In the normal course of events, with the engine operating at the correct temperature in defined conditions, the EMS will use load and engine speed to derive the correct ignition timing from the map. However there are circumstances under which the EMS may need to vary the ignition timing. These normally boil down to four circumstances, engine/coolant temperature, air temperature, knocking and start-up.
When the engine temperature is low the burn times within the cylinders are longer than with a fully warmed-up engine and the ignition timing will normally need to be advanced a little to adjust. The EMS usually has a small map of ignition timing adjustments graded by engine temperature that are added to the base timing figures. The engine temperature information is relayed to the EMS by an engine temperature sensor attached to the engine.
When air temperature varies, so does burn time of the inducted mixture since it is less dense. Again, a small map of ignition adjustments graded by air temperature is added to the base timing figures. The engine temperature information is relayed to the EMS by an air temperature sensor located near to the air inlet.
There may be times during the operation of the engine, even after adjustments have been applied, when the timing calculated does not meet the engine's requirements. Sometimes this may result in ‘pinking’ (a.k.a. ‘knocking’ or ‘pinging’) where the mixture burns so fast that it meets the piston just before TDC, while it is still on the compression stroke - rather than meeting the piston just after TDC on the power stroke. This is very harmful to the engine. Some EMS have acoustic sensors called a ‘knock sensor’ which listens for knocking and will inform the EMS when this occurs. The EMS is then able to make adjustments to the timing to prevent the occurrence of knocking.
Start-up or cranking
When starting an engine, its effective rpm is quite low - around 200rpm or so. If the ignition timing used at idle is set to around 25 degrees (which is about average for a mapped engine) the chances are that the piston will hit the ignited mixture while still on the compression stroke. This will have the effect of pushing the piston down against its normal rotation, effectively this is ‘knocking’ at cranking speeds. This is known as ‘kicking back’ and is normally characterised by the starter motor ‘straining’ and slowing right down, this makes the engine difficult to start and can easily destroy a starter motor in short order.
This is a common problem on engines equipped with mechanical ignition systems and more extreme cams, since the engine needs plenty of ignition advance at idle to run properly. Unfortunately this extra advance can also cause ‘kick back’ and there is no way with a mechanical system to differentiate the timing between cranking and idle.
EMS solve this problem by having a separate timing value for cranking/start-up which is normally set to around 5-8 degrees. This is low enough to prevent kick-back but is high enough to start the engine. The moment the engine fires, the appropriate ignition setting from the base map is used.