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Engine knocking
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==Knock detection== <!-- "Knock sensor" redirects here. --> Due to the large variation in fuel quality, atmospheric pressure and ambient temperature as well as the possibility of a malfunction, every modern combustion engine contains mechanisms to detect and prevent knocking. A control loop is permanently monitoring the signal of one or more '''knock sensors''' (commonly [[piezoelectric sensor]] which are able to translate vibrations into an electric signal). If the characteristic pressure peak of a knocking combustion is detected the ignition timing is retarded by steps of a few degrees. If the signal normalizes indicating a controlled combustion the ignition timing is advanced again in the same fashion keeping the engine at its best possible operating point - the so-called ″knock limit″. Modern knock control-loop systems are able to adjust ignition timings for every cylinder individually. Depending on the specific engine the boost pressure is regulated simultaneously. This way performance is kept at its optimum while mostly eliminating the risk of engine damage caused by knock (e.g. when running on low octane fuel).<ref>{{cite web |url=https://www.europa-lehrmittel.de/t-1/modern_automotive_technology_fundamentals_service_diagnostics-84/ |title=Modern Automotive Technology - Fundamentals, Service, Diagnostics |website=Europa-lehrmittel.de |publisher=[[Europa-Lehrmittel]] }}</ref> An early example of this is in [[Turbocharger|turbocharged]] [[Saab H engine]]s, where a system called [[Automatic Performance Control]] was used to reduce boost pressure if it caused the engine to knock.<ref>{{cite magazine |url=https://books.google.com/books?id=q0qVc8dQrpgC&pg=PA85 |page=85 |magazine=Popular Science |date=July 1982 |title=Turbocharger with a Brain |volume=221 |issue=1 |publisher=Bonnier |access-date=9 December 2023 }}</ref>
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