I could not tell you the last time it stumbled and that has been about 7 years since mag harness change.