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Testing for Ignition / Spark Plug Polarity

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Testing for Ignition / Spark Plug Polarity

Post by Fabian on Thu Feb 21 2013, 09:52




When current flow causes a spark to jump from a gap’s cathode electrode to the anode electrode, the cathode electrode eventually shows signs of wear. ["cathode" refers to the negative electrode which in this example is the ground electrode, otherwise known as the ground strap] So, on a well used spark plug, it can usually be seen that one electrode is worn more than the other. If the voltage polarity at the plug is negative, then wear will occur on the centre electrode.

This is why most spark plugs are made with a 'harder' metal on the center electrode than what is on the ground electrode. That is so the center electrode will not wear down fast. If the center electrode has positive voltage and the spark plug is not made with a 'harder' ground electrode then the ground electrode will wear down too fast. This is the case with Grubee engines. The standard ignition outputs positive high voltage to the center electrode of common spark plugs not designed to work with a reverse voltage system. Also the plug polarity affects the sparking voltage, with correct polarity causing a spark at lower voltage.

The spark plug has a thermally insulated center electrode (surrounded by ceramic). With engine running the center electrode runs substantially hotter than the exposed end electrode. Design of the ceramic insulator determines how hot the center electrode will run, leading to the designation of hotter or colder spark plugs. As electrons go, they love to jump away from a hot surface and fly toward a colder surface, so it is easier to drive them from hot to cold rather than from cold to hot. End result is a difference of 15 to 30 percent in voltage required to make spark "initially" jump the gap on the plug depending on which way it is going. So the spark plug prefers to see a voltage potential that is negative on the center electrode and positive on the end electrode for the very first hop of the spark. Oddly enough, this has nothing to do with polarity of the vehicle electrical system, but it is influenced by the common connection inside the ignition coil.

One of the major causes for hard starting or spark plug misfiring under load results when the ignition coil lead wire to the distributor is installed on the wrong side of the coil. This condition causes reversed coil polarity. Voltage at the spark plug terminals should always be negative. Whether it is or not depends on how the primary leads are attached to the coil. Remember, primary lead hook-up directly affects coil polarity, which in turn determines whether voltage at the spark plug terminals is negative or positive.
What difference does it make whether positive or negative voltage is supplied to the spark plug terminal? It directly affects the amount of voltage required to fire the spark plugs. When polarity at the spark plug terminals is positive, it's harder for the voltage to jump across the air gap than when polarity at the plug terminal is negative. Just why this is so is related to a pair of electrical theories--the electron theory and the theory of thermionic emission. According to the electron theory, all current flows from negative to positive. The theory of thermionic emission states essentially it's easier for electrons to leave a hot surface than a cold surface. Combining the two theories, one finds that electrons will always leave a negative charged surface for a positive charged surface, and they will leave the negatively charged surface with more ease when the surface is heated. Spark plug design is such that the center electrode almost always operates at a higher temperature than the ground electrode. Since it's easier for electrons to leave a hot surface, it is preferred to have the electrons "jump" from the hotter center electrode to the cooler ground electrode. When the center electrode is negatively charged (negative voltage at the spark plug terminal), this is what happens. Stated another way, putting the negative charge on the hotter center electrode causes the gap to be ionized at lower voltage. (Ionization is necessary to permit passage of the spark through the high resistance of the gases in the cylinder.) When positive voltage is supplied to the plug terminals, which happens when coil polarity is accidentally reversed, the hotter center electrode becomes positively charged. Consequently, electrons must leave the negatively charged ground electrode and move to the positively charged center electrode. But, since the ground electrode is cooler than the center electrode (and remember, it's easier for the electrons to leave a hotter surface), it takes more voltage to make the current jump the gap-in fact, up to 45 percent more.

Both the standard CDI/coil and the Jaguar CDI & coil output positive voltage to the spark plug center. This can be corrected by reversing the wires going from the CDI to the high voltage coil. But since the standard combo has those wires sealed within its casing they are not accessable. If you have a Jaguar setup you can reverse those wires as long as the high voltage coil does not have a wire going from its metal core to the bicycle frame or engine metal.
This should get longer spark plug life and a more consistent spark for better running.

An easy test to check the polarity of the ignition system is to use the graphite pencil method, as detailed in the image below:






Fabian
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