Interesting EGT.....

[Brantel]

Originally Posted by Captain Avgas

Paul, I might have been a bit hesitant to use an "insulating compound" such as Dow 4 on plug connectors but if you go to this technical blurb from Dow Corning you can see that it is in fact recommended for spark plug connectors.
http://www.dowcorning.com/content/ne...4_Compound.asp

Dielectric grease is used all over the world for these types of connections. Helps prevent corrosion and arcing....love the stuff!

[1:1 Scale]

Paul, did you try to lean more aggressively at all before going rich?

It obviously wouldn't have helped in this case, but the reason I ask is because I've had plugs foul in flight before, with the same rise in EGT and roughness. A couple of cycles of aggressive leaning has cleaned the plug and brought the errant EGT back in line.

I don't remember specifically what the CHT was doing, but the engines that it happened to me on run quite cool CHT's, so I was never concerned about them getting dangerously hot (both are Franklins, a 150 and a 220).

The EGT rise is due to the ignition timing effectively getting retarded by the one plug not firing, causing the mixture to still be burning as it's going out of the exhaust (imagine the flame front starting at each plug and meeting in the center of the cylinder for a properly timed combustion event, then imagine taking a plug out of the equation- the flame front now has to travel twice as far). I would expect the CHT's to lower a bit in the event of a fouled or not firing plug since the entire combustion event (and therefore the heat) is not taking place inside the cylinder....

[Ironflight]

Originally Posted by 1:1 Scale

Paul, did you try to lean more aggressively at all before going rich?

It obviously wouldn't have helped in this case, but the reason I ask is because I've had plugs foul in flight before, with the same rise in EGT and roughness. A couple of cycles of aggressive leaning has cleaned the plug and brought the errant EGT back in line.

I don't remember specifically what the CHT was doing, but the engines that it happened to me on run quite cool CHT's, so I was never concerned about them getting dangerously hot (both are Franklins, a 150 and a 220).

The EGT rise is due to the ignition timing effectively getting retarded by the one plug not firing, causing the mixture to still be burning as it's going out of the exhaust (imagine the flame front starting at each plug and meeting in the center of the cylinder for a properly timed combustion event, then imagine taking a plug out of the equation- the flame front now has to travel twice as far). I would expect the CHT's to lower a bit in the event of a fouled or not firing plug since the entire combustion event (and therefore the heat) is not taking place inside the cylinder....

I usually lean so aggressively that there isn't much more "lean" than I can go - but your idea might help someone in teh future.

I also figured that the reason that the EGT rises when one plug stops firing is the fire still goign on when the exhaust valve opens. Makes sense when you visualize it, but a serious combustion engineer woudl have to confirm it with science....

[lr172]

Originally Posted by Ironflight

I usually lean so aggressively that there isn't much more "lean" than I can go - but your idea might help someone in teh future.

I also figured that the reason that the EGT rises when one plug stops firing is the fire still goign on when the exhaust valve opens. Makes sense when you visualize it, but a serious combustion engineer woudl have to confirm it with science....

It doesn't matter, one plug or two, the charge will fully burn long before the bottom of the power stroke. Most engines only use one plug. EGT is going up due to the retarded timing associated with firing only one plug vs two (your "effective timing" is probably retarding 10 - 15*). Timing is not really relevant here on the exhaust theory. The flame speed is SO much faster than the piston speed, it is not possible to ignite it late enough for it to go out the exhaust port. If your WAY late, and the piston goes down, you lose the compression and then a spark cannot ignite it. The fuel air mixture in an uncompressed state cannot be ignited by a spark plug.

Larry

[scsmith]

Originally Posted by lr172

It doesn't matter, one plug or two, the charge will fully burn long before the bottom of the power stroke. Most engines only use one plug. EGT is going up due to the retarded timing associated with firing only one plug vs two (your "effective timing" is probably retarding 10 - 15*). Timing is not really relevant here on the exhaust theory. The flame speed is SO much faster than the piston speed, it is not possible to ignite it late enough for it to go out the exhaust port. If your WAY late, and the piston goes down, you lose the compression and then a spark cannot ignite it. The fuel air mixture in an uncompressed state cannot be ignited by a spark plug.

Larry

Well, not "still burning" but lets say, less time since it stopped burning. Engines with only one spark plug usually have it closer to the middle of the cylinder head. Flame front is much slower for very lean mixture, which is why it takes so much spark advance to get it all burned efficiently. Loosing a spark means only one flame front, with a lot farther to travel to get it all burned.

[lr172]

Originally Posted by scsmith

Well, not "still burning" but lets say, less time since it stopped burning. Engines with only one spark plug usually have it closer to the middle of the cylinder head. Flame front is much slower for very lean mixture, which is why it takes so much spark advance to get it all burned efficiently. Loosing a spark means only one flame front, with a lot farther to travel to get it all burned.

Most engines light the mixture around 35* before TDC and we hit peak power around 10* after TDC. I expect that flame has extinguished by 25-30* after TDC, though the gas is still hot and expanding.

The lycoming, at 25* timing, is getting about the same peak pressure point with dual spark. With only one plug, that moves to around 20* and the flame would extinguish by 40* after TDC.

The exhaust valve doesn't open until around 190* afterd TDC.

Larry

[DanH]

Originally Posted by lr172

The exhaust valve doesn't open until around 190* afterd TDC. Larry

Naaa. Typical would be more like 120~130 after TDC

[RV6_flyer]

Originally Posted by lr172

It doesn't matter, one plug or two, the charge will fully burn long before the bottom of the power stroke.

Larry

That is NOT what I was taught. The Lycoming cylinder diameter is so large that complete combustion does not occur when only one spark is used for ignition.

[humptybump]

Noob question.

I now know what "Dow #4" is.

What does "burp the plug wire" mean?

Thanks.

[DanH]

Originally Posted by RV6_flyer

That is NOT what I was taught. The Lycoming cylinder diameter is so large that complete combustion does not occur when only one spark is used for ignition.

Typical complete combustion (90 to 100% mass fraction burned) for a relatively non-turbulent four stroke (read "Lycoming") would be about 40 ATDC, well before exhaust valve opening. The delay due to single plug ignition is unlikely to be an additional 60 to 80 degrees.