Spark Under Compression

Jack Watson jacknade2@bigpond.com asked something we've all wondered about... Why don't plugs like to spark as well under compression? JW˛

Chuck Balyeat kerogas@the-i.net responded

Air is a resistor and the thicker it gets ,
the more it is so .

Over time it slowly becomes
easier for the current to flow across
soot on the plug insulator nose than jump the gap .
shorted plug .

Chuck Balyeat
Lake Corpus Christi
South Texas
E-mail me at
kerogas@the-i.net
Gas Engine Sundries and Supplies
http://www.angelfire.com/tx/hotube/indexps.html

Rick Strobel ricandkath@msn.com asked

I thot vacuum was the perfect insulator.
RickinMt.

Curt Holland curt@imc-group.com joined in with

I with you on that one Rick. More air molecules in a given space ought to make ionization easier.

Maybe it ain't in the air. Maybe the vaporized fuel acts as an insulator...

Curt Holland
Gastonia, NC

Rick Strobel ricandkath@msn.com said

I'm not disputin' what Chuck says, cause I've seen it on the old spark plug
cleaner/testers.
But here ya got the perfect air/fuel mixture in a preheated
chamber..just right for the "EXPLOSION." (luv that)......and now the spark
won't jump the gap. gotta be an insulator...headed for the books,
Rick

Elden DuRand edurand@scti.net responded

Guys:

From the deep, dark past, my mind dredges up the reason a plug will not fire under pressure. It is because the higher the air pressure is, the higher the voltage has to be to jump the gap. I -think- this is because the higher the pressure is, the more air molecules the spark has to pass through (and ionize) to get to 'the other side'.

When the insulator gets cracked or fouled with carbon, it won't fire under pressure. As for a crack, when the voltage reaches a point where it's easier for the spark to go through the crack, it does. As for the fouling, I think that after the plug has accumulated enough carbon, the resistance of this carbon is low enough to shunt the coil voltage down to a point where it can't jump the gap.

That's the benefit of my knowledge.......Remember that it was offered free of charge and may be worth perzackly whatcha paid fer it!

Take care - Elden DuRand
Kentucky, US of A
edurand@bellsouth.net
edurand@scti.net

John Culp johnculp@chartertn.net reopened the discussion

Not so. Ionizing molecules or atoms takes work, and lots of it. The voltage at which ionization starts is a characteristic of the particular gas and its temperature, mainly. The work expended in pulling each electron off is another characteristic of the atom/molecule, and has to be multiplied by the number of atoms/molecules being ionized. The more in a space, the more work has to be done to "break down" the gas in the space into a conducting plasma. Current flows quite readily through high-vacuum electron tubes. Take as an example a 5U4GB rectifier tube, which has a distance between the filament and plates comparable to a wide spark plug gap. When that filament's hot and emitting electrons, you can run 250 mA of current through the empty space between the filament and plates with only a 50V or so drop. Takes a lot more voltage than that to fire up an arc through normal atmospheric pressure air over that distance! (Now, if a LITTLE air leaks into that tube, you can get in trouble from the fast-moving free electrons ionizing gas molecules and humongously increasing the current flow through the tube, till either a fuse blows or something burns up, but if the tube breaks wide open and fills up with air at atmospheric pressure it'll stop conducting.)

John

Curt Holland curt@imc-group.com said
Elden,
What you wrote sounds convincing. But going back to what Rick said about a spark in a vacuum... In a perfect vacuum this would be the ultimate in insulation since there are no molecules to atomize.

Take a spark plug firing in a closed vessel. Using your analogy, the voltage to create the arc should drop off as a stronger and stronger vacuum is pulled on the vessel. At near 30"Hg then only the slightest voltage differential would be required to create an arc.

Is that what you are saying?

Curt Holland
Gastonia, NC

Edlen DuRand edurand@scti.net responded

Curt:

When I was a kid, I used a buzzcoil to draw about a six inch long spark in an evacuated glass tube with a stopper in each end, hooked to a reversed bicycle pump.

You can try this at home. Do the tube and stopper bit with the coil wires poked through the stoppers. Turn on the coil. With air in the tube, no spark (natch!). Now, pull up hard on the bycycle pump to draw air out of the tube. At the point where there are few enough air molecules, the spark will jump across the gap.

I believe that in a -perfect- vacuum, it would still take a finite (but small) voltage to jump a gap. The trick is to obtain a perfect vacuum. When you do this, make sure to tell me so I can go to another planet, very, very far away. I believe you will have created a black hole or something like it. :-)

Take care - Elden, the fizzicyst from Kentucky, US of A
edurand@bellsouth.net
edurand@scti.net

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This article is part of the Stationary Internal Combustion Engine Frequently Asked Questions (FAQ). This series is a combination of my views and knowledge and the views and knowledge of other people - most of them members of the Stationary Engine Mailing List (More info on ATIS). Those articles which were written by others are © Copyright to the author. Those articles written by myself are © Copyright to Paul Pavlinovich.

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