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  #1  
Old 01-31-2015, 01:43 AM
Airzen Airzen is offline
 
Join Date: Apr 2011
Location: Kirkland, WA
Posts: 167
Default EGT fluctuations during in-flight mag check (PMAGS)

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.



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?

Last edited by Airzen : 01-31-2015 at 01:58 AM.
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  #2  
Old 01-31-2015, 04:26 AM
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schristo@mac.com schristo@mac.com is offline
 
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Location: WA
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Default 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.
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  #3  
Old 01-31-2015, 04:48 AM
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N941WR N941WR is offline
 
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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.)
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  #4  
Old 01-31-2015, 07:22 AM
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erich weaver erich weaver is offline
 
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Default

Quote:
Originally Posted by schristo@mac.com View Post
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.
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
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  #5  
Old 01-31-2015, 08:31 AM
Airzen Airzen is offline
 
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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.
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  #6  
Old 01-31-2015, 10:18 AM
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N941WR N941WR is offline
 
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Quote:
Originally Posted by Airzen View Post
.... 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...
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.
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  #7  
Old 01-31-2015, 12:01 PM
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DanH DanH is offline
 
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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=j...85076809,d.eXY

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Last edited by DanH : 01-31-2015 at 12:05 PM.
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  #8  
Old 01-31-2015, 01:43 PM
Airzen Airzen is offline
 
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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
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  #9  
Old 01-31-2015, 02:18 PM
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DanH DanH is offline
 
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Quote:
Originally Posted by Airzen View Post
Thanks for the pointer. Here is my attempt at explaining the oscillations.....
Am I anywhere close?
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.
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  #10  
Old 01-31-2015, 06:04 PM
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RV10inOz RV10inOz is offline
 
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Quote:
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.
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.
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