Knock Sensor: Description and Operation

Ignition Maps - Knock Sensor Control:

One or two knock sensors are used to signal the ignition control unit when a sudden severe detonation (knock) occurs in the engine. When a knock is sensed, the ECU retards the ignition timing, in steps, until the knock ceases. The ignition control unit has two ignition "maps" programmed into its memory. One for premium fuel and one for lower octane fuels. The premium fuel ignition map is normally the one referenced by the ECU. If knocking cannot be eliminated by retarding the ignition timing the maximum number of degrees from the mapped point on the premium ignition map (12 degrees), the ECU switches to the other ignition map. If the knocking stops, the ECU advances the timing, in steps, back to the mapped point for that engine speed and load.

Knock Sensors:

The knock sensor consists of a piezo-ceramic crystal element. When mechanical forces act upon a crystal, causing distortion of the crystal, small electrical charges are generated on the crystals surface. When a knock occurs in the engine, the frequency and magnitude of the sound wave that is generated causes small distortions in the knock sensor crystal, generating a voltage on the surface of the crystal which the ECU reads. One of two types of knock sensors is used. Both operate in similar manner but differ in appearance (see diagrams). One type is fastened to the engine block by a bolt through its center, the other threads directly into the engine block.