On a recent flight(s) we have seen some unexpected fuel flow and EGT readings. Yesterday we were cruising at 6500 feet, with a density altitude of about 7800 feet. The engine rpm was 5000 and the fuel flow was 6.5 gph with the EGTs at about 1230 deg. At lower altitudes we typically see 4.8 gph and EGTs of 1320 deg for 5000 rpm. We have a carbureted 912 ULS engine. Descending to a lower altitude for our approach we found the fuel flow and EGTs to return to their expected values. Any thoughts?
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Fuel Flow
- Thread starter gemiller
- Start date
flightdoc101
Member
The lower EGT's are consistent with the ULS engine running generally richer the higher one goes. Operating temps run lower everywhere due to the cooling effects of the unburned fuel cycling through. All this is emphatically not true for the iS engine which is mapped and gets only the fuel it can burn delivered.
I can't speak to the fuel flow rates. The MaP would be more relevant to the fuel flow than the RPM.
I can't speak to the fuel flow rates. The MaP would be more relevant to the fuel flow than the RPM.
My recollection is that the mp was on the order of 21.4 in.
And how are you measuring your fuel flow with a 912ULS which has a return fuel flow line?
yes
We use the flow sensor interfaced with the Dynon display. We did a reasonably good job of calibrating the device so the numbers we get are reasonably accurate. The changes noted are significant ( less than 5 gph to well over 6 gph).
flightdoc101
Member
At a MaP of 21 inches I would have expected a flow of 4-5 g[h, lower with higher altitudes.
Piper J3
Well Known Member
This topic was discussed a while back in this forum... Perhaps do a search for fuel flow error at altitude. I can’t remember specifics, but it is a known deficiency with the early Dynon system where the fuel flow transducer shows higher flow rate at altitude when fuel burn is actually reduced...
Indeed it was in Van's SL 19-09-23. This addresses my issue completely. Thank you, sir.
I know Van’s has tried to solve this and their solution was to turn it off. Similar to when the “idiot light” in your car comes on, put a piece of black tape over the light, …. fixed. Has anyone read of any real solutions to this? My plane will read 8+ GPH at 9 - 10,000’ and I have replaced the red cube sensor so I know the paddle wear is not the issue. And it is interesting that it doesn’t always do this.
I guess you “calibrated out” the inaccuracy you are getting by using one fuel flow sensor, which does not reflect the portion of that fuel flow which is being returned to the tank. I believe some have installed a second fuel flow sensor on the return line and managed to figure out how to get Dynon to subtract it from the primary sensor’s value. I’m Garmin G3X and not savvy enough to figure out how to do that. Has anybody?
Are your transponder antenna and red cube mounted anywhere near one another? My transponder antenna is mounted on the lower forward corner of the fuselage, near to the firewall, and the red cube is mounted on the firewall only a couple feet away. I find that in congested airspace where my transponder is busy responding to interrogations, the indicated fuel flow rises (sometimes by as much as several tenths of a GPH).
It's very interesting to watch positive fuel flow during the bi-annual transponder cert when the engine isn't running
It's very interesting to watch positive fuel flow during the bi-annual transponder cert when the engine isn't running
We have not seen, nor looked for, fuel flow issues in busy airspace. We fly out of S50, well inside the Seattle Class B airspace. It is a very congested airspace.
Further investigation into my issue indicates the EI fuel flow transducer is mounted between the electric and mechanical fuel pumps, ignoring the installation instructions from EI. Vans no longer ships a transducer for the RV-12 with a 920/ULS engine. I suspect the issue is with cavitation of the fuel flowing through the transducer at certain altitude and temperature ranges - the trans ducer is counting not only blades but bubbles too that give the false readings. The bottom line is to ignore the fuel flow numbers as they are worthless.
Further investigation into my issue indicates the EI fuel flow transducer is mounted between the electric and mechanical fuel pumps, ignoring the installation instructions from EI. Vans no longer ships a transducer for the RV-12 with a 920/ULS engine. I suspect the issue is with cavitation of the fuel flowing through the transducer at certain altitude and temperature ranges - the trans ducer is counting not only blades but bubbles too that give the false readings. The bottom line is to ignore the fuel flow numbers as they are worthless.
