Spark Plug: Description and Operation

Spark Plug Showing Coated Insulator:

Purpose
To start the combustion process, resistor type, tapered seat spark plugs are used on all gasoline engines. No gasket is used on these tapered seat plugs. When replacing spark plugs, use only the type specified. Spark plugs are identified as shown in Figure 1.

Operation
Normal service is assumed to be a mixture of idling, slow speed, and high speed driving. Occasional intermittent high-speed driving is needed for good spark plug performance. It gives increased combustion heat, burning away carbon or oxides that have built up from frequent idling, or continual stop-and-go driving.

Construction
Spark plugs are protected by an insulating nipple made of special heat-resistant materials, which covers the spark plug terminal and extends downward over a portion of the plug insulator. These nipples prevent flash-over, which causes engine misfiring. Do not mistake corona discharge for flash-over, or a shorted insulator. Corona is a steady blue light appearing around the insulator, just above the shell crimp. It is the visible evidence of a high-tension field and as no effect on ignition performance. Usually it can be detected only in darkness. This discharge may repel dust particles, leaving a clear ring on the insulator just above the shell. This ring is sometimes mistakenly regarded as evidence that combustion gases have blown out between shell and insulator.