What Is Abnormal Combustion?
| To understand abnormal combustion, normal combustion must first be understood. Normal combustion is the complete burning of the air / fuel mixture in the combustion chamber in an even and steady manner. The flame front should begin at the spark plug roughly between 5 and 45 degrees before top dead center (TDC) and work its way outward and downward in a smooth three dimensional path. Think of a pebble being dropped into a pond, and the ripples that disperse from the entry point. The advanced ignition point allows time for pressure to rise smoothly until maximum pressure is obtained in the cylinder, ideally a few degrees after TDC. |  |
| Abnormal combustion is a term used to describe when an internal combustion engine is not running optimally. The process of improper combustion is commonly referred as detonation, knock, spark knock, pre-ignition, and pinging. These terms are commonly interchanged, but can be grouped into two major groups. Either group may cause severe engine damage if gone un-noticed or ignored. It is far too common are poorly assembled and tuned motors destroyed due to detonation and pre-ignition. Damages may include pounded rod bearings, a hole burned in top of the piston, cracked ring land, melted spark plugs, blown head gaskets and more.
The first group is detonation. Detonation occurs after the spark event, and is an instantaneous and explosive combustion of a pocket of air / fuel mixture outside of the flame front. When this flame fronts collides with the one created from the spark plug, a large pressure spike is created and a “ping” or “knock” can be heard. The ping can sound like a hammer hitting a large piece of metal. This is the noise of engine internals being damaged. The forces of this explosive collision can be devastating. The second grouping is Pre-ignition. Pre-ignition happens prior to the spark event, and is typically less violent, but no less destructive. Pre-ignition occurs when the air / fuel mixture is ignited by something other than the spark plug. In the harsh environment of a combustion chamber sharp edges can cause hot spots. Hot spots cannot dissipate the heat as well as a smooth surface, and will glow red hot. These spots can ignite the mixture during the intake stroke, or while the mixture is being compressed. The pressure spikes are not as high as detonation, but temperatures can get out of control. |
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Tags: detonation, knock, ping, pre-ignition, spark knock
