What is Knock?

Also known as engine ping or detonation, an engine knock is a sound and reaction that occurs when there is a second unplanned ignition or explosion within a cylinder that is separate from the regular controlled ignition from a spark plug. In other words, knock is bad news for your engine. A few things must happen for this to occur.


Basically, the spark plug ignition creates a flame front that travels through the remaining cylinder space. That flame front’s movement pressurizes the remaining air and fuel mixture. Increased pressure means increased temperature, and in some cases, it gets so hot that it creates a second ignition. The second ignition creates a second flame front, and when those two reactions collide, you get knock.
What Is a Knock Sensor?

A knock sensor is essentially a small “listening” device in or on the engine that detects these irregular vibrations and sounds that come from the engine block.

The knock sensor picks up vibration and sound coming from the engine block, turns it into an electronic signal and sends that signal to the engine control unit (ECU). The car’s computer then judges the information and determines whether or not ignition timing should be altered.
Where is a Knock Sensor Located?

A knock sensor is typically attached directly to the outside of the engine block, but in some cases, it is located underneath the intake manifold




How Does a Knock Sensor Work?

A large majority of knock sensors use piezoelectric ceramics or elements. According to Science Direct, “a piezoelectric ceramic is a smart material that converts a mechanical effect (such as pressure, movement, or vibration) into an electrical signal and vice versa. Due to the electromechanical effect, piezoelectric ceramics are used in a wide range of applications such as motion sensors, watches, ultrasonic power transducers, lithotripters, ultrasonic cleaning, ultrasonic welding, active vibration dampeners, high-frequency loudspeakers, actuators for atomic force microscopes, and many others.” Cool.
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