Spark plug fouling burn-off in cold weather

[sballmer]

I flew for the first time in -11deg C weather today, and came across a phenomena that I'd like to understand.

Conditions:
C182Q
Temp/DewP: -11/-18 C
The plane had flown earlier in the day, and was parked in a non-heated hangar, but was not plugged in.

I successfully was able to start the engine with 4 primer strokes. However it would quit again withing ~10 sec unless I was ready for another primer stroke. It took about one minute before the engine was warm enough to run by itself.

With all that priming, I wasn't surprised when my right mag turned up weak in the runup. I tried what I did many times before in warmer weather: run the engine up to ~2200RPM and start leaning aggressively in an attempt to burn off the spark plug fouling.

But this didn't work: To my big surprise the mixture setting was unstable at the lean end, i.e. if I pulled the mixture a little more, the RPM dropped dramatically, and didn't recover when I put the mixture back to where it was. I had to go almost full rich for the RPM to recover. Of course this prevented getting a clean burn-off. I eventually got the mixture check to within 200RPM, and decided to fly. After the flight I repeated the run-up, and the mag was just fine.

What bugs me is that I do not understand this unstable mixture behavior. Is that something known and/or normal? What causes it? I never observed it in warmer weather.

 

[Tom-D]

I flew for the first time in -11deg C weather today, and came across a phenomena that I'd like to understand.

Conditions:
C182Q
Temp/DewP: -11/-18 C
The plane had flown earlier in the day, and was parked in a non-heated hangar, but was not plugged in.

I successfully was able to start the engine with 4 primer strokes. However it would quit again withing ~10 sec unless I was ready for another primer stroke. It took about one minute before the engine was warm enough to run by itself.

With all that priming, I wasn't surprised when my right mag turned up weak in the runup. I tried what I did many times before in warmer weather: run the engine up to ~2200RPM and start leaning aggressively in an attempt to burn off the spark plug fouling.

But this didn't work: To my big surprise the mixture setting was unstable at the lean end, i.e. if I pulled the mixture a little more, the RPM dropped dramatically, and didn't recover when I put the mixture back to where it was. I had to go almost full rich for the RPM to recover. Of course this prevented getting a clean burn-off. I eventually got the mixture check to within 200RPM, and decided to fly. After the flight I repeated the run-up, and the mag was just fine.

What bugs me is that I do not understand this unstable mixture behavior. Is that something known and/or normal? What causes it? I never observed it in warmer weather.

Your rich mixture is a fixed orifice that when the OAT is extremely cold this orifice is just barely big enough to give you the proper mixture.

IOWs you were already lean with out pulling the mixture.

 

[flyingcheesehead]

I successfully was able to start the engine with 4 primer strokes. However it would quit again withing ~10 sec unless I was ready for another primer stroke. It took about one minute before the engine was warm enough to run by itself.

Another thing that can help on the 182 in extremely cold weather is to pull the carb heat. My theory is that the air filter has some frozen moisture in it and isn't letting enough air in, though that doesn't sound like what you were experiencing.

Tom knows that of which he speaks... Funny thing, the 182 I fly has started a lot faster and more reliably since we had Tom on Pilotcast 18! (Well worth a listen...)

 

[Dan Thomas]

Another thing that can help on the 182 in extremely cold weather is to pull the carb heat. My theory is that the air filter has some frozen moisture in it and isn't letting enough air in, though that doesn't sound like what you were experiencing.

Tom knows that of which he speaks... Funny thing, the 182 I fly has started a lot faster and more reliably since we had Tom on Pilotcast 18! (Well worth a listen...)

Cold gasoline doesn't atomize or vaporize well, especially in cold air. Carb heat aids that and will make a better warmup.

Priming: the checklist has the priming too far ahead of the actual cranking. Leave it until you are ready to crank. If you prime and then fool around with turning on the master and calling "clear" and all that, the fuel sprayed from the primer nozzles has fallen onto the induction system walls and doesn't get sucked into the cylinders properly. Prime it enough and it'll run down and out the airbox and maybe start a fire if the engine backfires.

Prime-start. Just about as fast as it takes to say it. get everything else set up first, with the mags on Both.

When it's really cold and the engine hasn't been preheated, frost can form on the plugs and short them. Won't get much action after that.

Continentals also let oil past the rings when the engine is cold and the bottom plugs will fill up with it. You probably had some of that, too.

Dan

 

[dmccormack]

Prime-start. Just about as fast as it takes to say it. get everything else set up first, with the mags on Both.

When it's really cold and the engine hasn't been preheated, frost can form on the plugs and short them. Won't get much action after that.

Continentals also let oil past the rings when the engine is cold and the bottom plugs will fill up with it. You probably had some of that, too.

Dan

Dan -- would this be true of hand-starting as well?

 

[Tom-D]

Another thing that can help on the 182 in extremely cold weather is to pull the carb heat. My theory is that the air filter has some frozen moisture in it and isn't letting enough air in, though that doesn't sound like what you were experiencing.

Tom knows that of which he speaks... Funny thing, the 182 I fly has started a lot faster and more reliably since we had Tom on Pilotcast 18! (Well worth a listen...)

Starting with the carb heat on, nothing will happen until the engine starts, then the muffler will provide warm air to the carb and the engine thinks it is a summer day. the trick is getting it to start.

Remember the 182, has a remote mounted carb, it is not influenced by the heat from the engine, That is why it is very important to have a carb heat gauge on the 0-470/182 and run enough heat to keep the needle out of the red.

 

[Capt. Geoffrey Thorpe]

Dan -- would this be true of hand-starting as well?

You need to slowly push the primer in as you flip the prop. How long are your arms?

Seriously, you need liquid fuel in the vicinity of the intake valve so that as the air is pulled into the cylinder, fuel is pulled off the edges of the intake port and back of the intake valve and get mixed in with the air. Not much is going to evaporate on it's own… Once fuel droplets are mixed with the air in the cylinder heat from compression will (with luck) evaporate enough that the engine will fire - lots of extra fuel is required to make sure enough actually evaporates. (You need lots of fuel to get enough of the more volatile components of the fuel.)

If the fuel from the primer is running back away from the intake valves, it is going to take a lot more cranking to get the air flow to move it back up. If you push in the primer as you crank, the fuel doesn't get a chance to run away. Some of it will get sucked directly into the cylinders during the intake stroke, but even then, the intake valve is only open ¼ of the total 4 stroke cycle so most of the primer fuel will be splashing on the port walls.

Even when the engine is warm, a significant fraction of the fuel from a carburetor will be flowing along the walls of the intake manifold and not mix with the air until it gets to the port. On engines with port fuel injection, a similar fraction of the fuel will be splashed against the port / intake valve and not get mixed in until the air flow shears the fuel off the walls in the valve opening.

Back in the olden days, things like clear plastic intake manifolds were used to visualize the fuel flow in automobile engine manifolds. During cold starts, it looked like a river of fuel flowing along the manifold from the carburetor – and the engine wouldn't fire until that river got to the ports.

 

[Dan Thomas]

Dan -- would this be true of hand-starting as well?

Yup. My A-65 seldom needs any prime, however, and when it's cold I have to preheat anyway. The small Continentals have the primer on the carb spider, way down low and a long ways from the intake valves. It doesn't work so well for hand-propping. Better to just pull the prop through with the throttle closed until it catches. The idle nozzle will supply atomized fuel better that way.

Dan

 

[Tom-D]

Cold gasoline doesn't atomize or vaporize well, especially in cold air.

What do you think happens to that air fuel charge when the piston starts up on the compression stroke? IOWs what is compression heating ?

Carb heat aids that and will make a better warm up.

Priming: the checklist has the priming too far ahead of the actual cranking. Leave it until you are ready to crank. If you prime and then fool around with turning on the master and calling "clear" and all that, the fuel sprayed from the primer nozzles has fallen onto the induction system walls and doesn't get sucked into the cylinders properly.

I've been preaching that for years

Prime it enough and it'll run down and out the airbox and maybe start a fire if the engine backfires.

Prime-start. Just about as fast as it takes to say it. get everything else set up first, with the mags on Both.

When it's really cold and the engine hasn't been preheated, frost can form on the plugs and short them. Won't get much action after that.

That's an old wives tail, what happens when air is compressed at a 6/1 compression ratio? that is what happens in a cylinder as the piston travels from BDC to TDC on the compression stroke.

Continentals also let oil past the rings when the engine is cold and the bottom plugs will fill up with it. You probably had some of that, too.

Dan

cold oil clings better than hot oil, so that will happen to all engines, not just continental.

also, the oil between the top two compression rings will drain out the end gap of the top ring into the combustion chamber, that will also happen to all engines that do not have upright vertical cylinders.

When you have excessive oil in the combustion chamber that contaminates the lower plugs, you need a new set of rings, and maybe a few valve guides.

 

[dmccormack]

Yup. My A-65 seldom needs any prime, however, and when it's cold I have to preheat anyway. The small Continentals have the primer on the carb spider, way down low and a long ways from the intake valves. It doesn't work so well for hand-propping. Better to just pull the prop through with the throttle closed until it catches. The idle nozzle will supply atomized fuel better that way.

Dan

My Lycoming O-145 will not start without prime.

It will if then engine's warm and I switch off mags. And then only about an hour. After that -- needs prime.

I guess it makes sense to prime then start, but I always pull the blades through 6 times on first startup. If I don't, it won't start and I'll end up pulling it through that many times anyway.

 

[flyingcheesehead]

Priming: the checklist has the priming too far ahead of the actual cranking. Leave it until you are ready to crank. If you prime and then fool around with turning on the master and calling "clear" and all that, the fuel sprayed from the primer nozzles has fallen onto the induction system walls and doesn't get sucked into the cylinders properly. Prime it enough and it'll run down and out the airbox and maybe start a fire if the engine backfires.

Prime-start. Just about as fast as it takes to say it. get everything else set up first, with the mags on Both.

Yep. Ever since that Pilotcast, my starting technique has been to do everything else first (master, beacon, mixture, fuel selector...) Then, I pull the primer out, call "clear," push it in once, pull it out, and crank on the second push. Works every time.

 

[Dan Thomas]

When it's really cold and the engine hasn't been preheated, frost can form on the plugs and short them. Won't get much action after that.

That's an old wives tail, what happens when air is compressed at a 6/1 compression ratio? that is what happens in a cylinder as the piston travels from BDC to TDC on the compression stroke.

I have taken plugs out of cold engine that had started and immediately quit. They were frosted. We have guys here who have worked in the North and have seen the same thing. When it's -20 or -30°C there's very little latent heat in the air and compressing it doesn't raise the temp much, and even if it did, inside a heavy metal device like an engine that's so cold the heat is absorbed instantly. The water vapor generated by combustion sublimates into frost right away and can short the plugs if the thing stops for any reason.

Dan

 

[Tom-D]

I have taken plugs out of cold engine that had started and immediately quit. They were frosted. We have guys here who have worked in the North and have seen the same thing. When it's -20 or -30°C there's very little latent heat in the air and compressing it doesn't raise the temp much, and even if it did, inside a heavy metal device like an engine that's so cold the heat is absorbed instantly. The water vapor generated by combustion sublimates into frost right away and can short the plugs if the thing stops for any reason.

Dan

Why are you trying to start any engine at -20 or-30. everything has frost on it at that temp.

 

[sballmer]

Your rich mixture is a fixed orifice that when the OAT is extremely cold this orifice is just barely big enough to give you the proper mixture.

IOWs you were already lean with out pulling the mixture.

Tom, looking back you are probably right, and that was my only issue. At the time it really felt wrong though - I guess I am simply not used to being lean of peak in the full rich position.

Tom knows that of which he speaks... Funny thing, the 182 I fly has started a lot faster and more reliably since we had Tom on Pilotcast 18! (Well worth a listen...)

Kent, I went back and listened to that episode again. Your description of staring at -13C sure sounded a lot like what I experienced yesterday.

One thing I heard conflicting information about is at which RPM you should let the engine warm. Tom said below 1000RPM on the Pilotcast, in order not to bust the oil pump. I also saw not below 1000RPM on a Lycoming flyer, because of spark plug fouling (as e.g. in my case...). Any thoughts on that?

 

[Dan Thomas]

Why are you trying to start any engine at -20 or-30. everything has frost on it at that temp.

When you live in a deep freeze like the Canadian prairies you do what you gotta do. We try not to have to start really cold engines but sometimes you also have to make a living. I'm just glad we don't have to do like some of the old bush pilots did before the semi-synthetic multigrades were available: Drain the oil while it was still hot after the last flight of the day, take it inside and set it on the woodstove all night. Put a tent over the engine and a torch under it in the morning to try to take the worst of the cold out of it, pour that warm oil in, and try to get it started before the oil stiffened. There was no electricity in many of those places, either. Rotten way to make a living.

Some guys had systems to pump gasoline into the crankcase before shutdown to thin the oil. The gas would boil off the next morning when the engine warmed up. I can just see the lead deposits left behind there...

http://historyoflight.wordpress.com/2010/12/22/in-canada-cold-is-the-mother-of-invention/

Dan

 

[gismo]

Yup. My A-65 seldom needs any prime, however, and when it's cold I have to preheat anyway. The small Continentals have the primer on the carb spider, way down low and a long ways from the intake valves. It doesn't work so well for hand-propping. Better to just pull the prop through with the throttle closed until it catches. The idle nozzle will supply atomized fuel better that way.

Dan

The A65 in my Porterfield was set up that way and the primer's main effect was dumping fuel on the ground. My mechanic disconnected that and ran the primer fuel to nozzles screwed into existing threaded holes on two of the cylinders and and the primer is way more effective now.

 

[gismo]

With all that priming, I wasn't surprised when my right mag turned up weak in the runup. I tried what I did many times before in warmer weather: run the engine up to ~2200RPM and start leaning aggressively in an attempt to burn off the spark plug fouling.

FWIW, running up at full power with a (normal) rich mixture seems to do a better job of clearing fouled plugs unless the problem is lead deposits shorting the gap in which case the only option is pulling the plug and replacing it or cleaning it manually.

 

[Tom-D]

When you live in a deep freeze like the Canadian prairies you do what you gotta do. We try not to have to start really cold engines but sometimes you also have to make a living. I'm just glad we don't have to do like some of the old bush pilots did before the semi-synthetic multigrades were available: Drain the oil while it was still hot after the last flight of the day, take it inside and set it on the woodstove all night. Put a tent over the engine and a torch under it in the morning to try to take the worst of the cold out of it, pour that warm oil in, and try to get it started before the oil stiffened. There was no electricity in many of those places, either. Rotten way to make a living.

Some guys had systems to pump gasoline into the crankcase before shutdown to thin the oil. The gas would boil off the next morning when the engine warmed up. I can just see the lead deposits left behind there...

http://historyoflight.wordpress.com/2010/12/22/in-canada-cold-is-the-mother-of-invention/

Dan

I've operated in the north and have done the remove the oil trick, and have actually built a fire under the NordY, to get it going.

when I couldn't do a preheat the old NordY would start right up when properly primmed and cranked.

I got smart early and quit working where it doesn't get light until spring.

When you remove the oil and place it on the stove over night, be sure it doesn't catch fire. BTDT not fun

 

[Tom-D]

Some guys had systems to pump gasoline into the crankcase before shutdown to thin the oil. http://historyoflight.wordpress.com/2010/12/22/in-canada-cold-is-the-mother-of-invention/Dan

That's an oil delusion systems not found in little engines. The smallest engine I've ever seen one on was a 1340. P&W

 

[Tom-D]

I can just see the lead deposits left behind there...Dan

Where do you think the lead scrapped off the cylinder walls end up?

ever pull a sump and see the gray stuff in the bottom?