SAAB WIS ONLINE

Normal ignition timing

Normal ignition timing


As soon as idle speed control stops, normal ignition timing is activated. The ignition timing now depends on engine speed and load.

The ignition must be advanced as engine speed increases because the combustion pressure would otherwise act too late on the pistons and hence reduce the efficiency of the engine.

A load increase means that a larger quantity of air is drawn into the cylinder during each induction stroke (mg air/combustion). This gives rise to a higher gas pressure with the result that the speed of combustion increases. For this reason, the ignition must be retarded when the load increases.

The ignition timing for every point of load and engine speed is written to a matrix in the control module memory. The values are optimized to give the highest torque at any one point, which also coincides with the best efficiency and, accordingly, the lowest fuel consumption.

The following section explains why the normal ignition timing must sometimes be advanced or retarded because also factors other than load and engine speed can affect it.