Bob_Belville Posted August 4, 2014 Author Report Posted August 4, 2014 The reason that making sure the line has air in it instead of being completely full of fuel is that the air is compressible and the fuel is not. If the line is completely full of fuel. It is going to show every pulse of the engine pump. It there is a snubber at the servo and the line is full of air, then the snubber is going to limit the amount of fuel that move back and fourth through it each time the engine pulses. The air is going to compress a small amount each time and and smooth out the pressure pulses. The small amount of fuel that is allowed through the snubber during each pulse will only have a negligible affect on a line full of air. I understand your point but the line had no fuel in it when we checked it. Since it wasn't working like that, we filled it with fuel and that seemed to help but after a short local flight that looked pretty good it went back to fluctuating. Can I ask you to tell us whether you know this from experience and can provide a technical/installation/service manual reference or if you are theorizing? No offence... We installed a snubber valve in the MAP line (air) and fixed a similar problem. The instruction there was not to put the snubber right at the transducer but to splice it into the line between the intake manifold and the sensor. I note that the fuel line sensor article I found instructs that the snubber should be right at the transducer. I'm guessing the difference is liquid fluid vs. air? Quote
N601RX Posted August 4, 2014 Report Posted August 4, 2014 It's from experience with industrial fluid flow measurement systems that I've designed and built. They usually have a Parker pressure accumulator to smooth everything out. Essentially think of the hose between the snubber and transducer as a storage tank and the transducer is measuring the pressure inside the tank. The tank can only be filled by what fluid enters and leaves through the snubber. The amount of filtering available is a ratio of the size of the tank vs the size of the hole trying to fill and empty it. If the tank becomes filled with liquid then it looses its ability to smooth out the pressure pulses. Another advantage to putting the snubber on the engine side for oil and fuel systems is that if a hose comes loose or burst it will limit how fast the oil is lost and the amount of fuel being sprayed around. Quote
Bob_Belville Posted August 5, 2014 Author Report Posted August 5, 2014 Dr. Bill, Lee Layton, and any other resident EEs and A&Ps, what's the possibility that RF or EMI is going on with the indicator oscillations? Do you see any correlation between the rpm (red) and the fuel pressure (green)? Quote
N201MKTurbo Posted August 5, 2014 Report Posted August 5, 2014 According to Nyquist you have to filter out any effects with a frequency above 1/2 the sampling frequency of your system. If you don't you will get unwanted signals which are a combination of the measured signal and the sampling rate. That being said, you need to add a low pass filter to your system either electronic or pneumatic. http://en.wikipedia.org/wiki/Nyquist_frequency Quote
Bob_Belville Posted August 5, 2014 Author Report Posted August 5, 2014 It has been suggested that the vertical mounting of the hose @ the transducer might allow a harmonic due to vibration. I'm not sure that explains the graphs but it might. If so, I suppose a snubber would help and reorienting the mounting of the transducer would certainly change the amplitude of the "slosh". Quote
Bob_Belville Posted August 21, 2014 Author Report Posted August 21, 2014 Update: I have had considerable contact with JPI. Started with email exchange, sending data files and swapping info. They called me several times and have been quite helpful. In all I've talked to four people. Their diagnosis was a bad electrical connnection. I shipped the EDM and the transducer to JPI (CA). They checked both out finding no problem. They updated the firmware. While the plane was down I checked the cable with an ohm meter. Continuity is fine. I rerouted a few wires that might have been close to the wire from the transducer to the EDM. The EDM arrived here today. I cleaned all the plugs with contact cleaner and reassembled. I did not have time to fly but I ran the engine with everything turned on for 10 minutes. FP graph is steady @ 24 psi! Hopefully something I did, probably reconnecting a plug, fixed the problem. (Or as my mechanic reports, devices that go back to the factory work properly upon return even though the factory finds nothing wrong.) 1 Quote
Bob_Belville Posted August 22, 2014 Author Report Posted August 22, 2014 Update: I have had considerable contact with JPI. Started with email exchange, sending data files and swapping info. They called me several times and have been quite helpful. In all I've talked to four people. Their diagnosis was a bad electrical connnection. I shipped the EDM and the transducer to JPI (CA). They checked both out finding no problem. They updated the firmware. While the plane was down I checked the cable with an ohm meter. Continuity is fine. I rerouted a few wires that might have been close to the wire from the transducer to the EDM. The EDM arrived here today. I cleaned all the plugs with contact cleaner and reassembled. I did not have time to fly but I ran the engine with everything turned on for 10 minutes. FP graph is steady @ 24 psi! Hopefully something I did, probably reconnecting a plug, fixed the problem. (Or as my mechanic reports, devices that go back to the factory work properly upon return even though the factory finds nothing wrong.) Took it up for a short flight yesterday. The FP was still very steady. Hopeful... If it is fixed, the most likely solution among all the things I did was simply cleaning the electrical plug connections. I need to take Nancy back to 6B6 tomorrow so I'll get to watch it for 10 hours over the next few days. (While the EDM was out I replaced the wire that controls the overhead vent. Spent $1.32 for .058" inner wire. The EDM stuff cost me the one way UPS/insurance to ship it to JPI )CA). Total out of pocket <$50. I guess I qualify as a member cheap ba*rd club.) 1 Quote
N601RX Posted August 22, 2014 Report Posted August 22, 2014 Yep that .058 inner cable is straight from the lawn and garden section at Lowes. Quote
Bob_Belville Posted August 22, 2014 Author Report Posted August 22, 2014 Yep that .058 inner cable is straight from the lawn and garden section at Lowes. $0.20/ft. @ "Burke Outdoor". www.burkeoutdoor.com/ Quote
Bob_Belville Posted August 24, 2014 Author Report Posted August 24, 2014 Took it up for a short flight yesterday. The FP was still very steady. Hopeful... If it is fixed, the most likely solution among all the things I did was simply cleaning the electrical plug connections. Rats. Quote
M016576 Posted September 24, 2014 Report Posted September 24, 2014 flights_8-23-2014.jpg Rats. Fuel pump or fuel selector? Quote
Bob_Belville Posted September 24, 2014 Author Report Posted September 24, 2014 Fuel pump or fuel selector? Plane is down for annual and more so I am not sure but we found several frayed ignition wires and am optimistic that the problem is RFI. There is nothing about the fuel flow or engine performance to indicate the problem is really the actual fuel pressure. Quote
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