Hey Ed, now might be the time you cleaned a few SAG plugs and
tested them?
It seems hard to believe that plugs need be thrown out after 25
hours.
BTW, up here in Ottawa, I had to pay $17 each ($68 per set) for
stock Mazda plugs, on a trip south I picked up some in Ogdensburg NY around $5
each.
Jeff
From: Rotary motors in
aircraft [mailto:flyrotary@lancaironline.net] On Behalf Of Ed Anderson
Sent: Tuesday, July 07, 2009 7:10 AM
To: Rotary motors in aircraft
Subject: [FlyRotary] SAG was: [FlyRotary] Re: Muffler
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?