| Gary:
To further the educational exchange:
*** I made the assumption that the timing was originally "correct" with the peak pressure occurring at the right time - not always a valid assumption. If the burn rate is slowed, the peak cylinder pressure will occur at a later point in the cycle, reducing the power, but only if it was originally correct. **
With the thing correct, at takeoff power the thetaPP is closer to TDC than desirable. So, moving the thetaPP away from TDC increases power. IN some applications, it would produce MORE power to takeoff on one mag when the thetaPP would be 16dATDC rather than about 10dATDC. I am NOT recommending this, just pointing out that the physics is what it is.
*** I'd challenge Walter to do a quick dyno test at maybe 15 inches MAP with only one plug firing. What does the EGT vs fuel flow curve look like?***
It looks the same. What I think you are referring to is the situation where the EGT begins to rise when VERY LOP and leaning continues. This is a normal phenomenon in the AT THAT POINT the exhaust valve beings to open while a minimal amount f burning is still taking place. This is normal but MUCH leaner than any mixture that would be useful on the lean side of peak. This occurs significantly past the BSFC(min) mixture.
The problem with doing the mag check after the flight at low power is that is is poorly diagnostic compared to the in-flight, high power LOP mag check. Thousands of pilots are currently doing them routinely. The piston airlines did them routinely in flight with paying passengers on board. If one is not comfortable with anything, I suggest either not doing it or getting comfortable. That is a personal choice. It is helpful to know that there is no scientific reason not to do the in-flight mag check. On Oct 24, 2013, at 7:07 AM, Gary Casey wrote: It is with great reluctance that I would contradict - or even question - anything said by Walter, but here goes...
I said:
The slower burn rate on one plug means that a significant portion of the =
burn happens during the expansion stroke and less heat is converted to =
power. =20
Walter replied:
Uh, not really. You can actually end up with MORE power, depending on =
where the thetaPP started.
My reply to his reply: I made the assumption that the timing was originally "correct" with the peak pressure occurring at the right time - not always a valid assumption. If the burn rate is slowed, the peak cylinder pressure will occur at a later point in the cycle, reducing the power, but only if it was originally correct.
I said:
Further, when the mixture is leaned past the best power (or maybe peak
=
EGT) the flame travel is even slower and consequently the exhaust =
temperature will likely just continue to go up when leaning, not peak =
and go down.
Walter replied:
NO! EGT will always rise to peak, then fall when LOP. ALWAYS. Unless, =
of course, Sir Isaac Newton was wrong.
My reply to his reply: I'm not sure Walter conclusion is always correct, especially when running on one plug(which was how the conversation started) and at low BMEP (naturally aspirated at high altitude. I have tested my Lycoming at 15,000 feet (both plugs firing) and found that, while there was a noticeable change in slope of the EGT when going LOP, with the slope leveled off and then continued to go up, not down. In another case I was
completing a flight with the Lightspeed not firing on all cylinders. The only way I could keep the EGT down to a reasonable value was to run ROP - when going lean the EGT of that cylinder would seem to just keep rising as I leaned. My conclusion is that the EGT doesn't ALWAYS decrease LOP. If the flame travel is already slow with the thetaPP occurring after the optimum, the opposite can happen. I'd challenge Walter to do a quick dyno test at maybe 15 inches MAP with only one plug firing. What does the EGT vs fuel flow curve look like?
Gary
ps: There was another post that
concerned itself about doing a mag test in flight. It suggested it be done at only certain conditions. Actually a single mag should fire the mixture reliably under ALL conditions. Otherwise there is something wrong. I agree with the idea of doing inflight mag checks, but I'll admit I have a fundamental problem shutting things off in flight, so I rarely do them. It makes all the sense in the world to do a quick mag check after flight. What better time to find out if maintenance is required? |