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?