Preignition

What is preignition?

Preignition in an internal combustion engine is combustion that starts from a heat source other than the spark plug. Most preignition is caused by carbon buildup or a sharp edge inside the combustion chamber. As the engine heats up, the carbon or sharp edge gets so hot that it can start the combustion event at any time. In addition to carbon buildup, preignition can also occur if the wrong spark plug has been installed in the engine. Using the wrong octane fuel can also cause preignition.

Every spark plug is rated for a certain heat range. To change the heat range of a spark plug, the manufacturers alter the design of the porcelain insulator, vary the length of the center and side electrodes or change how far the electrodes extend into the combustion chamber. The insulator and spark plugs threads are critical design elements to draw heat away from the electrode tips and keep them within a certain operating range temperature.

When the wrong spark plug is installed, the electrodes and insulator retain too much heat and can act as an ignition source before the spark is generated.

When does preignition start?

An air/fuel mixture ignites more easily at low pressure and resists ignition at higher pressures. So preignition usually occurs at bottom dead center (BDC) or slightly higher, during the compression stroke. This is why preignition is so destructive to an engine; it’s literally burning and expanding as the engine is trying to compress the mixture on its way up to TDC. So the fire burns much longer, resulting in excessive heat inside the combustion chamber.

What does preignition do to an engine?

The high heat from preignition can melt holes in the tops of pistons, burn the electrodes off of spark plugs and overheat and disintegrate the edges of valves.

Preignition piston damage

©, 2019 Rick Muscoplat