Why are there many different types of spark
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
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
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
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
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