There are undoubtedly numerous possibilities
as to what is happening/causing SAG. However, two things are very clear:
- 100
LL does definitely reduce the time spark plugs can be used before SAG
occurs. The nominal time period that I have found in my engine is
24-30 hours. You can take the plug out and examine the ceramic cone
and see the glint of what I believe (but have not done a chemical
analysis) of lead crystals. Folks using MOGAS (such as Tracy) have reported
going over 150 hours without SAG. So I do believe there is a
connection between 100LL and the short interval of use before SAG.
I even attempted to use
TCP which is used in some aircraft engines and is reportedly suppose to reduce
lead deposits. It simply make the problem worst, but upon further
research I discovered that for this chemical to work the cylinder head temperature
has to be much higher than what our water cooled sparkplugs typically
see. In my case, the chemical caused further deposits (of some sort) on
the sparkplug and I barely got 5 hours out of the set I used. This may in
part explain why our sparkplugs seem to foul more with 100LL than aircraft
sparkplugs – they don’t get hot enough?
- If
you encounter SAG, then replacing the sparkplugs immediately eliminates
the SAG problem (for a period of time).
So both of those factors lead me to
believe it is fouling of spark plugs rather than pre-ignition. If pre-ignition
then my dozens of times of SAG (like every 15 minutes on the 5 hour trip from Louisiana) would surely
have resulted in damage to the engine.
In any case, whatever the cause, since I
have been replacing my sparkplugs regularly at 25-30 hour intervals I have
encountered no SAG. YMMV
Ed
From: Rotary motors in aircraft
[mailto:flyrotary@lancaironline.net] On
Behalf Of William Wilson
Sent: Tuesday, July 07, 2009 12:09
AM
To: Rotary
motors in aircraft
Subject: [FlyRotary] Re: Muffler
While the rotary design
is a little more resistant to the onset of preignition, once it happens, the
rotary is damaged just as easily as a piston engine or even more so. Drag
racers suffering preignition have dismantled their motors and found rotors
dented in from excess chamber pressure. It is possible that you have been
having mild preignition not enough to damage the engine, I suppose. Do
you suppose you have been running really excessively rich? That would
also contribute to plug fouling, low power and low EGT. If you have fuel
injection, a fuel map problem showing up only at high manifold pressures
(higher than you normally attain in your usual operating regime) maybe?
You should get WAY more than 10 hours on spark plugs even running 100LL.
If combustion were still taking place as you speculate then it would ignite the
incoming mixture during the intake cycle and cause a backfire. So I think
that is not it.
I assume you've seen this but I found it interesting:
http://www.wankel.org/74_Ignition/74-22%20Spark%20plug%20life%20.htm
On Mon, Jul 6, 2009 at 7:17 PM, Steven Boese <sboese@uwyo.edu> wrote:
I never found the broken muffler part in the outlet of the
muffler where it might have caused increased back pressure even though it was
possible for it to end up in that location by standing the muffler on its
end. The symptoms after repairing the muffler were the same as before the
repair was made so at this point, I don’t think the broken part was a
factor in the performance issue. Looking back at the data log, the #2
rotor EGT dropped by about 250 degrees during the power deficit. Back
pressure would probably affect both rotors in a similar fashion so that would
also point to SAG as the culprit.
In response to your and Bobby’s questions, at the onset
of SAG, there were a total of 15 hours on the plugs, 106 gal total of fuel
burned, 53 gal of that was 100LL.
Ed is most likely right that operating at the increased
manifold pressure available at low altitude made the SAG show up while it
didn’t at lower manifold pressures that I normally see.
The relatively small amount of time and fuel that the plugs
had seen when SAG occurred may also be due to the low power levels and
correspondingly lower combustion temperatures permitting more rapid lead
deposition on the plugs. It looks like I should routinely change plugs
every 10 hours or so. I did have extra plugs along on the Texas trip and it would
have been easy to have installed them at the rotorfest.
The nature of SAG is puzzling. The symptoms of
decreased EGT, decreased power output, and increased heat transfer to the oil
and coolant are all consistent with descriptions of pre-ignition more than that
of spark plug misfire. In piston engines, pre-ignition can rapidly lead
to destruction, but the rotary engine may be more tolerant of it. Maybe
it isn’t a matter of no ignition due to the spark plug not firing, but a
matter of the fire not going out between sparks. The lead in 100LL
increases the octane rating by slowing the combustion rate and thereby
decreasing the tendency for detonation. Maybe there is enough gas phase
lead in the vicinity of the spark plug with lead deposited on it that the
combustion rate is slowed enough to persist from one cycle to the next while
the normal combustion rate occurs in the rest of the chamber. This could result
in a process similar to pre-ignition. This theory probably has as much
probability of being right as the one I had concerning the apex seals clicking.
Maybe I can accumulate a pile of SAGGING spark plugs as big
as Ed’s. That would be verifiable at least.
Steve Boese
-----Original Message-----
From: Rotary
motors in aircraft [mailto:flyrotary@lancaironline.net]
On Behalf Of Todd Bartrim
Sent: Monday, July 06, 2009 6:23
PM
To: Rotary
motors in aircraft
Subject: [FlyRotary] Re:
Muffler
Hi Steve;
So you now assume that your power loss is directly
attributable to SAG and not to the partially blocked muffler? Is this correct?
I'm curious as I also use a homemade muffler (SpinTech
copy) and have wondered what the result would be if any of the internal came
loose. I wonder if your increased backpressure contributed to the onset of SAG?
And do you only burn 100LL, occasionally, or never?