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
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?