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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
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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
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