Fuel Flow sensor Install

by Jack Martin in General Tech Discussion 6 years ago

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Re: Fuel Flow sensor Install

by Bill Hertzel » 5 years ago

Bill Hertzel

13 1.95k 7.96k

I have been trying to avoid this argument, But!, And with all due respect...

I do not know if your engine is Fuel Injected or Carbuetted.
If it is carbureted then your Indicated flow will be about 10 times the true fuel burned rate at idle and about 2 times at full power.
If it is Fuel injected the total flow will be a nearly constant 20-30 gph depending on how many fuel pumps are running.
Or about 50 times the true fuel burned rate at idle and about 5 times at full power.

By averaging out and scaling down these extremely over indicated flow rates, what you have essentially created is a fuel flow powered clock.
The flow meter is going to over-indicate at low power settings and under-indicate at high power settings and at the end of the flight they all average out to what you expected in the first place.
You are never indicating the True Flow Rate except at one power setting somewhere around cruise.
And because you spend most of the time at cruise and about an equal amount of time Idling and Climbing, you get a result that appears to correct but only because you have intentionally recalibrated the Flowmeter to give you the result that you want to see.

This is not a system that is to be recommended.
I know you will strongly disagree, and I am sorry if I have offended.

Bill Hertzel
Rotax 912is
North Ridgeville, OH, USA
Clicking the "Thank You" is Always Appreciated by Everyone.

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Re: Fuel Flow sensor Install

by James Ott » 5 years ago

James Ott

22 322 358

Bill, certainly no offense taken; I think this is a good discussion. However I do disagree with you-here's why: I have no idea where you got the information that bypass flow is roughly equal to engine consumption flow at cruise conditions. My 912uls bypass system was installed per Rotax instructions and includes A 1mm restrictor orifice in the bypass line, which allows nowhere near the 4 gph my engine consumes at cruise conditions. I am a hydraulics engineer and have worked with orifice flow calculations for 40 years. Using a 1mm orifice in a 1/4" fuel bypass line, at 3.5 psi fuel pressure differential, with a 0.85 fuel density, and a 0.6 sharp edge orifice coefficient, I calculate a fuel bypass flow rate of 0.7 gph. This is only about 1/6 of the engine consumption rate and is a small enough value to easily calibrate it out of the total flow rate without losing much accuracy at all. Also, my idling and climb time is much less than my typical cruise time. Now if you are running a fuel injected engine with much higher fuel pump pressures, then I tend to agree with your analysis.

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Re: Fuel Flow sensor Install

by Bill Hertzel » 5 years ago

Bill Hertzel

13 1.95k 7.96k

I stand corrected.
You are correct!
I am used to working on the fuel-injected engines with 45psi of pressure behind the orifice and bypassing 3-5gph.
And so your bypass flow will be closer to idle fuel flow than WOT flow.

But respectfully, I will still continue to not recommend the recalibration method.
Out of curiosity, what K-Factor did you find produced a reasonable display?

Bill Hertzel
Rotax 912is
North Ridgeville, OH, USA
Clicking the "Thank You" is Always Appreciated by Everyone.

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Re: Fuel Flow sensor Install

by James Ott » 5 years ago

James Ott

22 322 358

I don't wish to belabor this discussion, but I must correct an error I made in my calculation above. I mistakenly used 1 mm for the fuel return orifice size when the actual correct size is 0.35 mm. Redoing the flow calculation using this correct size gives a bypass flow rate of 0.079 gph instead of the 0.7 gph reported above. As you can well imagine this EXTREMELY small bypass flow is almost small enough to be ignored, but I calibrated it out anyway. In my view, these facts make it quite unnecessary and a waste of money and complexity to bother with a second fuel flow sensor.

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Re: Fuel Flow sensor Install

by James Ott » 5 years ago

James Ott

22 322 358

Bill, I ended up changing the FT-60 "K" factor from the default 68,000 to 103,800 pulses/gal which seems to be very accurate in my particular system. I think most of the K factor change was to match my system plumbing, pressures, restrictions, etc. rather than to compensate for the very small bypass flow.

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