HI Bill,
Yes the Normal/Max numbers were swapped in my email (copied/pasted too fast) and there are indeed:
- Min fuel pressure: Airbox pressure + 2.18 psi (0.15 bar): 4.43 in
- Max fuel pressure: Airbox pressure + 5.08 psi (0.35 bar): 10.3 in
- Normal fuel pressure: Airbox pressure + 3.63 psi (0.25 bar): 7.38 in
This week we figured it out with my A&P and the reason is indeed what you explained (which I had got a reply earlier to save his time). But here is what I wrote to my manufacturer last Wed, Sep 7:
We watched the Rotax video explaining how to adjust the fuel pressure regulator, and it requires a differential pressure gauge (connected to the airbox). I believe I finally understood this AM how this works: our fuel sensors (I checked the Kavlico web site) like the analog gauge we used this AM, measure fuel pressure vs atmospheric pressure. A pressure (like a voltage) is always differential, whether the base is the atmosphere, some sealed cage preset value or a vacuum. The P4055-15G on my plane is a “G” model, meaning it is atmospheric (there must be a little hole in it somewhere), Kavlico also makes a “A” like absolute and a “S” sealed versions.
That is why a disconnected gauge (or when the pump is not running) shows 0 and not 14.7 PSI (14.7 PSI is the standard sea level pressure, or 29.92 in Hg, as we all know). Therefore, on a Rotax 912, the fuel pressure sensor is always measuring the fuel pressure going to the carburetor vs the air pressure around the sensor (which also happens to be, on the 912, the air pressure in the carburetors, which is the atmosphere, and roughly the same as the static pressure entering the altimeter). The measured pressure is always a differential pressure between the fuel line and the atmosphere. On a Rotax 914 the fuel pressure sensor is wired the same, that means it reports the difference of pressure between the fuel line and the atmosphere, same as in the 912. The Rotax manual says that the fuel pressure should be between 2.2 and 5.1 PSI (3.6 is the average) ABOVE the air box pressure (also the air pressure in the carburetors). That is not what the sensor is measuring (since the sensor is not a differential sensor with two inputs). Again, the sensor only measures the pressure vs the atmosphere.
So, that totally explains why my fuel pressure indication rises when the MAP rises or when the altitude rises. Let’s say we are on a standard day and the fuel regulator is working correctly (so delivering fuel at 3.6 PSI above air box pressure):
- At sea level, if power is set to MAP = 29.92" then the sensor will report 3.6 PSI (the sensor is measuring 3.6 PSI between the fuel line that has an absolute pressure of 18.3 PSI and the ambient air that has an absolute pressure of 14.7 PSI)
- At 12500 ft, if power is boosted to MAP = 29.92" then the sensor will report 9.1 PSI (the sensor is measuring 9.1 PSI between the fuel line that has an absolute pressure of 18.3 PSI and the ambient air that has an absolute pressure of 9.17 PSI). The fuel line has 18.3 PSI and the airbox 14.7 PSI (29.92”) so the difference is 3.6 PSI, as expected, the carburetors are getting the exact same fuel pressure as at sea level (so 3.6 PSI) but the reported indication is now 9.1 instead of 3.6
- At 12500 ft, if power is bumped to MAP = 36" then the sensor will report 12.1 PSI (the sensor is measuring 12.1 PSI between the fuel line that has an absolute pressure of 21.3 PSI and the ambient air that has an absolute pressure of 9.17 PSI). The fuel line has 21.3 PSI and the airbox 17.7 PSI (36”) so the difference is 3.6 PSI, as expected, the carburetors are getting the exact same fuel pressure as at sea level (so 3.6 PSI) but the reported indication is now 12.1 instead of 3.6
So, either the Rotax 914 should use a differential fuel pressure sensor (with one end connected to the air box) if those exist and then the tape could be calibrated to be green between 2.2 and 5.1 PSI. Or the tape should be changed to be green between 2.2 and 15.5 PSI (if the plane can maintain 36” to 16000 ft, as Rotax says, and if the max acceptable differential pressure can be 5.8 PSI, as Rotax says, that means the indicated value can reach 15.5 PSI, follow the math above: static is 7.97, airbox is 17.7, fuel line is 17.7+5.8=23.5). But wait, the sensor is 15 PSI max so it will blow up? Someone did not think this installation right???
What this means today is that the fuel pressure indication is 99% useless (unless it shows less than 2) in any similar Rotax 914 installations (incl. the Tecnams) unless the pilot does some complicated math to derive the actual fuel pressure differential vs the air box pressure vs the atmospheric conditions. I suspect if a certified P2006T was offered with the 914 it would have to use differential gauges. I will do some researches for other planes using the TSIO-550 to see how they report fuel pressure.
So, to summarize, the fuel pressure can be stable and within specs at 3.6 PSI above the airbox pressure and yet it could be displayed as 3.6, 9.1 or 12.1 (in my examples). Hence my comment that this kind of sensor installation is pretty useless. Perhaps using an absolute sensor and showing its value as in of Hg (like the one form the MAP, against a vacuum) would be easier as the pilot would always expect the value to be between MAP+4 inches and MAP+10" inches. I will probably do that.
Thanks.
Jeff
