Spark Plugs Explained

SPARK PLUG BASICS

 

     So, what is there to know about spark plugs? Plenty! You’d be surprised how many different types are out there. As important as a spark plug is and as simple a part it may seem to be, there is much to learn about Spark Plugs Explained.

 

Definition: A spark plug is an electrical device that fits into the cylinder head of some internal combustion engines and ignites compressed fuels such as, aerosol gasoline, Ethanol, and Liquefied petroleum gas by means of an electric spark. Spark plugs have an insulated center electrode which is connected by a heavily insulated wire to an ignition coil or magneto circuit on the outside, forming, with a grounded terminal on the base of the plug, a spark gap inside the cylinder.      The first commercially available high-voltage spark plug was produced in 1902 by Gottlob Honold. Historians note that the first spark plug was invented by Edmond Berger in 1839.   Karl Benz was also given credit for this invention.
Spark plugs have (8) main parts.

                                                            

Terminal:     This part is located at the top of the spark plug and is used to connect it to the ignition system. Some applications use a spade connection while others use a snap-on connection.
Insulator:     The insulator in made of porcelain.  It has main two functions. The porcelain provides a mechanical support for the for the electrode and it insulates the high voltage. It also serves as an extension of the electrode for easy access.
Ribs:Because of the extended distance of the electrode to the grounded metal case, the ribs improve the electrical insulation and prevent electrical energy from leaking along the insulator.
Insulator Tip:The sintered aluminum oxide ceramic insulator is located between the metal case and the center electrode.  It will withstand 600° C and 60,000 volts.  The exact composition and length determines the heat range. Short insulators are cooler while longer insulators run hotter.
Seals:Seals insure that there is no leakage from the combustion chamber.  It made by a multi-layer braze. Special alloys of are used to work with the metal case and the ceramic insulator.
Metal Case:Also known as a “Jacket”, its purpose is to torque the spark plug into the cylinder head.  It also grounds the the spark going through the center electrode.  Another duty it has is to cool the heat passing through the insulator.
Center Electrode:It is connected to the terminal through an internal wire and usually through a ceramic series resistor to reduce radio noise.  The tip can made from a combination of copper, nickel-iron, chromium or precious metals. The center electrode is the hottest part of the plug which emit electrons (aka cathode). Newer longer lasting plugs now use electrodes made of yttrium, iridium, platinum, tungsten, or palladium.
Ground Electrode:The side or ground electrode is made of high nickel steel and welded to the metal case.
Spark Plug Gap:Although many people assume that you can use the plugs unchanged right out of the box is true because the plug may be used for several applications which require different gaps. A simple gap gauge is all that is needed to adjust the gap to factory specs. for your application.Some issues gapped plugs are:

Narrow Gap risk –
spark might be too weak/small to ignite fuel
Narrow Gap benefit –
plug always fires on each cycle
Wide Gap risk –
plug might not fire, or miss at high speeds
Wide Gap benefit –
spark is strong for a clean burn
Heat Range:The operating temperature of a spark plug is the temperature at the tip while the engine is running.  The lower the number, the cooler the heat range of the spark plug. Some factors that determine a spark plug’s heat range are:

Surface area and/or length of the insulator nose
Thermal conductivity of the insulator, center electrode, etc.
Structure of the center electrode such as a copper core, etc.
Relative position of the insulator tip to the end of the shell (projection)


The best firing temperature is between 500° C (932° F)  and 800° C (1472° F) approx.  The most common problems with spark plugs is carbon-fouling and overheating.

Causes of Carbon Fouling:

 

Continuous low speed driving and/or short trips
Spark plug heat range too cold
Reduced compression and oil usage due to worn piston rings / cylinder walls
Over-retarded ignition timing
Air-fuel mixture too rich
Ignition system deterioration

 

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