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EGT fluctuations during in-flight mag check (PMAGS)

A

Airzen

Well Known Member
Hi Folks,

I am seeing fluctuating EGTs when performing inflight mag checks. See the image below that shows EGT rise when the left and the right mag is grounded.

9fzsoXD.png


The fluctuation on left mag grounding (the first EGT rise) is less pronounced (approx 4 deg.) than the right (approx.15 deg). In fact, during the right mag grounding there is a perceptible pulse in the amount of power been generated by the engine.

Here are the system details:
Dual PMAGs
Jumper-in (A curve)
Timed at 3 degrees After TDC (due to light weight prop).
This test was done at 25 inches / 2500 RPM.

Any ideas why is this happening and what might be the next course of action?
 
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less complete burn...

this is expected... with only one set of plugs firing there is less complete burn in the cylinder and some of the energy is lost to the exhaust. the different profile for your left and right mag check graph suggests that you have top plugs on one mag and bottom on the other rather than a cross mix. if you perform the mag check well LOP there should be a much more pronounced temperature rise.
 
Everything Stephen said is correct.

In addition, get your P-mags in for the Version 40 upgrade so you don't have to clock the P-mags for easy starting. V40 doesn't fire the plugs until 4 degrees past TDC for easier starting.

Did you time the P-mags with one blow or did you blow in them individually? It is best to have the MAP lines tied together and time them at the same time. I don't know why but with our EICommander customers, we have found it can make a difference, even though it shouldn't.

How many hours are on your spark plugs? We had some EICommander customers who were experiencing odd behavior, including myself, with over 100 hours on the auto plugs. We sent the plugs out for analysis and found some very minor deterioration in the insulator. This is nothing you would feel and the only way any of us knew there was an issue was because the EICommander was indicating something was going on. Changing the plugs with a new set solved that. (My current set have 140 hours on them and are in need of changing.)
 
[email protected] said:
this is expected... with only one set of plugs firing there is less complete burn in the cylinder and some of the energy is lost to the exhaust.
Click to expand...

that may explain why the egt rises during a mag check but I don't see how it explains a fluctuating EGT, which is NOT normal. I would check the plugs as Bill suggests.

Erich
 
Thanks for the responses.

The timing was set simultaneously on both pmags, by blowing once through the tied map lines. I am wondering if I need a restrictor/filter in the map tube in order to dampen those fluctuations. But this still would not explain higher oscillations on one side.

I will replace the spark plugs and see if anything changes.
 
Airzen said:
.... I am wondering if I need a restrictor/filter in the map tube in order to dampen those fluctuations. But this still would not explain higher oscillations on one side...
Click to expand...
Yes, put in some type of restrictor. I drilled the smallest hole I could through the top of an AN426 rivet and put it in the AN fitting and tightened it down. This smoothed out my MAP readings and I suspect also smoothed out the P-mag firing angle.

As for your EGT reading, it may solve it, it may not. It is possible that the flame front is moving around in the cylinder with only one firing the fuel charge, allowing more and varied flames out the exhaust valve. I'm not really sure on this one.
 
Study the following illustration, take from a thesis at MIT under the supervision of John Heywood. The research explores the effects of very late spark timing. You're looking at measurements for a retarded-but-conventional timing, spark at TDC, and a spark well after TDC. Note that in all cases, combustion ends long before exhaust valve opening (EVO).

The data was taken at 1500 RPM. EOC would indeed be a little later in crank angle at 2700 RPM, but not much.

http://www.google.com/url?sa=t&rct=...RElJ0e54eqpSUVB1fwzMf8A&bvm=bv.85076809,d.eXY

 
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Dan,

Thanks for the pointer. Here is my attempt at explaining the oscillations.

Since the PMAGS have an advanced spark, it follows from the first column, last row in the figure that the instantaneous Mass fuel burn (MFB) rate is going to fluctuate. (My guess is this is mostly because fuel kind of runs out towards end of the cycle).

The EGT fluctuations reflect this changes in MFB rate. And since one set of spark plugs are more efficient than the others (due to asymmetry effects), these oscillations are more pronounced when one of the mags is grounded.

Am I anywhere close?

Ashish
 
Airzen said:
Thanks for the pointer. Here is my attempt at explaining the oscillations.....
Am I anywhere close?
Click to expand...

We often read that EGT rise on one ignition is due incomplete combustion, or combustion so delayed that it is still taking place when the exhaust valve opens. Not so, thus the illustration. Nothing specific there regarding the oscillation.
 
We often read that EGT rise on one ignition is due incomplete combustion, or combustion so delayed that it is still taking place when the exhaust valve opens. Not so, thus the illustration. Nothing specific there regarding the oscillation.
Click to expand...

CORRECT! And I wish the OWT's would go away. The rise in EGT is to do with the expansion of the gas from a lower ICP (less expansion to atmosphere). Pv=nRT.
 
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.
 
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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
 
Bevan said:
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
Click to expand...

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

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