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-   -   EGT fluctuations during in-flight mag check (PMAGS) (https://vansairforce.net/community/showthread.php?t=122230)

BillL 01-31-2015 07:07 PM

The MIT data was taken at lambda=1.0 . There would be complete burn here due to burn rate (heat release). At a more fuel rich mixture, it is possible to get residual burning at exhaust valve opening. Just a technical point, but that is probably not affecting the OP's situation. Additionally, the port shape, combustion chamber and compression ratio affects the burn rate. A swirl imparted to the cylinder charge would increase burn rate/heat release rate. Tumble with a 4 valve configuration would also have an effect. Swirl would reduce best torque timing advance, but also increase heat rejection to cylinder, piston and head.

I was told on some Conti turbo charged engines, the peak exhaust temp occurred 12-14 inches from the port. They had to move the temp probe for that engine.

Interesting data here, an investigation would be some fun.

Bevan 02-01-2015 03:49 AM

If the combustion cycle is completely done by the time the exhaust valve opens, why can you see a blue flame at the exhaust pipe exit at night on short exhaust engines such as radials? Are radials different in this regard?

Bevan

BillL 02-01-2015 07:28 AM

Quote:

Originally Posted by Bevan (Post 955356)
If the combustion cycle is completely done by the time the exhaust valve opens, why can you see a blue flame at the exhaust pipe exit at night on short exhaust engines such as radials? Are radials different in this regard?

Bevan

The combustion and heat release is already done at 99.9%+, the blue is residual thermal plasma of ionized gasses. If it was a little rich then carbon in the exhaust would give a yellow-red-orange color. Just because we see something does not mean there is any signifiant "burning"i.e. oxidation of H and C left. One would need to instrument an exhaust pipe and measure the gas temps at various distances from the port to see what heat release is taking place post exhaust valve.

DanH 02-01-2015 08:43 AM

Returning to the oscillation, Airzen, are we looking at a time hack in minutes? I assume yes, so the visible peaks suggest an EGT oscillation with a period of about 8 seconds per cycle...pretty long. You mentioned a preceptible pulse in power output. Is it also a long 8 second cycle?

Airzen 02-01-2015 09:25 AM

Yes, the pulsating power is around the same 6-8 second cycle.

I am thinking of sending the pmags out for an inspection.

DanH 02-01-2015 10:38 AM

Have a laptop? Make the necessary connections (three wires), run the EICAD program, and look for an oscillating firing angle. Or do the same with an EI Commander.

schristo@mac.com 02-01-2015 11:46 AM

Test without MP input
 
Disconnect the manifold pressure input from the PMags and run the same test first.

Airzen 02-19-2015 12:16 AM

Resolved
 
The issue is resolved.

Based on the suggestions the following actions were taken:

1) Changed all the spark plugs. I did check the older ones using an ohm meter and all of them showed around 5K ohms, so unlikely that this was an issue.

2) Sent both the units back so that Brad at EMAG air could check the units and advise as appropriate. His tests did not reveal any problems. The units were update with the latest V40 firmware as a part of the process.

3) A restrictor was added to the manifold line to the PMAGS.

I went up again and did the same ignition check at the altitude. The problem was gone. Did several tests since then and the mag check is perfectly fine.

It appears that lack of the restrictor was causing some variation in the sensed manifold pressure (at least in one of the mags) and causing the firing angle to change.


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