How a Knock Sensor Works

A car engine.

A knock sensor is a device used to prevent detonation or spark knock. A spark knock or detonation, as it is commonly referred to, is a form of combustion that can cause different kinds of damage and failure to the engine. This occurs when the heat and pressure in the combustion chamber of the engine is excessive, causing the air-fuel mixture to auto-ignite.

What is a Knock Sensor

The knock sensor in an engine is used to send signals to the engine-control computer if this action is detected, preventing a detonation from happening. In a sense, the knock sensor is a microphone placed in the engine to listen to any unusual noises that the engine makes.

A knock sensor is often bolted on the center of the wall of the cylinder block. There are also some vehicles where the knock sensor is installed on the cylinder head itself. The location of the knock sensor depends entirely on how effective it will be in catching the noise an engine makes.

Most cars that are manufactured today have knock sensors installed in the engine. However, there are a few vehicles that may not have a knock sensor. It is advisable to check if the knock sensor is in place to prevent any potential damage to the engine.

The knock sensor may be mistaken for a spark plug because it looks just like one only smaller. This part has a threaded part on one end and a small pole-like or pin projection on the other end. Most of the parts of the knock sensor are located at its center. The threaded part is used to bolt the device on the engine, although some do not have this part installed. However, a knock sensor without the threaded part will have a threaded bolt hole instead.

How a Knock Sensor Works

The knock sensor works by sensing the “knocks,” or noises, the engine makes when the pressure and heat in the engine is too much. When this happens, the sensor picks up the noise or the vibration being made. Low volume reverberations from the engine which ranges from 6 to 8 kHz are common.

These low reverberations are picked up by the resonating plates inside the knock sensor. This is then relayed to the piezoelectric crystalline element of the sensor. Once the element receives the shock wave or signal, it will generate a small amount of voltage which will go to the electronic control unit of the vehicle.

As a result, the electronic control unit of the vehicle will delay the firing of the spark in the combustion chamber, preventing it from detonating or creating a spark knock.

The knock sensor essentially has two uses. The first is to detect any spark knock for an optimum engine performance and the second is to protect the engine from power-robbing early detonations.

Levels of a Spark Knock

There are certain “noise” levels made in the engine that a knock sensor can detect. A mild knock often creates a pinging sound that can sound like a small ball is bouncing around the engine. This type of knocking can easily be detected by a knock sensor.

However, if the engine of a vehicle is not properly cared for, severe knocks, which can sound like someone banging on a door, may not be detected instantly and that can lead to a damaged engine.