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 spoark plug was inveented
by Edmond Berger in 1839. Karl Benz was also given credit for this
invention.
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Spark
plugs have (8) main parts.
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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.
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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 extention of the elcetrode for easy access.
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Ribs:
Because of the extended distance of the electrode
to the grounded metal case, the ribs improve the elcetrical insulation and
prevent electrical energy from laeking along the insulator.
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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 compsition and length
determines the heat range. Short insulators are cooler while longer insulators
run hottter.
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Seals:
Seals insur that there is no leakage from
the combustion chamber. It made by a mulit-layer barze. Special alloys
of are used to work with the metal case and the ceramic insulator.
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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 pasing throuigh the insulator.
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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 electodes made of yttrium, iridium, platinum, tungsten, or palladium.
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Ground
Electrode:
The side or ground electrode is made of high
nickel steel and welded to the metal case.
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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:
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Narrow
Gap risk -
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spark might
be too weak/small to ignite fuel |
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Narrow
Gap benefit -
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plug always
fires on each cycle |
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Wide
Gap risk -
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plug might
not fire, or miss at high speeds |
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Wide
Gap benefit -
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spark is strong
for a clean burn |
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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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