Knock Sensor (KS) System Description: Operation

Fig 1: Non-Resonant Type Knock Sensor Component View

Courtesy of GENERAL MOTORS CORP.

The KS detects when the engine is experiencing detonation. The sensor then signals the PCM to reduce the spark advance until detonation is no longer detected. A conventional resonant type KS uses a vibration plate that has the same resonance point as the expected knocking frequency of the engine. The non-resonant type KS has a steel weight (1) with an insulator (4) separating the weight from a piezoelectric element (3). The vibration caused by engine detonation is transferred to the weight, whose inertia applies pressure to the piezoelectric element. The weights action against the element, generates an electromotive force that is modified and transmitted to the PCM. The non-resonant design sensor is able to detect detonation vibration over a wide frequency band, from 6 kHz to 15kHz. In comparison, the resonant type KS detects only vibrations that are within a narrow frequency band.

In response to the KS signal the PCM retards the spark advance in order to reduce the detonation. The amount of timing retard that the PCM applies is based on the engine speed and the length of time that the engine detonation is detected. Once the spark timing is retarded, the KS circuitry in the PCM performs calculations in order to determine how much spark advance should be re-introduced. Normally the ignition timing advance is increased until zero retard, or normal ignition timing, is re-established. If detonation occurs again, the whole cycle will repeat. The alteration of the ignition timing by the KS often occurs continuously while the engine is running, even though no detonation is heard by the vehicle's operator.