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 electrode.
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 cooler one.
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 head.
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 fouling.
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 standardized.
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 fouling
deposits.
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 standard plugs.
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