How to detect Knock?

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Knock Detection:

Due to the massive variation in fuel quality, gas pressure, and ambient temperature moreover because of the possibility of a malfunction, every modern combustion engine contains mechanisms to detect and stop knocking.

A control loop is permanently monitoring the signal of 1 or more knock Sensors (commonly piezoelectric sensors which are able to translate vibrations into an electrical signal). If the characteristic pressure peak of knocking combustion is detected the ignition timing is retarded by steps of some degrees. If the signal normalizes indicating controlled combustion the ignition timing is advanced again within the same fashion keeping the engine at its very best operating point the so-called ″knock limit″. Modern knock control loop systems are able to adjust ignition timings for each cylinder individually. Counting on the precise engine the boost pressure is regulated simultaneously. This manner performance is kept at its optimum while mostly eliminating the chance of engine damage caused by knocking e.g. when running on low octane fuel.

An early example of this is often in turbocharged Saab H engines, where a system called Automatic Performance Control was wont to reduce boost pressure if it caused the engine to knock.

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