spark plugs again

flying belgian · Oct 27, 2020

After many hours of operation the ground electrode on spark plug of my grain dryer burnt off. This plug has electrodes about 1 and a quarter inch long. That happened to me years ago and I just cut a quarter inch off positive electrode and bent a new end on negative and away we went for many years. So that's what I did again, made it another quarter inch shorter. Now realize this plug is not in a cylinder or under compression so I can just do this until there are no electrodes left. The LP comes out of the nozzles in about a 3 square ft. pattern so an inch more or less position of plug makes no difference.
Any way this got me to thinking. A local voe-tech tested different brands of spark plugs to see which one would fail first as they increased compression. They did this with a cylinder with an inspection glass peak hole. Now my question. Why would a plug fail with more compression? It's just a gap between a positive and ground electrode. Just like your welder. Why does compression matter?
Incidentally in there experiment the Champions failed first and NGKs failed last.

shaun wallace · Oct 27, 2020

That is same plug used in oilfired torpedo heaters. It will get to short to be in fuel path and then it wont lite. It arcs all the time the unit is running.

Janicholson · Oct 27, 2020

As compression increases, the cylinder temperature increases. Detonation and the intense pressures involved can double head temperatures, sometimes melting holes in the top of cylinders. I have seen sparkplugs removed from engines with ruined pistons with nothing showing of the electrodes, and it was firing like a diesel till it seized. Jim

flying belgian · Oct 27, 2020

That is correct. I've shortened it twice in the 25 years I had it so this one should take me to the end.

Jaden · Oct 27, 2020

I believe that the air/fuel mixture doesn't ionize as easily under higher pressure. That's the visible spark when the air ionizes.

moresmoke · Oct 27, 2020

I’ve got an old Champion plug cleaner/tester. Has a buzz coil and you can put compressed air to the plug and see what the spark does under pressure. Some plugs will start sparking around the porcelain at higher pressures.

pete 23 · Oct 27, 2020

The simple non scientific explanation of why a spark plug won't fire as well in a pressurized cylinder as it does under atmospheric pressure is, it takes more voltage to jump the gap when in a pressure chamber.

That is why an spark plug will missfire in an engine under a load but run fine with no load or light load because at no load, very little air and fuel is in the compression chamber. Under load the throttle is allowing more air, along with the fuel, thus much more pressure in cylinder.

You can easily see the increase in voltage required during load or acceleration with proper instrument hooked to high tension system. Then, with higher voltage it takes an easier path like fouled center electrode or insulation around plug.

dr sportster · Oct 27, 2020

The Champion plug testers always had compressed air added to test spark. Same with other brands.

Mark-Mi · Oct 27, 2020

If your dryer works like my Farm Fan burner the sparkplug only ignites the gas on start up. The burning gas maintains the flame without the use of the sparkplug.

wore out · Oct 27, 2020

"I've shortened it twice in the 25 years I had it so this one should take me to the end."

Did you note the part number marked on the igniter while your were mucking around with it?

I would have, and looked for a NOS unit on the 'net at a reasonable price and bought it and put it on the shelf "just in case" vs. paying a $$$$$ price to the dryer Co. if it fails.

MarkB_MI · Oct 28, 2020

> The simple non scientific explanation of why a spark plug won't fire as well in a pressurized cylinder as it does under atmospheric pressure is, it takes more voltage to jump the gap when in a pressure chamber.

Which is why reciprocating engines in aircraft that operate at high altitude have pressurized magnetos.

showcrop · Oct 28, 2020

(quoted from post at 14:08:25 10/27/20) The simple non scientific explanation of why a spark plug won't fire as well in a pressurized cylinder as it does under atmospheric pressure is, it takes more voltage to jump the gap when in a pressure chamber.

That is why an spark plug will missfire in an engine under a load but run fine with no load or light load because at no load, very little air and fuel is in the compression chamber. Under load the throttle is allowing more air, along with the fuel, thus much more pressure in cylinder.

You can easily see the increase in voltage required during load or acceleration with proper instrument hooked to high tension system. Then, with higher voltage it takes an easier path like fouled center electrode or insulation around plug.

Pete, your explanation is perhaps actually an observation. Here is what is happening: Combustion is a reaction which consists of the ionization of the fuel, causing dramatic expansion which is what generates the energy. The first stage of combustion is the spark. in order for the spark to propagate across from one electrode to the other the molecules of fuel in the path have to ionize which takes high voltage. The cylinder pressure is a dampening force on the expansion that ionization requires. The spark is, so to speak, being delayed by the squeezing of the molecules under pressure. Smaller center electrodes will propagate a spark better under high pressure, but they erode away faster unless they are made of iridium.

Earl-IL · Oct 28, 2020

A lot of the old "D" John Deere tractors you had to open the plug to .050 gap to get them to idle smoother. If you put in higher compression pistons they would also help the idle.

kencombs · Oct 28, 2020

My minimally educated guess at an explanation is this:
As others have stated the spark jumps the gap because the high voltage present has ionized the air in the gap creating a path for the electrical arc.

the voltage must be high enough to 'knock' some electrons free in order to do that

The voltage present at the plug is determined by the ignition system and (this is key) the rate of rise. If the plug has a leakage to ground due to faulty construction or fouling some voltage will be lost before the air molecules are ionized.

Under load, most if not all engines will have a method to admit more fuel, or richen the mixture. Some of the air molecules in the arc path are replaced with fuel molecules that don't readily ionize. And, due to the added air being admitted by the throttle, there are more air molecules in the arc path, requiring higher voltage to ionize.

The mentioned leakage will drain off part of the current delaying the voltage rise.

Best I can guess.

spark plugs again

flying belgian

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shaun wallace

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Janicholson

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flying belgian

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Jaden

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moresmoke

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pete 23

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dr sportster

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Mark-Mi

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wore out

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MarkB_MI

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showcrop

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Earl-IL

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kencombs

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