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recapen(at)earthlink.net
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 Posted: Mon Dec 17, 2012 1:53 pm Post subject: More combustion concepts |
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Did some vacuum hunting prior to my most recent flight and rebuilt the MAP sensor hoses that go to the suspect cylinder. Still have consistently high EGT on that cylinder only.
Cycling the magnetos shows elevated egt regardless of which magneto is running....even the LASAR electronic ignition. I have verified the base timing for when in the magneto mode.
My conceptual thinking here is that both plugs for that cylinder are firing.
Leaving mixture as the culprit...
CHT's are within the range for the other three cylinders.
My conceptual thinking here is that the ignition is lean and completely igniting.
When I returned, I opened everything up again and pulled the intake runner for that cylinder. The rubber connecting hose looked fine but the top gasket ahad a couple of spots that looked like they were not forced in to contact with the head - and could leak. I replaced the gasket and carefully aligned the tube while putting it back together.
I'm also replacing the EGT sensor just in case. I have had a sensor failure before but it indicated lower than normal readings instead of higher.
My conceptual thinking here is that the thermocouple effect would fail low instead of high - but I don't plan to take that one to the bank....
Thoughts?
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carl.froehlich(at)verizon
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| Posted: Mon Dec 17, 2012 2:58 pm Post subject: More combustion concepts |
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Not sure I follow. The actual EGT reading is dependent on many factors, so the reading itself is of little value other that to demonstrate very abnormal conditions (a dead cylinder as example).
As you report the CHTs are comparable, and without knowing relationship between cylinder to cylinder fuel flow at peak EGT, I would guess you don't have a problem. If on the other hand the cylinder in question is first to peak and there is a significant fuel flow change to get the next cylinder to peak, then I would agree with the conclusion that the cylinder in question has a leaner mixture than the others.
The data run I suggest you do is to establish cruise conditions at 5K-7K feet, 2400 RPM, 23"-24" MP. Lean to get the first cylinder to peak EGT and record fuel flow. Continue leaning to get to the second cylinder to peak and record fuel flow and degrees lean of peak on the first cylinder, repeat for the next two cylinders.
A nicely matched engine will run smooth LOP with all cylinders within 20 to 50 degrees on the lean side of whatever temperature was peak EGT for that cylinder.
Again, as the EGT reading is very sensitive to sensor location and clocking on the exhaust pipe, the EGT value itself has little meaning. The "Degrees LOP or ROP" comparison between the cylinders as well as having each cylinder peak at about the same fuel flow tells you if you have a problem or not.
Carl
On Dec 17, 2012, at 4:49 PM, "Ralph E. Capen" <recapen(at)earthlink.net> wrote:
| Quote: |
Did some vacuum hunting prior to my most recent flight and rebuilt the MAP sensor hoses that go to the suspect cylinder. Still have consistently high EGT on that cylinder only.
Cycling the magnetos shows elevated egt regardless of which magneto is running....even the LASAR electronic ignition. I have verified the base timing for when in the magneto mode.
My conceptual thinking here is that both plugs for that cylinder are firing.
Leaving mixture as the culprit...
CHT's are within the range for the other three cylinders.
My conceptual thinking here is that the ignition is lean and completely igniting.
When I returned, I opened everything up again and pulled the intake runner for that cylinder. The rubber connecting hose looked fine but the top gasket ahad a couple of spots that looked like they were not forced in to contact with the head - and could leak. I replaced the gasket and carefully aligned the tube while putting it back together.
I'm also replacing the EGT sensor just in case. I have had a sensor failure before but it indicated lower than normal readings instead of higher.
My conceptual thinking here is that the thermocouple effect would fail low instead of high - but I don't plan to take that one to the bank....
Thoughts?
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recapen(at)earthlink.net
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| Posted: Tue Dec 18, 2012 4:39 am Post subject: More combustion concepts |
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Carl - thanks for your insight - prior to the teardown (hangar door blown in to prop by tornado), I had the cylinders balanced for LOP to within .1gph fuel flow. I have the instrumentation to record the data you have described and had already used it to balance the flows. I haven't run the peak test you've described yet due to fear of toasting my cylinder.
I will be replacing the EGT probe prior to next flight since I have a spare on hand.
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