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.

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It can also cause a check-engine light (CEL) to appear or potentially shut down part of the motor in order to save itself from further damage.

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. 

What Causes Engine Knock?

There are multiple reasons engine knock could occur. Here are a few potential causes:

  • Poor timing: The spark is not igniting at the correct time.
  • Improper air and fuel mixture: If the ratio of air to fuel is incorrect, this could create ignition problems.
  • Deposits inside the cylinder: Dirt, grime, and contaminants can enter cylinders and create all sorts of issues.
  • Faulty, unhealthy, or incorrect spark plugs: The wrong type of spark plug, spark plugs with deposit build-up, or incorrect spark plug gaps could cause poor spark or incorrectly timed spark. 

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What is Pre-Ignition?

Pre-ignition is often confused or mixed up with knock, but they are two different things. While knock often occurs around the same time as the spark plug ignition, pre-ignition occurs before the spark plug ignition occurs. It most commonly occurs when the piston is moving upward. 

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