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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 the 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.