Jon Wilder
Pre-takeoff checklist
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- Oct 19, 2015
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Jwylde
I'm asking here because the AFMS doesn't mention anything about leaning procedures. But...this system is a variable advance system, which changes everything.
A lean mixture burns slower, yet the fixed timing of a traditional two magneto system lights the mixture at exactly the same point of rotation on the compression stroke (for sake of this conversation, we'll use 25°). Regardless of mixture setting, the mag always lights the mixture at 25° before TDC. Thus, as you lean, the point of rotation after TDC at which peak cylinder pressure occurs on the power stroke changes due to the slower burning of the leaner mixture. As you approach stoichiometric, some of this mixture is still being consumed as the exhaust valve opens, causing the EGTs to increase.
Traditionally, you would lean until peak EGT occurs, then turn it back rich of peak by 50-100°F.
However, this electronic direct fire high energy ignition system has variable timing that adds 2° of advance for every 1 in hg of manifold pressure drop. According to Mike Kobylik of Electroair, the peak EGT point, in regards to where it happens on the mixture control travel, actually shifts, and this exhibits itself as a greater pull of the mixture control to get to peak EGT.
Pilot reports from those who have flown our Electroair equipped 172M have noted that moving the mixture control doesn't seem to have an effect on EGT, however.
Perhaps they're not inputting enough of a lean input?
If I'm understanding what is happening, as the mixture is leaned out, manifold pressure decreases, causing the timing to advance and the mixture to light earlier. As the advance matches the burn rate, peak cylinder pressure is still occuring at the optimum point, and the mixture has already been completely burned by the time the exhaust valve opens, thus there would be no increase in EGT. CHT, however, would increase as the engine is now doing a better job of burning the mixture in the combustion chamber instead of in the exhaust manifold.
But...at some point in the leaning, and perhaps further back than it would be with a fixed timing magneto, you start to lose that ideal mixture. Manifold pressure would start to increase again, causing the timing to retard. The mixture is now burning even slower, and is being lit at a later point in the cycle. The mixture, at this point, would still be burning as the exhaust valve opens, cuasing a rise in EGT. From here you should be able to find peak EGT, if you haven't yet run into lean misfire.
For those who are flying the EIS-41000 ignition system, what say you? What has been your experience leaning the mixture with this system? Electroair says there are no different leaning procedures, but that peak EGT will require a greater pull of the mixture control than with a fixed timing magneto. It just seems to me that the advance cancels out the mixture control input, thus EGT never rises. Are we just not going lean enough because of the advance?
Any input on this would be greatly appreciated. Thanks in advance.
A lean mixture burns slower, yet the fixed timing of a traditional two magneto system lights the mixture at exactly the same point of rotation on the compression stroke (for sake of this conversation, we'll use 25°). Regardless of mixture setting, the mag always lights the mixture at 25° before TDC. Thus, as you lean, the point of rotation after TDC at which peak cylinder pressure occurs on the power stroke changes due to the slower burning of the leaner mixture. As you approach stoichiometric, some of this mixture is still being consumed as the exhaust valve opens, causing the EGTs to increase.
Traditionally, you would lean until peak EGT occurs, then turn it back rich of peak by 50-100°F.
However, this electronic direct fire high energy ignition system has variable timing that adds 2° of advance for every 1 in hg of manifold pressure drop. According to Mike Kobylik of Electroair, the peak EGT point, in regards to where it happens on the mixture control travel, actually shifts, and this exhibits itself as a greater pull of the mixture control to get to peak EGT.
Pilot reports from those who have flown our Electroair equipped 172M have noted that moving the mixture control doesn't seem to have an effect on EGT, however.
Perhaps they're not inputting enough of a lean input?
If I'm understanding what is happening, as the mixture is leaned out, manifold pressure decreases, causing the timing to advance and the mixture to light earlier. As the advance matches the burn rate, peak cylinder pressure is still occuring at the optimum point, and the mixture has already been completely burned by the time the exhaust valve opens, thus there would be no increase in EGT. CHT, however, would increase as the engine is now doing a better job of burning the mixture in the combustion chamber instead of in the exhaust manifold.
But...at some point in the leaning, and perhaps further back than it would be with a fixed timing magneto, you start to lose that ideal mixture. Manifold pressure would start to increase again, causing the timing to retard. The mixture is now burning even slower, and is being lit at a later point in the cycle. The mixture, at this point, would still be burning as the exhaust valve opens, cuasing a rise in EGT. From here you should be able to find peak EGT, if you haven't yet run into lean misfire.
For those who are flying the EIS-41000 ignition system, what say you? What has been your experience leaning the mixture with this system? Electroair says there are no different leaning procedures, but that peak EGT will require a greater pull of the mixture control than with a fixed timing magneto. It just seems to me that the advance cancels out the mixture control input, thus EGT never rises. Are we just not going lean enough because of the advance?
Any input on this would be greatly appreciated. Thanks in advance.
