lml@lancaironline.net Mailing List Archive

I agree with Scott. I'm certainly not an expert on the certification requirements, but as I recall after an electrical failure (like a failure of one mag) the engine has to produce at least 90% power and be free from detonation, overheating and other bad stuff. Just imagine a circle with two ignition points spaced from the center. With both igniting fuel at the same time, two ever-expanding circles of flame will gradually burn all the fuel. With only one lighting the fuel, it's "circle of fire" will have to expand clear to the opposite side of the chamber, which will take longer. One problem with that is that the air/fuel mixture at the far end of the chamber will have longer to get hot from the high pressure in the chamber and heat radiated from the flame front. Consequently, I'm sure that the detonation margin will be less when running on one plug. When running one EI, one plug will likely fire before the other. Not as bad as the second not firing at all, but complete combustion is still delayed a little. Does the mag still contribute? I'm sure it does, as the flame certainly hasn't progressed to the location of the mag-fired plug. I'm also sure the mag actually fires, even though the pressure in the chamber is somewhat higher than it would be if the EI hadn't fired its plug first. But does the EI produce a "more reliable" spark? No, I don't think so - the mag lights the fire every time, just as the EI. Is the EI-produces spark "better"? In some ways. The CD system used by Lightspeed will certainly fire a fouled plug better. When my engine was new it would foul plugs really bad during ground running, and the mag couldn't fire its plugs at all, while the Lightspeed would keep the engine running just fine, thank you. The E/P-mag uses an inductive system and it won't be quite as good with fouled plugs, but it will be better at running far lean of peak. In normal operation with reasonably good plugs the mag will do just fine. So you pays your money and takes your choice :-).

If you have increased the compression ratio beyond what Lycoming or Continental had designed into the engine, don't be misled into thinking that they would just too dumb to do it themselves. They had to guarantee freedom from detonation under the worst condition, which is most likely 115F ambient, dry air, redline cylinder head temperatures while running on one mag. When we raise the compression ratio like I have we have to recognize that we have probably reduced or destroyed that margin. If one ignition fails, reduce manifold pressure as much as possible, run rich and land (running LOP pretty much requires that 2 plugs be firing because of the slow flame travel).

Just my opinion,

Gary

From Scott:

Chris,

Besides the two plugs being backup, the flame front is more effective with 2 ignition sources. Some dual EI's that communicate with each other change the timing slightly if one of them has ceased operating. As can be seen from my experience, 25 DBTDC (both mag and single EI) cover a broad range of cruise flight conditions and the EI is producing a better and more reliable spark. This is probably why there is a more modest cruise performance improvement from only one EI. At high altitudes or low power, the mag is lagging the advanced EI spark and the flame fronts are started differently or the mag is already firing into an already started combustion event. This still improves the power delivered from a better timed fire.

To experience your question, turn off one of your mags in cruise. You know what happens at the pre flight mag check - RPM drops and RPM is a component of power. The loss of power is related to the entire combustion event timing not matching the twin flame front condition.

Scott

In a message dated 11/28/2011 9:53:59 A.M. Central Standard Time, chris_zavatson@yahoo.com writes:

Gary, Scott,
A little off-thread, but...
Do our Lycoming cylinders require that each spark plug fires to properly propagate the combustion event throughout the entire cylinder? On those planes that have one mag and one EI (and while the EI has advanced its spark), does the mag contribute or can it even still fire?

Chris Zavatson