Combustion

Gasoline engine combustion is:

The rapid, thorough burning of a compressed air fuel mixture, initiated by a spark from the engine’s ignition system

It is a flame that starts at the start plug and then:

Moves rapidly across the combustion chamber, heating the gases and building pressure in an even, controlled manner

Normal combustion:

Does not involve an explosion, and it is always initiated by an electric arc at the spark plug

If combustion is started by something other than an electric arc at the spark plug:

This is an abnormal condition known as pre ignition

When does pre ignition occur?

When the air fuel mixture is ignited prematurely(before the spark plug would normally be fire)

Where could pre ignition take place

Typically in the combustion chamber by a hot spot, such as a glowing spark plug electrode or a hot piece of carbon

Detonation:

Is when an air fuel mixture explodes, rather than burns

Why can Detonation take place when an engine’s air fuel mixture is lean?

Because lean mixtures burn very slowly

As the flame moves across the combustion chamber:

The unburned gases become heated due to the advancing flame and increasing compression in the cylinder

If the temperature of these unburned gases rises sufficiently:

They will “auto ignite” and the flame front from this explosion will collide with the original flame front

The pressure spike from this abnormal combustions results in a:

Brutal shock to the engine assembly, and can be heard as a “knock”

Severe engine damage can result from:

Unchecked Detonation