M20J - Fuel Burn, Inverse of expectation

[d0tnet]

I replaced the fuel servo a few months back in my M20J, io360-a3b6d engine.

Shortly thereafter, I noted an anamoly with fuel burn that is the inverse of what is expected - I burn more fuel as altitude increases.  From memory, down at 2500 feet, 100 ROP yields approx 10.5 gph indicated at 24 squared.   At 6000, same power settings require 11.5-11.7.   I have a hoskins ft-101 that I am getting my data points from on fuel burn.

I had a discussion about this with my mechanic and he explained that as a fuel servo goes bad, it dumps a bunch of extra fuel into the cylinder so when leaning prior to new fuel servo, I had a bad baseline to compare to.

I challenge that, however, since I am not concerned with either a higher or lower gph reading, but rather what seems to be a mixture issue since I am requiring a richer mixture as I climb.  

Any thoughts?  

[Bob - S50]

One question.  When you say you are leaning to 100F ROP at 2500 and 6000, are you really doing that or just leaning to the same EGT in both cases?

I'd suggest you try something different.  Try leaning to the same RPM, MP, and FF at both altitudes, then compare the TAS to see if you are faster at 6000'.

In my plane, I'm not sure I could get 24" MP at 6000'.  It would be close.

When ROP, power is basically determined by RPM and MP because the engine is oxygen limited.  Well, that's true until you get well ROP when the mixture won't burn well.  When LOP, power is determined by FF because the engine fuel limited.

[takair]

56 minutes ago, d0tnet said:

I replaced the fuel servo a few months back in my M20J, io360-a3b6d engine.

Shortly thereafter, I noted an anamoly with fuel burn that is the inverse of what is expected - I burn more fuel as altitude increases.  From memory, down at 2500 feet, 100 ROP yields approx 10.5 gph indicated at 24 squared.   At 6000, same power settings require 11.5-11.7.   I have a hoskins ft-101 that I am getting my data points from on fuel burn.

I had a discussion about this with my mechanic and he explained that as a fuel servo goes bad, it dumps a bunch of extra fuel into the cylinder so when leaning prior to new fuel servo, I had a bad baseline to compare to.

I challenge that, however, since I am not concerned with either a higher or lower gph reading, but rather what seems to be a mixture issue since I am requiring a richer mixture as I climb.  

Any thoughts?  

Why did you change the servo?  When you are at 6000, you are releaning, slowly, to 100ROP?  Is ram air used in either case?

[N201MKTurbo]

The servo has nothing to do with fuel burn. It just supplies a constant fuel pressure to the flow divider. And except near idle the flow divider supplies the same pressure to the individual injectors. 

You control the fuel pressure at any particular altitude with the mixture control. All the servo does is keep a ratio between impact tube pressure and outlet fuel pressure. This will ultimately maintain a linear relationship between throttle setting and fuel flow.

So it is unlikely the servo has anything to do with your symptoms.

What you are saying is that Hoskins reads differently then it did. I have seen this happen with the flowscan. If it gets tiny bubbles in it, it will read high. 

Have you verified that the fuel flow readings are correct by tracking fuel consumption? 

Do you have an engine scanner and verified all your cylinders peak at he same place they did befor? 

[d0tnet]

One question.  When you say you are leaning to 100F ROP at 2500 and 6000, are you really doing that or just leaning to the same EGT in both cases?

I'd suggest you try something different.  Try leaning to the same RPM, MP, and FF at both altitudes, then compare the TAS to see if you are faster at 6000'.

In my plane, I'm not sure I could get 24" MP at 6000'.  It would be close.

When ROP, power is basically determined by RPM and MP because the engine is oxygen limited.  Well, that's true until you get well ROP when the mixture won't burn well.  When LOP, power is determined by FF because the engine fuel limited.

I am leaning to ROP at both altitudes. Manifold pressure may be a bit less than 24" at 6k but rpm in which case would be firewall forward.

Sent from my XT1254 using Tapatalk

[d0tnet]

56 minutes ago, d0tnet said: I replaced the fuel servo a few months back in my M20J, io360-a3b6d engine.

Shortly thereafter, I noted an anamoly with fuel burn that is the inverse of what is expected - I burn more fuel as altitude increases.  From memory, down at 2500 feet, 100 ROP yields approx 10.5 gph indicated at 24 squared.   At 6000, same power settings require 11.5-11.7.   I have a hoskins ft-101 that I am getting my data points from on fuel burn.

I had a discussion about this with my mechanic and he explained that as a fuel servo goes bad, it dumps a bunch of extra fuel into the cylinder so when leaning prior to new fuel servo, I had a bad baseline to compare to.

I challenge that, however, since I am not concerned with either a higher or lower gph reading, but rather what seems to be a mixture issue since I am requiring a richer mixture as I climb.  

Any thoughts?  

Why did you change the servo?  When you are at 6000, you are releaning, slowly, to 100ROP?  Is ram air used in either case?

Fuel servo replaced due to reaching stop limit of adjustment for ground idle mixture.

Yes, leaning at 2500 and then separately leaning again at 6000. The altitude references are also just to illustrate the point that fuel flow increases with altitude. I haven't been higher than 7500 yet to test if it continues past that point. Ram air closed in all scenarios.

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[d0tnet]

The servo has nothing to do with fuel burn. It just supplies a constant fuel pressure to the flow divider. And except near idle the flow divider supplies the same pressure to the individual injectors. 

You control the fuel pressure at any particular altitude with the mixture control. All the servo does is keep a ratio between impact tube pressure and outlet fuel pressure. This will ultimately maintain a linear relationship between throttle setting and fuel flow.

So it is unlikely the servo has anything to do with your symptoms.

What you are saying is that Hoskins reads differently then it did. I have seen this happen with the flowscan. If it gets tiny bubbles in it, it will read high. 

Have you verified that the fuel flow readings are correct by tracking fuel consumption? 

Do you have an engine scanner and verified all your cylinders peak at he same place they did befor? 

Good point.

Yes, that is indeed my symptoms. I haven't been diligent in documenting actual fuel burned vs what the Hoskins says. Will do it when I get plane back from annual. I have an engine monitor and cylinders are peaking similar to before - all around 365-370 F with 2400 RPM, cowl flaps closed.

Any specific procedure to clear air bubbles from fuel flow transducer? Maybe when servo was installed air entered the system that hasn't been purged? This somewhat validates what I recently saw - total fuel burned according to the Hoskins didn't match wing gauges. I. E. Hoskins said 59 gallons burned since last reset and I was showing 10 gallons approximately in the tanks

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[N201MKTurbo]

The flowscan has pipe thread fittings. I would take it all apart, clean all the fittings and reassemble it with permatex or fuel lube instead of Teflon tape. My theory is that air bubbles are being drawn into the sensor through a tiny leak. Which doesn't make a lot of sense because all of the lines should have positive pressure and any leak would just leak out fuel instead of sucking in air, but if there is flow, the restriction of the turbine wheel could cause a bit of Burnulli pressure drop. I wouldn't rush out to do any of this until you verify the fuel flows, but if the indication is wrong, it is something cheap to try. In my case cleaning and redoing the fittings made the flow indication correct again. Just not sure why that is.

[d0tnet]

8 minutes ago, N201MKTurbo said:

The flowscan has pipe thread fittings. I would take it all apart, clean all the fittings and reassemble it with permatex or fuel lube instead of Teflon tape. My theory is that air bubbles are being drawn into the sensor through a tiny leak. Which doesn't make a lot of sense because all of the lines should have positive pressure and any leak would just leak out fuel instead of sucking in air, but if there is flow, the restriction of the turbine wheel could cause a bit of Burnulli pressure drop. I wouldn't rush out to do any of this until you verify the fuel flows, but if the indication is wrong, it is something cheap to try. In my case cleaning and redoing the fittings made the flow indication correct again. Just not sure why that is.

Thanks for the suggestion.  Will complete as suggested tomorrow.  Won't be able to confirm function until mid-week when annual is done and plane is back in the air.  Will also top off tanks and keep track of fuel burn manually to compare expected book numbers to the fuel totalizer reading.

[d0tnet]

So, it seems after about 20 hours of keeping track, the fuel flow indicated is showing 1.0 GPH higher than actual.

I had the sensor cleaned during annual hoping that would solve it, but no luck.

Any thoughts on how to approach this?

[jetdriven]

On 4/18/2016 at 8:17 PM, N201MKTurbo said:

The flowscan has pipe thread fittings. I would take it all apart, clean all the fittings and reassemble it with permatex or fuel lube instead of Teflon tape. My theory is that air bubbles are being drawn into the sensor through a tiny leak. Which doesn't make a lot of sense because all of the lines should have positive pressure and any leak would just leak out fuel instead of sucking in air, but if there is flow, the restriction of the turbine wheel could cause a bit of Burnulli pressure drop. I wouldn't rush out to do any of this until you verify the fuel flows, but if the indication is wrong, it is something cheap to try. In my case cleaning and redoing the fittings made the flow indication correct again. Just not sure why that is.

Regarding Teflon tape, it has killed a few people. Some of that stuff migrates off the threads and into fuel injectors where it can clog them solid, or a main bearing passage in an engine and causes engine failure. Or a prop governor... You get the idea.  Or it gets into a vacuum pump where it kills the pump.   I know if you are careful you can keep it all inside the threads, but when it is disassembled later, it shreds and gets loose where it can cause serious trouble, and not that day, but maybe next year. That stuff doesnt belong on an airplane.