spark plug fouling

traumachicken

traumachicken

Well-Known Member
Could someone explain to me what exactly causes this. I know that on warm days, by not leaning the mixture while taxiing, it can cause carbon and lead deposits to build. But I want to know why.

Thanks.
 
traumachicken said:
Could someone explain to me what exactly causes this. I know that on warm days, by not leaning the mixture while taxiing, it can cause carbon and lead deposits to build. But I want to know why.

Thanks.
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Think about it, you need three things for fire; fuel, O2, and ignition. For a given mixture setting an engine is going to use the same amount of fuel for each cylinder for a given throttle setting. When the air warms it expands, think density altitude, there are fewer O2 molecules going into the engine which mean less to burn. Same reason you lean the mixture at altitude. If you don't lean the mixture, at low power settings like on taxi when the engine is cool, the unburnt fuel does break down to carbon and is deposited around the cylinder head, which includes areas around the valves and sparkplugs
 
Just talked to an A&P about an hour ago and was told that you can also get fouled plugs by frequently running too lean since this leads to damaged rings and engine oil getting past them.
 
Wastegate said:
Just talked to an A&P about an hour ago and was told that you can also get fouled plugs by frequently running too lean since this leads to damaged rings and engine oil getting past them.
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I doubt that would be a problem on an engine that is well cared for. You can get fouling on the ground on a hot day in anything.
 
Another factor is the most aviation engines use 100LL ie Low Led. Lead/carbon can get built up over time, due to running too rich a mixture during any phase of engine operation, grounding out the spark plug.
 
Every 50 hours I check spark plugs on my new Lycoming O-320. I run it lean, rich, hard, easy, on mogas, vodka and LL and I always get lead balls on ALL BOTTOM spark plugs. I don't know why but I always check, clean and rotate them.


plug1.jpg



plug4.jpg
 
The reason any of this matters on an aviation piston engine is because you're running 1930's technology with absolutely nothing to control the combustion process but your right hand on that little red knob. At full rich, the mixture is WAY too rich, even at sea level at max takeoff power. This is because these engines rely on the adiabatic cooling of the extra fuel to control cylinder temperatures at higher power settings. At lower power settings, you are so overly rich that you literally have completely unburnt fuel leaving in the exhaust (which is what creates that very distinct exhaust smell when taxiing). However, running this rich leads to very incomplete combustion, and cooler burn temperatures. This causes significant carbon build up, and, because AvGas contains lead, it also leaves lead deposits on things like spark plug electrodes. These things can run very clean at higher power settings, and especially, if lean of peak. 135 piston bird engines look completely different when torn down when compared to flight school planes because of this.

Also, keep in mind, that any air cooled engine runs much looser tolerances to allow for the huge temperature swings of normal operation, so things like the piston ring gaps are huge, which allows for a decent amount of blow-by, which is why these things dirty their oil up so quickly, and why they consume quite a bit. Oil doesn't burn very cleanly, so this leaves even more carbon in there to build up (this is true even on a brand new engine).

This entire issue is largely what has the EPA so upset. You want a clean burning piston engine? It'll be computer managed and liquid cooled.
 
Wastegate said:
Just talked to an A&P about an hour ago and was told that you can also get fouled plugs by frequently running too lean since this leads to damaged rings and engine oil getting past them.
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I don't know how to put one of those face palm pics, but this needs one.

Running lean will not do anything to the piston rings. What fouls spark plugs is lead, not oil.
 
USMCmech said:
I don't know how to put one of those face palm pics, but this needs one.

Running lean will not do anything to the piston rings. What fouls spark plugs is lead, not oil.
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Unless the rings are already hosed anyway and it's super cold out and the oil is congealed on the plug... I had to clean a set because of that once.

At any rate.

Mixture is rich on the ground for a reason. I know it bugs the crap out of Patrick, but for the moment the engines we have are the engines we have ;) Some say you should lean on the ground aggressively. I personally am more in favor of making sure your idle mixture is set properly so that you don't have to (Single engine restart Cessna owners, it's an AD for you).

Nothing you can do will prevent lead fouling, and you can't burn off lead either. All you can do is clean the plugs once in a while. The fouling you tend to get on the ground is carbon fouling and is easily burned off.
 
trafficinsight said:
Unless the rings are already hosed anyway and it's super cold out and the oil is congealed on the plug... I had to clean a set because of that once.

At any rate.

Mixture is rich on the ground for a reason. I know it bugs the crap out of Patrick, but for the moment the engines we have are the engines we have ;) Some say you should lean on the ground aggressively. I personally am more in favor of making sure your idle mixture is set properly so that you don't have to (Single engine restart Cessna owners, it's an AD for you).

Nothing you can do will prevent lead fouling, and you can't burn off lead either. All you can do is clean the plugs once in a while. The fouling you tend to get on the ground is carbon fouling and is easily burned off.
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I think to the OP, if you read my first response, and this one... You will have your answer. This is why when you run the engine up to a higher powersetting for a minute it clears it
 
trafficinsight said:
Unless the rings are already hosed anyway and it's super cold out and the oil is congealed on the plug... I had to clean a set because of that once.

At any rate.

Mixture is rich on the ground for a reason. I know it bugs the crap out of Patrick, but for the moment the engines we have are the engines we have ;) Some say you should lean on the ground aggressively. I personally am more in favor of making sure your idle mixture is set properly so that you don't have to (Single engine restart Cessna owners, it's an AD for you).

Nothing you can do will prevent lead fouling, and you can't burn off lead either. All you can do is clean the plugs once in a while. The fouling you tend to get on the ground is carbon fouling and is easily burned off.
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Nevermind the fact that it "bugs the crap out of me" (which it does, but that's another topic), I am in favor of aggressively leaning on the ground. You're absolutely correct though, that ground fouling is carbon and/or unburnt fuel, not lead. Either way, running leaner (and hotter, even if only slightly) on the ground will help avoid these problems.
 
Patrick said:
Nevermind the fact that it "bugs the crap out of me" (which it does, but that's another topic), I am in favor of aggressively leaning on the ground. You're absolutely correct though, that ground fouling is carbon and/or unburnt fuel, not lead. Either way, running leaner (and hotter, even if only slightly) on the ground will help avoid these problems.
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I don't disagree with you, in theory anyway, I said that because I remember having this discussion before ;)


Sent from 1865 by telegraph....
 
sorry for this hijack, but it intrigues me. someone once told me to lean the mixture while practising stalls.
-it was a warm day, I think the METAR was suggesting around 27C.
-ground was about 2000ft, we were about 5500ft
-in an old, crappy, 152.

discuss
 
fbm2009 said:
sorry for this hijack, but it intrigues me. someone once told me to lean the mixture while practising stalls.
-it was a warm day, I think the METAR was suggesting around 27C.
-ground was about 2000ft, we were about 5500ft
-in an old, crappy, 152.

discuss
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That is because your density altitude would be ~7500'
 
if that was his reason for it then fine. But it wasn't.
I suppose I also didnt give the full context. its late here in Australia now, forgive me ... The schools 152's needed plug cleaning every 50 hours. his obvious explanation was that we weren't leaning the mixture when practising stalls and so THAT was why the plugs where becoming fouled so quickly. and NOTHING else, because according to him, taxiing rich doesnt matter all that much.

I guess the question should have been, would running the engine with full rich (standard training flow for PPL manoeuvre's- fuel on, mix rich, throttle as needed, lights on, seatbelts) for a whole i dont know, 10 minutes per flight cause the plugs to die like that?
 
fbm2009 said:
standard training flow for PPL manoeuvre's- fuel on, mix rich, throttle as needed, lights on, seatbelts
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No where I have taught used full mixture as a standard operating procedure for any maneuvers. Even takeoffs in winter time are leaned slightly because of density altitude.
 
That's kinda silly. 152 lower plugs need cleaning every 50 hours no matter how you run them


Sent from 1865 by telegraph....
 
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