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Discussion Starter · #1 ·
Somehow my piece on Sparks and Such got removed from the FYI section.
Is it some where else? where?
If it got lost here it is:

Sparks and such
It’s quite normal for Non-Quantum Physics’ to think of sparks as a thing that happens instantly like, Wham Bam Thank You Mam. Not so.

A spark as in a spark plug in an engine, has a pre. period (think foreplay) a strong build up, (intercourse) , a climax (well yeah) and even an after glow.

OK enough of sexual innuendo.

When you engine’s timing system sends a message to the Ignition system that it would be nice if you fired a plug about now, a lot happens is sequential order. For this description we’ll start after the coil is saturated and has received a signal to fire a plug.
Electrons of energy start to flow down the plug wire, happily on there way till they reach the break in the wire (the plug gap). Then they stop and mill around like wildebeest waiting to cross a river. As more and more electrons pile up behind the ones at the edge and build up in the plug wire and the center conductor in the plug. As this pressure builds an intense electric field is created in the gap to the ground element. When the ignition system has fed enough electrons down the wire the ones piled up at the gap are milling around faster and faster and the crowd gets thicker and thicker, the intense field created just starts knocking electrons off the edge. The gap then becomes Ionized. The Ions in the gap provide a set of stepping stones. The electrons then begin to flow across the gap. This flow becomes a rush and the space is so crowded the there is a lot of friction created by the mass of electrons tying to go somewhere. The heat created by the friction is what we call “Spark”
That’s the simple explanation , Now about Spark Plugs. Electrons in the center conductor of the plug are bound there by an electrical force call “work function”. This can be overpowered by sufficient energy. The binding force is lessened the hotter the metal is. This is the biggest reason the center wire of a plug is insulated from the body, more so than for electrical insulation. The hotter the wire is, the better the spark action. Now you can quickly exceed the melting point of common metals, so exotics are chosen for this part, again not for their electrical conductive properties but for their heat resistance. These metals are expensive, like Platinum, and others and you would think that the reason the electrodes are tiny in the pricey plugs is because of the cost of the metal. Nope wrong again.

The ease in which the electrons are knocked off the electrode is improved by the sharpness of the edges. As each electrons leave the electrode to journey across the gap is take a bit of the electrode with it ( like packing a sandwich for a trip) As the edges of the electrode become rounded the force necessary to cause a spark to jump the gap is greatly increased. Couple this activity with the heat thing , and you have the reason for the exotic metal. The hotter the electrode is made to run the less likely it is to foul. So when you look at your plugs, don’t worry too too much about the color but look very hard at the sharpness of the edges of the center electrode when you can see a rounded shape, discard that plug.

Right here is a good place to bring up heat range marking of plugs. Contrary to popular notion that the spark strength is what’s indicated , it’s really the temperature that the electrode runs at. This can be judged by examining a plug’s insulator . The deeper the groove around the center wire that the insulator is , the longer the path the heat traveling to the body of the plug has , the hotter the electrode runs. Conversely if you examine a Cold plug designed for racing, the insulator is nearly flush with the tip of the electrode.

The tiny size of the electrode is also not to conserve expensive metal but to reduce the size of the electrode to provide a small an “Edge” for the electrons to pile up against. And for this primary reason to resist thermal breakdown longer. This and un leaded gas is why you have plugs that last 100 K miles.

OK back to sparks: The obvious reason for a spark is to start a fire in the combustion chamber. This sounds easy but can be deceptively difficult to do. In today’s lean engines the chances that a bit of combustible fuel will find it’s was into the plug gap is increasingly rare. If the spark were an instantaneous thing your engine probably wouldn’t run at all. As we learned in the previous paragraph the build up to a spark is a slow process, The first part of the spark is call the Capacitive part, because the current stored in the metal can be regarded as a capacitor. Then the energy gradually diminishes till the spark is not as effective. Then there is a residual part of the spark that can aid in lighting the fire too. This latter part is the part that causes electrical noise that can be heard in your radio. But can also elongate the duration of a spark to help engines with poor combustion chambers to burn better. Our beloved EX has a resistor in the plug cap to clip this latter part of the spark. I don’t know why except there must be a Federal Regulation the limits Spurious radiation along with gas emissions.

Ok if any of you are still awake, I hope you have learned something of use, as you think about your motorcycle. There’s a lot more to this stuff that most mechanics understand, and this is what gives rise to the bad decisions that I read about on this forum.

Bill FOG Martovich Senior Engineer
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