Spark Plug: Description and Operation

Typical Spark Plug Construction:

PURPOSE
The spark plugs provide spark to ignite the compressed air/fuel mixture.

LOCATION
The spark plug is located in the cylinder head.

NOTE: There are many different manufacturers of spark plugs and in some cases, any brand will do. Always use the correct plug type for the application as prescribed by the manufacturer. The following TECHNICAL HIGHLIGHTS are provided as a guide to proper spark plug choice.


TECHNICAL HIGHLIGHTS
Thread Size
The diameter and pitch of the threaded portion of the spark plug.

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

Spark Plug Heat Range Illustrated:

Heat Range
The ability of the spark plug to cool its electrodes.
a. 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.
b. 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.

Resistor type
Most spark plugs are resistor type plugs. These have a carbon resistor or auxiliary air gap built into them to provide radio static suppression and to ensure a higher sparking voltage is reached in the secondary ignition circuit before the spark occurs. Resistor type spark plugs are always used with high energy electronic ignition systems.

Non-resistor type
These are rarely used any more due to their poor compatibility with electronic ignition systems. 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 are used in older breaker point type ignition systems that are not designed to produce the high sparking voltages required by today's emission controlled gasoline engines.

Platinum, Silver or Palladium plugs
Some spark plugs have center electrodes coated with or made of special alloy metals to reduce wear.