Starting ignition timing

Fig 1: Identifying Starting Ignition Timing Graph Pattern

Courtesy of SAAB-SCANIA OF AMERICA, INC.

Starting ignition timing is active from the point when the engine starts until the engine speed has reached the requested idling speed.

Ignition timing is normally approx. 8° BTDC, but is retarded when the intake air temperature rises. When the temperature of the intake air has reached 100°C the pre-ignition is 5° BTDC. The reason that the ignition is retarded when the air temperature rises is that the combustion velocity also rises.

When starting, the control module triggers each cylinder with a 200 Hz signal for 60 crankshaft degrees, whereby a so-called multi-spark occurs. When the engine speed exceeds 450 RPM, the frequency will be increased to 330 Hz. The function ceases when the engine speed exceeds 650 RPM. If the engine speed drops below 450 RPM again, the function will be restarted provided the coolant temperature is below -10°C.

This is to ensure ignition in unfavorable starting conditions and to prevent the engine from stalling directly after start.