Advance and retard mechanisms are used to vary the ignition timing to suit different engine operating conditions.
The centrifugal advance mechanism rotates with the distributor shaft inside the distributor housing and advances ignition timing as the engine speed increases.
Ignition advance in relation to engine load is catered for by a vacuum advance unit which has a vacuum line connection to a port on the intake throttle body.
If the engine ignition timing has been retarded too far then the engine will tend to knock or ping.
Ignition timing and Knocking
Knocking (also called pinking or pinging)—technically detonation—in internal combustion engines occurs when fuel in the cylinder is ignited by the firing of the spark plug and smooth burning proceeds but some of the unburned mixture in the combustion chamber explodes before the flame front can reach it, combusting suddenly before the optimum moment of the four-stroke cycle. The resulting shockwave reverberates in the combustion chamber creating a characteristic metallic "pinging" sound.
Detonation
The fuel/air mixture is normally ignited slightly before the point of maximum compression to allow a small time for the flame-front of the burning fuel to expand throughout the mixture, so that maximum pressure occurs at the optimum point. The flame-front moves at roughly 110 feet/second during normal combustion. It is only when the remaining unburned mixture is heated and pressurized by the advancing flame front for a certain length of time that the knocking effect occurs. It is caused by an instantaneous ignition of the remaining fuel/air mixture in the form of an explosion. The cylinder pressure rises dramatically beyond design limits. If allowed to persist detonation will cause vibration and damage to engine parts.
Detonation can typically be prevented by:
Correct ignition timing is essential for optimum engine performance and fuel efficiency. Modern automotive and small-boat engines have sensors that can detect knock and delay the spark plug firing to prevent it, allowing engines to safely use petrol of lower octane ratings, with the consequence of reduced power and efficiency.
A knock sensor consists of a small piezoelectric microphone on the engine block, connected to the engine's electronic control unit. Spectral analysis is used to detect the trademark frequency produced by detonation at various RPM. When detonation is detected, ignition timing is retarded, reducing the knocking and protecting the engine. See also Automatic Performance Control (APC).
Pre-ignition
Detonation, explained above, is a different phenomenon from pre-ignition, which occurs when the air/fuel mixture in the cylinder (or even just entering the cylinder) ignites before the spark plug fires. Pre-ignition is accompanied by a loud hacking noise and caused by an ignition source other than the spark. Heat can buildup in engine intake or cylinder components due to improper design e.g. spark plugs with too low a heat rating or due to maintenance, e.g. carbon deposits in the combustion chamber, or also due to overheating of the air/fuel mixture during compression. This heat buildup can only be prevented by eliminating the overheating (through redesign or cleaning) or the compression effects (by reducing the load on the engine or temperature of intake air). As such, if pre-ignition is allowed to continue for any length of time, power output reduces drastically and engine damage can result.
Pre-ignition may lead to detonation and detonation may lead to pre-ignition or either may exist separately.
Detonation in otto-cycle engines is caused by the detonation of the unburnt portion of the fuel (knocking) (due to its overly high sensitivity to heat and pressure of the particular fuel under certain conditions) resulting in a pressure wave. This force is extremely destructive to engines, and often results in holes blown through the top of pistons or engine blocks. This is different from knocking in diesel engines, which use heat and pressure as the primary source of ignition. In diesel engines, low sensitivity to these conditions results in ignition being delayed while a highly detonable mixture accumulates, causing detonation when ignition finally occurs. Diesel fuel has a cetane number much like gasoline has an octane rating, except that it represents increasing sensitivity while the octane rating represents decreasing sensitivity.
Source: CDX Global & Wikipedia - en.wikipedia.org
