Detonation problems from junk on spark plug

[Skybrd]

I learned a lesson years ago that might be helpful for others to learn. Years ago, I took off at Lompoc airport with my Experimental Wittman Tailwind and shortly after take off, the engine ran terrible and started shaking. Fortunately the Tailwind can run on reduce power and doesn't fall out of the sky when the engine power is removed. I nursed the plane back around and landed as quickly as possible. Before takeoff I did my pre-checklist test and no engine problem showed. -- To try to find the problem, I tied the plane down on the ramp and ran the engine up to full power. I was able to get it to repeat the same problem and then tried to do a mag check to see if that would help. The mag check when the problem was occuring didn't make any difference. I did try something else that worked. I pulled the power back to less power and as the engine cooled the problem cleared up. Next I turned off one of the magnetos and ran the power up again to try to get it to run bad again and if I had selected the magneto off on the bad sparkplug the problem would not show up again. -- I found out the problem was small piece of junk embedded into the electrode of the spark plug that acted as a ember when the temperatures were hot enough and caused combustion firing at the wrong time. If this ever happens to you and your at a safe altitude try to pull the power back to cool off the cylinders and turn off one of the magnetos off and then run it back up again. If it gets bad again then do it again by pulling the power back to cool it again and then change the mag switches to the other one. Hopefully this might save your engine and life. I just watched a show on TV where this might have been their problem. The show had a Curtis C-46 Aircraft with engine problems in flight and the engine appeared to be backfiring. The pilot tried to pull the power and feather the prop but it wouldn't work and had to continue flying with a inflight emergency condition and poor weather.

[Kwixdraw]

Was that a carbureted engine on the Tailwind?

[Skybrd]

One thing I forgot to write about concerning ways to test for detonaton in flight is to lean the engine under high power. This will quickly heat up the cylinder and if there is junk in the spark plug to cause detonation it will happen.  I've seen this to happen when I ran my Tailwind engine at full power and leaned for more power. Low altitude airports will show up detonation on takeoffs more that higher elevations.

[Skybrd]

Quote: Kwixdraw

Was that a carbureted engine on the Tailwind?

[Shadrach]

This is actually a preignition situation... I am surprised that switching mags had any affect,  as I would think that the "ember" was being heated by the combustion event, and not the plug firing...

[DaV8or]

I don't think this was pre-ignition or detonation. Pre-ignition for very much time usually would have resulted in a forced landing with a melted piston or some other serious failure. I think more likely that the junk in the electrode was causing the plug to ground under higher combustion pressures and not make a spark. This would cause one of your cylinders to make less power than the rest and so run rough. Spark plugs are sometimes funny beasts. I have seen this sort of thing before in engines other than airplanes before. Just a guess though, who knows what really happened.

[Skybrd]

Quote: DaV8or

I don't think this was pre-ignition or detonation. Pre-ignition for very much time usually would have resulted in a forced landing with a melted piston or some other serious failure. I think more likely that the junk in the electrode was causing the plug to ground under higher combustion pressures and not make a spark. This would cause one of your cylinders to make less power than the rest and so run rough. Spark plugs are sometimes funny beasts. I have seen this sort of thing before in engines other than airplanes before. Just a guess though, who knows what really happened.

[Kwixdraw]

Also with all due respect Skybrd, if this Was detonation, you were flirting with disaster trying to diagnose this problem in flight and then risking your engine to go there again on the ground. The pressures involved in true detonation are extremely unhealthy to engines and I would not recommend anyone follow these methods to find out what the problem was. How long did the engine fly after this episode? Detonation puts severe loads on everything and it's not uncommon for rod bolts and other parts to fail after a bout of detonation.

[Shadrach]

Quote: Skybrd

Respectfully no. It wasn't a grounded plug but ember that ignited fuel at the wrong time. Also from the previous post, it didn't clear up with a mag test while the plug was acting as a ember.

[Skybrd]

Quote: Kwixdraw

Also with all due respect Skybrd, if this Was detonation, you were flirting with disaster trying to diagnose this problem in flight and then risking your engine to go there again on the ground. The pressures involved in true detonation are extremely unhealthy to engines and I would not recommend anyone follow these methods to find out what the problem was. How long did the engine fly after this episode? Detonation puts severe loads on everything and it's not uncommon for rod bolts and other parts to fail after a bout of detonation.

[Shadrach]

Detonation

Detonation is the spontaneous combustion of the end-gas (remaining fuel/air mixture) in the chamber. It always occurs after normal combustion is initiated by the spark plug. The initial combustion at the spark plug is followed by a normal combustion burn. For some reason, likely heat and pressure, the end gas in the chamber spontaneously combusts. The key point here is that detonation occurs after you have initiated the normal combustion with the spark plug.

Pre-ignition

Pre-ignition is defined as the ignition of the mixture prior to the spark plug firing. Anytime something causes the mixture in the chamber to ignite prior to the spark plug event it is classified as pre-ignition. The two are completely different and abnormal phenomenon.

[DaV8or]

Quote: Shadrach

Detonation

Detonation is the spontaneous combustion of the end-gas (remaining fuel/air mixture) in the chamber. It always occurs after normal combustion is initiated by the spark plug. The initial combustion at the spark plug is followed by a normal combustion burn. For some reason, likely heat and pressure, the end gas in the chamber spontaneously combusts. The key point here is that detonation occurs after you have initiated the normal combustion with the spark plug.

Pre-ignition

Pre-ignition is defined as the ignition of the mixture prior to the spark plug firing. Anytime something causes the mixture in the chamber to ignite prior to the spark plug event it is classified as pre-ignition. The two are completely different and abnormal phenomenon.

[Shadrach]

Thanks for linking article, I did not read it yet, it was the first concise description of both that I found when trying to get a definition...

A final add on though, pre-ignition can  at times cause the same rapid pressure spikes that detonation does, giving us the same ping/pinking sound (which is the sonic boom of the cumbustion event/flame front breaking the sound barrier as it explosde under extreme pressure). The consequeces are often the same, it is the genesis of the ignition that is different.