Engine management system - V8 (Ignition timing): Description and operation

Ignition timing

The ignition timing is an important part of the ECM adaptive strategy. Ignition is controlled by a direct ignition system using two four-ended coils operating on the wasted spark principle.

When the ECM triggers an ignition coil to spark, current from the coil travels to one spark plug, then jumps the gap at the spark plug electrodes, igniting the mixture in the cylinder in the process. Current continues to travel along the earth path (via the cylinder head) to the spark plug negative electrode at the cylinder that is on the exhaust stroke. The current jumps across the spark plug electrodes and back to the coil completing the circuit. Since it has simultaneously sparked in a cylinder that is on the exhaust stroke, it has not provided an ignition source there and is consequently termed 'wasted'.

Conditions The ECM calculates ignition timing using input from the following: l CKP sensor. l Knock sensors (KS). l MAF sensor. l TP sensor (idle only). l ECT sensor.

Function At engine start up, the ECM sets ignition timing dependent on ECT information and starting rev/min from the CKP. As the running characteristics of the engine change, the ignition timing changes. The ECM compares the CKP signal to stored values in its memory, and if necessary advances or retards the spark via the ignition coils.

Ignition timing is used by the ECM for knock control.

Knock control

The ECM uses active knock control to prevent possible engine damage due to pre-ignition. This is achieved by converting engine block noise into a suitable electrical signal that can be processed by the ECM. A major contributing factor to engine 'knock' is fuel quality, the ECM can function satisfactorily on 91 RON fuel as well as the 95 RON fuel that it is calibrated for.

Conditions The ECM knock control system operates as follows: l Hot running engine. l 91 or 95 RON fuel.

Function The ECM knock control uses two sensors located one between the centre two cylinders of each bank. The knock sensors consist of piezo ceramic crystals that oscillate to create a voltage signal. During pre-ignition, the frequency of crystal oscillation increases which alters the signal output to the ECM.

If the knock sensors detect pre-ignition in any of the cylinders, the ECM retards the ignition timing by 3° for that particular cylinder. If this action stops the engine knock, the ignition timing is restored to its previous figure in increments of 0.75°. If this action does not stop engine knock then the ECM retards the ignition timing a further 3° up to a maximum of -15° and then restores it by 0.75° and so on until the engine knock is eliminated.

The ECM also counteracts engine knock at high intake air temperatures by retarding the ignition as above. The ECM uses the IAT signal to determine air temperature.