Why are there many different types of spark plugs
Spark plugs need 5,000 to 40,000 volts from the ignition
coil before a spark will jump across its electrode gap. It
takes a lot of volts to push the spark across the gap
because air doesn't conduct electricity unless it is ionized
first. The spark jumps from center electrode to side ground
The reason why a plug fires from center electrode to side
ground electrode, instead of vice versa, is because it's
easier for a spark to originate at a hot electrode than a
The center electrode runs much hotter than the side
electrode because the center electrode is encased in ceramic
(a good insulator of heat as well as electricity). This
slows down heat transfer from center electrode to cylinder
If ignition polarity is reversed, it can take up to 40% more
firing voltage to send the spark from ground electrode to
center electrode. The result can be misfiring under load and
poor engine performance.
Keeping the center electrode hot also helps burn off fuel
and oil deposits that form on the insulator tip. Deposits
can conduct voltage away from the gap causing the plug to
misfire, so keeping the center electrode hot helps prevent
If the plug is too hot for the application, it can become a
source of pre-ignition. If the plug is too cold, it can
experience fouling problems.
The operating temperature of a spark plug depends on a
number of variables. The two most influential are cylinder
head temperature and the relative richness or leanness of
the fuel mixture. Given such variables, it is impossible to
have a single spark plug that would work well in every
application, even if thread sizes and reach were
Heat range is determined by several design features, one of
which is the distance heat must travel from center electrode
tip to the plug's shell. A plug with a short ceramic
insulator between electrode tip and shell runs cooler than
one with a long nose insulator.
A cold plug is good for high speed, high load operation
because it sheds heat quickly and is less likely to overheat
and cause pre-ignition. Colder heat ranges are used most
often in high performance and turbocharged engines.
For short-trip, stop-and-go driving, a cold plug may not run
hot enough to keep itself clean. A hotter heat range plug
may be needed to resist fouling.
For sustained high speed or high load running, a hotter plug
may become too hot and cause preignition. The trick is to
use a plug hot enough to prevent fouling yet cold enough so
there is no danger of pre-ignition.
One way to extend or broaden the heat range of a spark plug
is to extend the tip of the plug further into the combustion
chamber. The longer insulator makes the tip run hotter for
better self-cleaning at low speeds and light loads. It also
exposes the tip to more of the incoming air/fuel mixture,
keeping it from overheating at high speeds and loads. An
extended tip spark plug typically has a much broader heat
range than a standard spark plug.
Another way to increase heat range is to use a center
electrode with a copper core. Copper is an excellent
conductor of both heat and electricity. With a copper core
center electrode, heat is carried away from the plug tip
through the electrode during high speed, high load
operation. This allows the plug to dissipate heat more
quickly like a colder plug, yet stay hot enough to burn off
Because of the increased heat range copper core plugs offer,
one plug can be used in applications formerly requiring
several different plugs with narrower heat ranges.
The use of a platinum or gold palladium center electrode is
another design innovation that improves fouling resistance
while greatly extending plug life. The special alloy at the
tip of the center electrode is more wear and corrosion
resistant than standard electrode metal. It allows the use
of a longer insulator, helping plugs reach a self-cleaning
temperature of 750 degrees F in only a few seconds.
Spark plug manufacturers avoid making specific mileage
claims for such premium plugs, but many experts say the
plugs will often last up to 60,000 miles. Other benefits
include better cold starting, less cold fouling, and
improved operation during both stop-and-go and highway
driving. These plugs are considerably more expensive than
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