11. Normal Combustion
Under ideal conditions the common internal combustion engine burns
the fuel/air mixture in the cylinder in an orderly and controlled fashion.
12. Abnormal Combustion
When unburned fuel/air mixture beyond the boundary of the flame front is
subjected to a combination of heat and pressure for a certain duration (beyond
the delay period of the fuel used), detonation may occur. Detonation is
characterized by an instantaneous, explosive ignition of at least one pocket of
fuel/air mixture outside of the flame front. A local shockwave is created around
each pocket and the cylinder pressure may rise sharply beyond its design
limits.
13. Knocking
Knocking (also called knock, detonation, spark knock, pinging or pinking) in spark-
ignition IC engine occurs when combustion of the air/fuel mixture in the cylinder
starts off correctly in response to ignition by the spark plug, but one or more pockets
of air/fuel mixture explode outside the envelope of the normal combustion front.
The fuel-air charge is meant to be ignited by the spark plug only, and at a precise
point in the piston's stroke. Knock occurs when the peak of the combustion process
no longer occurs at the optimum moment for the 4 stroke cycle. The shock wave
creates the characteristic metallic "pinging" sound, and cylinder pressure increases
dramatically. Effects of engine knocking range from inconsequential to completely
destructive.
