Spark Plug: Description and Operation

PURPOSE
Spark plugs protrude into the combustion chambers. When the ignition coil discharges, a high voltage spark jumps across the air gap between center and ground electrodes, igniting the compressed air/fuel mixture.

Typical Spark Plug Construction:

CONSTRUCTION
There are different types of spark plugs for different applications. Each type has specific advantages for its applications. Features include, but are not limited to:

Thread Size
The diameter and pitch of the threaded portion of the spark plug varies with different applications.

Reach
The length of the threaded portion and how far the electrode protrudes into the combustion chamber varies with different applications.

Spark Plug Heat Range Illustrated:

Heat Range
The ability of the spark plug to cool its center electrode.

Hot plugs have longer insulators around the center electrode and less cross-sectional area for heat to transfer from the electrode to the cylinder head. Heat is conducted away from the center electrode more slowly and the electrode tip stays at a higher operating temperature.

Cold plugs have shorter insulators around the center electrode and a larger cross-sectional area for heat to transfer to the cylinder head. Heat is conducted away from the center electrode quickly and the electrode remains at a cooler operating temperature.

It is important to use spark plugs with the correct heat range for the application. If a spark plug is used with too high a heat range for a particular application, pre-ignition may occur. Spark plugs with heat range that is too cold for the application will tend to foul easily.

Resistor Type
Most spark plugs are resistor type plugs. These have a carbon resistor built into them to provide radio static suppression and to ensure a higher sparking voltage. Resistor type spark plugs are always used in vehicles with computerized engine controls because the reduced radio static is less likely to interfere with operation of sensitive components.

Non-Resistor Type
These are rarely used any more due to their poor compatibility with electronic engine control systems. Also, spark plugs without auxiliary resistance built in will allow the coil to discharge at much lower voltages (5K-10K volts) resulting in a weaker (lower energy) spark than electronic ignitions are capable of. These type plugs are used in older breaker point type ignition systems that do not have computer controls and are not designed to produce the high sparking voltages required by today's emission controlled gasoline engines.