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Spark Plugs
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 spark plug (in British English: a sparking 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 an electric spark. The spark plug is quite simple in theory: It forces electricity
to arc across a gap, just like a bolt of lightning. The electricity must be at a
very high voltage in order to travel across the gap and create a good spark.
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Spark Plug Construction A spark plug is composed of
a shell, insulator and the central conductor. It pierces the wall of the combustion
chamber and therefore must also seal the combustion chamber against high pressures
and temperatures without deteriorating, over long periods of time and extended use.
Parts of the Plug
Insulator The
insulators are made of a highly purified alumina powder, which gives exceptional
dielectric strength and thermal conductivity at high temperatures, as well as greater
physical strength.
Ribs By lengthening the
surface between the high voltage terminal and the grounded metal case of the spark
plug, the physical shape of the ribs functions to improve the electrical insulation
and prevent electrical energy from leaking along the insulator surface from the
terminal to the metal case. The disrupted and longer path makes the electricity
encounter more resistance along the surface of the spark plug even in the presence
of dirt and moisture. A few spark plugs have insulators that are not ribbed, but
this is rare. Terminal The top of the spark plug contains
a terminal to connect to the ignition system. The exact terminal construction varies
depending on the use of the spark plug.
Insulator tip The tip
of the insulator, the part from the metal body of the plug to the electrode protruding
into the combustion chamber, must resist high temperatures while retaining electrical
insulation. To avoid over-heating the electrode, it must also provide good thermal
conductivity. The porcelain of the main insulator is inadequate and so a sintered
aluminium oxide ceramic is used, designed to withstand 650°C and 60,000 V.
Seals/Gaskets Because the spark
plug also seals the combustion chamber or the engine when installed, the seals ensure
there is no leakage from the combustion chamber. The seal is typically made
by the
use of a multi-layer braze because there are no braze compositions that will wet
both the ceramic and metal case and therefore intermediary alloys are required.
Metal case/Shell The metal
case (or the "jacket" as many people call it) of the spark plug withstands the torque
of tightening the plug, serves to remove heat from the insulator and pass it on
to the cylinder head, and acts as the ground for the sparks passing through the
central electrode to the side electrode. As it acts as the ground, it can be harmful
if touched while igniting.
Central electrode
The central electrode is connected to the terminal through an internal wire and commonly a ceramic series resistance to reduce emission of RF radio noise
from the sparking. The tip can be made of a combination of copper, nickel-iron,
chromium, or precious metals. The central electrode is usually the one designed
to eject the electrons (the cathode) because it is the hottest (normally) part of
the plug; it is easier to emit electrons from a hot surface, because of the same
physical laws that increase emissions of vapor from hot surfaces (see thermionic
emission).
Side electrode, or ground electrode
The side electrode is made from high nickel steel and is welded to the side
of the metal case. The side electrode also runs
very hot, especially on projected
nose plugs. Some designs have provided a copper core to this electrode, so as to
increase heat conduction. Multiple side electrodes may also be used, so that they
don't overlap the central electrode. |
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