Spark Plug: Description and Operation

Spark Plug Cutaway View:

PURPOSE
Spark Plugs allow high-voltage secondary current to arc across a small air gap to ignite the air/fuel mixture in the combustion chamber.

HEAT RANGE
The temperature range of a spark plug is determined by its ability to dissipate heat created during combustion. Heat dissipation is affected by the length of the insulator around the center electrode, and the size of the heatsink area. The longer the insulator, the smaller the heatsink area will be. This causes the spark plug tip to operate at a hotter temperature.

If too cold a plug is used, the possibility of fouling is increased. A plug that is too hot can cause preignition. Hotter plugs tend to burn cleaner with less deposit build-up. They can be used for stop-start city driving, but not for extended high speed or load conditions as engine damage can occur.