Engine Knock - "Detonation Danger"

Detonation, also called knocking, pinging or pinking occurs in petrol engines when one or more pockets of the air-fuel mixture explodes (detonates) in an uncontrolled manner within the combustion chamber but outside the normal zone of combustion.  The normal combustion process should grow progressively from the point of ignition (the spark plug) and burn all the available fuel so that maximum power and efficiency is gained from the fuel-air charge and peak cylinder pressure is reached at the optimum part of the cycle, generally a few degrees after the piston has reached the top of the compression stroke.

Detonations create shock waves within the combustion chamber that are heard as the characteristic metallic "pinging" sound and cause the cylinder pressure to increase dramatically and at the wrong part of the cycle.  Allowed to continue, detonation or “engine knock” will cause serious long-term damage to the engine and even catastrophic failure.

The smaller more regular detonations can erode the surface of the piston and cylinder head, wearing away metal fragments which end up in the oil causing damage to other components.  The larger more violent ones can rupture the combustion chamber by breaking sections of the pistons away and punching holes right through pistons or the cylinder head itself.

The main causes of detonation are:

1.         Using fuel with too low an octane rating for the engine.  Octane is the index of a fuel’s resistance to detonation (see previous articles on fuel)

2.         Fuel-air mixture too lean causing higher combustion chamber temperature.

3.         Ignition timing overly advanced.

4.         Excessive engine load caused by incorrect gear selection or throttle opening too high for current engine speed.

Detonation can (and should) be avoided by:

1.         Using fuel with the correct or higher octane rating.

2.         Ensuring that fuel mixture is correct – i.e. correct carburettor or injection system settings.  (Richer air-fuel ratio will help limit detonation).

3.         Ensuring that ignition settings are correct.

4.         Correct gear selection allowing engine to operate in the optimum range and not be lugged under load in higher gears.

Engine design is also important in terms of the proclivity for detonation.  Factors such as compression ratio, the amount of turbulence generated in the combustion chamber during induction and compression, the shape of the combustion chamber and position of the spark plug are some of the factors which are considered.  These factors are generally outside the control of vehicle owners however and in most cases the problem can be avoided by considering points 1.& 4. In the above causes and strategies for avoidance.

The images below show some of the damage detonation can cause -