Spark Plug: Description and Operation
CONSTRUCTION AND OPERATIONNormal service is considered to be a mixture of idling, slow-speed and high-speed driving. Occasional or intermittent highway driving is needed for good spark plug performance. The higher combustion temperature generated during highway driving burns away any deposits of carbon or oxides that build up from frequent idling or continuous stop-and-go driving.
CORONA DISCHARGE
Spark plugs are protected by an insulating nipple made of special heat-resistant material, 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 discharge 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 has no effect on ignition performance.
SPARK PLUG HEAT RANGES
Spark plugs must operate within certain temperature limits if they are to provide the performance and service life expected. The spark plug selected for an engine is based on the normal service for which the engine was designed and may not perform satisfactorily under other than normal operating conditions. For almost exclusive city driving, a spark plug one step higher in the heat range might deliver longer service life than the spark plug recommended. Conversely, a spark plug one step colder will perform better for heavy loads or continual high-speed driving.
SPARK PLUG SELECTION
There are three rules to follow when selecting spark plugs for an engine in good condition:
1. Select a spark plug with the specified heat range.
2. If spark plug overheating occurs, select a spark plug that is one heat range lower than the specified heat range.
3. If fouling is a problem, select a spark plug that is one heat range higher than the specified heat range.