Automotive Electronics

Knock Sensors

Basic Description

Knock sensors are used to detect irregular vibrations of the engine caused by uneven combustion. Generally, there are two approaches used to sense engine knock: vibration detection and pressure measurement. Most knock sensors operate based on the first method. Vibration detection sensors can be divided into three types according to their sensing mechanisms: inductive resonant sensors, piezoelectric resonant sensors, and piezoelectric non-resonant sensors. Both the inductive and piezoelectric resonant sensors are comprised of a vibration plate that has the same resonance frequency as the knock vibration. When knocking occurs, the plate achieves its maximum amplitude of vibration, which is converted to an electric signal through an electromagnetic inductive effect or a piezoelectric effect. Resonant sensors can only detect a particular frequency of engine knock due to their narrow-band response characteristics. On the other hand, the piezoelectric non-resonant type uses a spring-mass system to measure the vibration directly and thus has a wider bandwidth (typically from about 5 kHz - 15 kHz). Since the knock frequency may vary a little bit with the engine speed, non-resonant sensors tend to perform better than resonant ones. Vibration-based knock sensors are generally installed on the engine block or cylinder head.

Another way to detect engine knock is measuring the internal pressure of the cylinder directly. Pressure type sensors are usually integrated into the spark plug by mounting a ring-shaped piezoelectric element to the washer. When engine knock occurs, high-frequency noise is generated and superimposed on the normal combustion pressure waveform. The knock detection is achieved by filtering this waveform.

Bosch, Continental, Delphi, NAPA Echlin, Kistler, RAE
For More Information
[1] Engine Knocking, Wikipedia.
[2] Cylinder Pressure Sensor, Kistler data sheet, 2003. [pdf]
[3] Knock Sensors Do Two Jobs, Mark Hicks, Wells Counter Point, Oct. 2000. [pdf]
[4] Engine Knock Sensor, Part 2, Mark Hicks, Wells Counter Point, 2008. [pdf]
[5] Auto Knock Sensor, YouTube, Jan. 2008.
[6] Tip Clip: Knock Sensor Testing, YouTube, June 15, 2009.