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Why Do Engines Have Knock Sensors? Novice Series

Picture this: modern combustion engines and their ECUs are like neurotic hypochondriacs (healthily) obsessed about the smallest things (like a knock sensor signal).

They’re so paranoid about everything that could potentially cause knocking – fuel quality, atmospheric pressure, ambient temperature, mass airflow, intake temps, you name it – they’ve installed all sorts of sensors to prevent engine knock from happening and sensors to detect engine knock when it happens. And just like a neurotic hypochondriac, the ECU is constantly monitoring themselves (the sensors) with a control loop that’s always on the lookout for any signs of trouble.

The engine knock control loop usually relies on knock sensors (usually piezoelectric type), which are basically the engine’s equivalent of a doctor’s stethoscope. The engine ECU listens for the telltale pressure peak of a knocking combustion from the knock sensor. If the knock frequency and volume is above the set threshold the ECU freaks out and retards the ignition timing in small increments. Once the engine calms down and starts behaving normally again, the ignition timing is advanced once more, and everyone can breathe a sigh of relief until the engine detects a relevant signal from the knock sensor and the process is repeated. The engine ECU can tune the engine automatically as it gets to know what environmental and engine conditions lead to engine knock. By knowing what conditions have caused knock in the past it can prevent engine knock from happening so often and it can also set ignition timing close the the optimum setting.

And if that wasn’t enough, some/most modern engine ECUs can even adjust the ignition timing for each individual cylinder, like a bunch of tiny doctors each with their own stethoscope. Except the engine only needs one stethoscope. Because the ECU knows the rotational position of the crankshaft and it knows which part of the rotation is most likely to give knock for each cylinder it can identify which cylinder is causing knock. It’s like they’re running their own little clinic under the hood! All in all, it’s a pretty impressive setup – and definitely more effective than just crossing your fingers and hoping for the best, like we used to do in the old days.

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This page was last modified Feb 25, 2023 @ 2:56 pm

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