Ed,
You take good notes.
Just a couple of clarifications: The decrease in the thermal conductivity of the copper core may be the result of thermal cycling causing development of porosity within the copper. This is independent of the corrosion going on between the Mn alloy electrode sheath and the insulator. So there are two essentially independent mechanisms contributing to the heat range change of the spark plug with age.
The development of the porosity within the copper is not particularly affected by the type of fuel whereas the corrosion between the Mn alloy sheath and the insulator is accelerated by the use of 100LL.
The cure is still to install new spark plugs.
I have colder spark plugs from Racing Beat that are recommended for prolonged use in high RPM high power applications. At best, the colder plugs will merely delay the onset of SAG but won't prevent it from happening eventually. It will be some time before the results of using those plugs will be known.
Steve Boese
From: Rotary motors in aircraft [flyrotary@lancaironline.net] On Behalf Of Ed Anderson [eanderson@carolina.rr.com]
Sent: Thursday, September 30, 2010 8:46 AM
To: Rotary motors in aircraft
Subject: [FlyRotary] New SAG Theory Discussions of SAG
In a nut shell, Steve's excellent data gathering (how many of use have access to a scanning electron microscope or spectrum analyzer {:>)) and analysis indicates a possible alternative reason for SAG other than fouling.
It basically boils down to a theory of pre-ignition based on the deteorating heat removal capacity of the plug.
There is a copper core (the photos showing the two cores) in the center of the ceramic cone which is responsible for conducting 90% of the heat away from the center electrode. Based on Steve's' electron microscope viewing of two cores (one new and one having been used in a 100LL environment), it appears that the plug using leaded gasoline has corrosion occurring on the boundary between this copper core and the surrounding (hot) elements. This layer of corrosion hinders heat removal from the plug leading to parts of the plug becoming hot enough to trigger pre-ignition.
This SAG tends to happened at high power settings and reducing power tends to make it go away (for a while). Steve's theory is that the plug encounters max heat and temperature at high power settings and that reducing the power permits the plug to cool off enough for the pre-ignition to go away (for a while).
So we now have two theories - one is the fouled plug and one of the overheated plug causing SAG. Preignition generally leads to detonation which does not appear to be happening (at least no one is finding the typical detonation damage to the rotary engine). A possible reason is the quenching effect of the less than optimum shape of the rotary combustion space may tend to delay or prevent the onset of detonation damage that one would expect from continuous preignition caused by a too hot plug.
I believe Steve intends to see if he can find some colder plugs (the ones that conduct heat better) and see if that extends the time between SAG. Currently I find that roughly 25-30 hours is about the max I can get using 100LL before SAG sets in, whereas Tracy and others have well over 150 hours using unleaded Mogas. So if the colder plugs extend the time before SAG, I would say that pretty much would suggest Steve's theory is correct - if Not, then perhaps the fouled Plug theory still has merit or it could be some other phenomena happening.
Steve, my apologies if I did not present your case as well as you could
Ed
Edward L. Anderson
Anderson Electronic Enterprises LLC
305 Reefton Road
Weddington, NC 28104
http://www.andersonee.com
http://www.eicommander.com
.
Sent: Thursday, September 30, 2010 9:59 AM
Subject: [FlyRotary] Discussions of SAG
I understand that there was a presentation on SAG at the meeting. Can someone go over the alternative theory with the ones of us that were not in attendance? It looked like someone had a scanning electron microscope photo that they were showing! Wow! Are we getting sophisticated or what!! Was this an explanation of the cause, or ideas for prevention, both??
Bill B