In a period engine the ignition timing is perhaps not everything, but it's certainly one of the two most important variables influencing performance. (The other is mixture strength, which on modern engines is being fine-tuned by the oxygen sensor, as we saw last week.)
By ignition timing we mean the exact moment in the piston's upward travel on the compression stroke when the spark jumps across the gap between the spark plug electrodes.
This must happen before the piston reaches the top of its travel so that the firestorm has time to spread throughout the combustion chamber and reach its peak soon after the piston has started its downward travel on the power stroke, just when it needs the strongest possible shove from the hot, expanding gases above it to help it along its way.
To get this right is not so simple, because the speed at which the flame front spreads through the combustion chamber increases as:
l The cylinders receive a fuller air-fuel charge due to higher inlet manifold pressure; or
l The inlet mixture becomes more fuel-rich.
Furthermore, as the engine revs rise, there is simply less time for the combustion to reach its peak before the moment comes around when maximum pressure is needed.
The spark timing therefore needs to be adapted to changing conditions in the engine. Retarding the spark is never a good idea because it results in a loss of power and higher exhaust gas temperatures, a common cause of burnt exhaust valves. But advancing the spark slightly (making it jump earlier) increases power and reduces fuel consumption up to the point where the engine starts pinging (knocking) under heavy load at low revs. Knocking occurs when the combustion in a cylinder starts normally from the jumping spark, but instead of it spreading evenly from there throughout the combustion chamber, outlying pockets of gas get so hot that they ignite spontaneously before the flame front reaches them.
This is also known as detonation and it leads to sharp, premature pressure rises which not only waste power, but will also cause expensive engine damage if not corrected quickly. The aim of engineers is therefore to advance the timing to a point just short of the knock limit.
Before the advent of electronic engine management, it was necessary to leave a clear safety margin between the knock limit and the actual spark advance which was often crudely linked to engine revs in those days, although the vacuum advance was supposed to cater for variations in manifold pressure.
The safety margin had to ensure that even in the most knock-prone cylinder, on the hottest day and with somewhat inferior fuel, knocking would not occur.
Nowadays engineers can control spark timing far more precisely. Firstly, they have a host of sensors monitoring all the factors influencing the optimal point of ignition and whose inputs are all correlated by the electronic control unit.
Secondly, they have "knock sensors" which allow the spark timing to be fine-tuned. These sensors, of which there are often more than one on an engine, thread into the side of the engine block. They contain a sliver of piezo-electric material which generates an electrical voltage when pressure is exerted on it, for instance, from the vibrations in the engine block caused by detonation in a cylinder.
The voltage signals from the knock sensors then go by wire to the engine control unit where electronic wizardry is used to filter out signals from other sources of vibration and focus only on knock signals. The ECU can also identify the cylinder where the detonation is happening and instantly it can back off the spark advance in that cylinder a fraction until the detonation just stops.
Symptoms of a defective knock sensor are either pinging or lack of power and perhaps some hesitation .
When installing a knock sensor it's important to follow the manufacturer's torque specifications to the letter.