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Common Spark Plug Problems

Let's explore three common issues that occur during the firing sequence, and how spark plugs manufacturer's work to overcome these problems in plug design.

Flashover

Flashover occurs when the voltage sent to the spark plug does not fire between the center and ground electrodes within the combustion chamber, instead shorting between the metal shell and terminal nut, causing the engine to misfire.  This can occur when the air is highly ionized (such as on a stormy day) or when the insulator is dirty.

An elongated insulator extends the electrical path to help reduce flashover.  Many plugs also include 4 or 5 ribbed corrugations to create an even longer path along the insulator surface from the terminal to the metal shell, creating added resistance. For more information, see the article, "Spark Plug Insulator Design".

Quenching

Quenching is caused when the heat generated by a spark is absorbed  by the center or ground electrodes instead of igniting the air/fuel mixture.

To understand quenching and how it is reduced it is first necessary to review the basic purpose of the spark plug: to ignite the air/fuel mixture in the combustion chamber. To do this your vehicle ignition  system generates tens of thousands of volts to jump the gap between the center and ground electrodes. However, it is not the actual electricity that ignites the air/fuel mixture, it is the heat energy generated by  that electricity or spark. Therefore, you want as much of the heat from  that spark to be used to ignite the air/fuel mixture during its generation, and not have the heat from that spark be re-absorbed by the center and ground electrodes.

Plug manufacturers use a variety of designs to try and reduce quenching by reducing the contact surface area between the electrodes and flame nucleus.

 

Shadowing

Just as putting a board in front of a flashlight will block the light from everything behind that board, so the ground electrode can block a portion of the air/fuel mixture from exposure to the spark. Thus a variety of ground electrode designs are available, such as a cut back ground, low angle ground and surface discharge plug, all created to help reduce shadowing. For more information, see the article, "Bosch Ground Electrode Designs".

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