SAAB WIS ONLINE

Knock control

Knock control


A high compression ratio is favourable for engine efficiency and thereby fuel consumption. The tendency of the engine to knock increases with the compression ratio, however, so a compression ratio is selected that is as high as possible with regard to the knock resistance of the engine's recommended fuel. Knock control in modern engines is not a safety function but a normal one. Therefore, there is nothing wrong with knock control being active during normal operation.

Trionic T7 does not have a conventional knock sensor. Instead, the ignition discharge module analyses the ionization current across the spark plug gap on all cylinders and sends a knock signal from each cylinder to control module pin 38.

From the combustion signals, the control module will be aware of which cylinder has fired and if a knock signal above a certain level is registered at the same time by the control module, it will gradually retard the ignition for the cylinder in question until the knocking ceases. Timing correction then returns slowly to zero.

If the mean value of the ignition retardation on all cylinder exceeds a certain level, fuel enrichment will take place. If the mean value of the ignition retardation on all cylinders increases further despite fuel enrichment, the maximum permissible air mass/combustion will be limited.

The control module expects the knock signal to be at a certain level as the spark is produced, so it will therefore start the actual knock detection in the respective cylinder only after the spark has been produced.

Diagnostics


• If there is an open circuit or a short circuit on the knock lead between the ignition discharge module and the control module, diagnostic trouble code P0327 will be generated.