What Is a Knock Sensor?

Parents knocking, good. Engines knocking, bad.

Medium 16621 Maserati Nettuno Engine
Maserati

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There are a million and one things that could go wrong with a car engine. In old times, it was solely up to the owner and/or technician to drudge through possible issues until they determined what was broken or malfunctioning. Modern cars have become so intelligent, though, that they can detect problems as they happen, that’s what sensors are for.

There are dozens of sensors all throughout a vehicle that constantly read a car’s functionality and health. Some read oxygen, air, or temperature levels, while others such as knock sensors, keep tabs on vibrations and sound levels so you don’t end up with a piston-sized hole in the engine itself.

But there’s more to a knock sensor than just that, so please allow The Drive’s crack informational team to explain and answer the question, what is a knock sensor?

Knock sensors are often bolted directly onto the engine block.
Autozone

A Duralast knock sensor for a 2003 Acura RSX.

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 Does Engine Knock Sound Like?

Engine knock typically sounds like a knock,  ping, or click coming from the engine. The sound often becomes more noticeable during throttle input and/or acceleration.

An example of a knocking noise is pretty apparent in this video, though we do not endorse any methods or actions taken in this clip: 

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. 

A knock sensor monitors irregular cylinder ignitions
Bosch

A Bosch Knock Sensor.

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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