Re: spark coil questions

Posted by electro on November 24, 2021 at 17:34:59 from (206.74.76.168):

In Reply to: spark coil questions posted by Bob N.Y. on November 23, 2021 at 14:06:20:

JMOR is correct. There may be a slight output voltage change based on the internal wiring but is small. The thermionic emissions itself has only a small effect at spark plug temperatures.

I only remember one study that looked at fuel air ratio, compression ratio, type of electrodes on the spark plugs, turbulence inside the cylinder, and spark plug polarity. The study found the effect of switching the polarity was around 30 percent. So the 10,000 volts or so needed to jump the gap goes to 13,000 volts. That sounds like it would be a problem but a coil should be able to produce at least 20,000 volts so producing 13,000 volts will not be a problem.

There are lots of duel spark coils on cars and motorcycles that fire one plug positive center and one plug negative center and they still seem to be able to run just fine. Magnetos for multi-cylinder engines typically will fire every 180 degrees of magnet or rotor rotation. They will fire with the magnet in a certain position relative to the coil and produce a spark of a certain polarity. The rotor will then turn 180 degrees and now the magnet is reversed from the first spark position so the polarity of the spark is reversed. They still manage to run correctly even with that alternating polarity and the supposed large effects from thermionic emissions.

The effect from thermionic emission is real but small and not significant for coil or magneto that is in good condition.

I suppose you could have a failing coil that produces just enough voltage to initiate a spark with one polarity but that may not be enough for the other 30% more voltage for the other polarity.

I am missing the theory behind why one polarity would be more efficient than another. Both polarities would be the same as far as efficiency, i.e. power in vs power out.

Even though the effect of thermionic emissions is small you still have to wire the coil in one direction or the other some way so you may as well wire it so the terminal listed as positive is connected in some way to the positive side. Coils with terminals labeled dist and ground or grd are typically for neg ground systems and therefor the dist terminal would be positive and the grd terminal would be negative.

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