Testing for Spark

[Richard Milhous]

I found an article on Wikipedia (Paschen's Law) which says (in a very confusing way) that the breakdown voltage of an air gap is a function only of air density and gap length (specifically, the product thereof).

So, if you are comparing the ability to get a spark (achieve breakdown voltage over the gap) in open air you could multiply the plug gap by the compression ratio to simulate the same breakdown voltage.

0.045" x 6 = 0.27", almost exactly a quarter inch.

But:

The spark occurs before TDC, when the density isn't this high. At 8° BTDC compression is only 5.8.

The compression stroke doesn't start at ambient temperature. Even at cranking speed there will be some vacuum in the manifold, and the pressure in the cylinder will be lower than in the intake. Otherwise, the air wouldn't flow. I don't know how much vacuum a lawnmower pulls but I'm betting this would get the final ratio <5.

OTOH the presence of gasoline would probably raise the breakdown voltage. I haven't found anything on this.

Is 1/4" gap for open-air testing of spark plugs too conservative? I'm thinking 3/16" might be enough.

 

[bertsmobile1]

Again Richard, OVERTHINKING
The plug either fires or won't fire .
The only thing the owner can do is change the plug or adjust the air gap.
There is also the specific resistance of the plug , the thickness and chemical composition of the oxide on both of the surfaces, the grain size & orientation in the metal.
Systemaic resistance from the coil to the plug tip , temperature of the magnet & speed the magnet field both builds up & collapses .
But again as far as the owner is concerned it is a yes it works or no it don't situation.

 

[Richard Milhous]

Either it fires or it doesn't... under what conditions? A plug may fire consistently in open air, but not at all under compression. That's the whole point of Slomo's 1/4" gap test.

 

[bertsmobile1]

I have a variable gap spark tester as well, in fact I have 3 of them, 4 if you include the B & S 3 electrode tester
It gets used for chasing down misfires
However with a non starting engine it does not matter if the magneto can throw a 1/4 " spark or not.
All that matters is will it make the engine go bang
So, sparks in air = coil works & safetys work
No bang in engine = weak coil so replace it

Thus it is a 2 minute job
Sparks when grounded to block, then go to carb cleaner test
No spark when grounded to block try again with kill wires removed
Still no spark = dead magneto ( you can go one step further & verify you have a sound secondary circuit but they very rarely fail )
Spark = problem with safety circuit
Sparks in air but fails carb cleaner test, then check the valves .
Valves working OK then toss the coil

These are not NASCAR race engines of F1 open wheelers they are mowers.
Mower engines are the cheapest nastiest engine that it is possible to make and hold together till the warranty has expired .
All the rest are parrameters for engine designers to consider.
For instance the stuff inside the cylinder is not air but an air:fuel mix which may be all gassified or could only be the low temperature aormatics that have evaporated so you have a mix of air with methyl benzine + micro droplets of the aspirated fuel that was not volatile at the current temperature.
The specific resistance of all of these are different .
Then the specific resistance of "air" will not be the same anywhere because things like humidity, dust & pollen all come into play.
Equations are good but they are only correct for tests done in labs using clean filtered & conditioned air ( usually from a bottle ) at standard temperature & pressure with 0% humidity.

My high school had an appartus similar to the old Champion spark tester with a couple of electrodes in a bell jar that we could change the gas composition , temperature & pressure.
We did simplified calculations like the ones you found and for the fun of it introduced different bottled gasses so made lots of pretty coloured coronas .

However none of this will help D_H to get his mower going.

 

[Hammermechanicman]

Yup. Overthinking it.
IF THE IGNITION WILL JUMP A 1/4" GAP IN AIR IT WILL RUN THE ENGINE. folks watch Utube vids of guys measuring spark plug resistance and coil resistance and making claims about all kinds of irrelevant or incorrect claims. If you are dealing with a modern self contained solid state magneto ignition if it will jump a 1/4" gap it has all the energy needed to run an engine. No math or analysis needed. If it doesn't then replace the coil. Coil may be failing when it gets hot but that can't be diagnosed with a meter.