There are two primary functions for spark plug:

1. Ignite the air/fuel mixture
2. Remove heat from the combustion chamber

In short, spark plugs transmit electrical energy that turns fuel into working energy. The ignition system should supply a sufficient amount of voltage to cause it to spark across the spark plug’s gap. This is called “Electrical Performance”.

The temperature at the firing end of the spark plug must be kept low enough to prevent preingnition, but the temperature should be high enough to prevent fouling. This is called “Thermal Performance”. The Thermal Performance is determined by the heat range selected.

Spark plugs do not create heat, they are designed only to remove heat.  By pulling unwanted thermal energy away from the combustion chamber, and transferring the heat to the engine’s cooling system, the spark plug works as a heat exchanger. The spark plug’s ability to dissipate heat is termed as heat range.

There are certain factors that determine the rate of heat transfer:

1. The insulator nose length
2. Gas volume around the insulator nose
3. The materials/construction of the center electrode and porcelain insulator

The heat range of a spark plug has no relationship to the actual voltage transferred through the spark plug. The measure of spark plug’s ability to remove heat from the combustion chamber is termed as heat range. There are several factors that determine the heat range measurement. The factors that determine the heat range measurement are:

• The length of the ceramic center insulator nose.
• The spark plug’s ability to absorb and transfer combustion heat.
• The material composition of the insulator.
• The central electrode material.

Related posts:

1. Spark Plugs: Heat Rating and heat Flow Path
2. Tip Temperature
3. Spark Plugs