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Knock control

Knock control


A high compression ratio is favourable for engine efficiency and hence fuel consumption. However, the tendency of the engine to knock increases with a higher compression ratio. Therefore a compression ratio is selected which is as high as possible in view of the knock resistance of the engine's recommended fuel. Knock control on modern engines is not a safety function but a normal function. It is not a fault if the knock control operates during normal running.

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 knows which cylinder has fired and if at the same time a knock signal above a certain level is registered by the control module, the control module retards the ignition in steps for the cylinder concerned until th e knocking stops. Timing correction then gradually returns to zero.

If the mean value of ignition retardation on all cylinders exceeds a certain level, fuel enrichment takes place. If the mean value of ignition retardation on all cylinders rises further despite fuel enrichment, the maximum permitted mass air flow/combustion will be limited.

Since the control module expects the knock signal to have a certain level as the spark is produced, actual knock detection in the cylinder concerned begins only after a 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.

• If the ignition discharge module sends a knock signal during fuel shut-off, trouble code P0328 will be generated.