What is an Engine Knock Sensor and How Does it Work?

Engine Knock Sensor is used in the engine block to detect the knock or vibration coming out from the engine. When fuel ignites before the top of the compression stroke in a gasoline engine, it tries to expand and pushes the piston back down on the incorrect side of the stroke, causing a “knock.” In this article, I will discuss the function and usefulness of the Engine Knock Sensor.


Engine Knock Sensor

The engine is the most important component of a vehicle. An engine needs to be protected in any case. There are a lot of things that happen in the engine that cause the vibration or the knock. This knock sensor is important to determine the frequency of the vibration and control accordingly.

Engine knock is an undesirable condition where the air-fuel mixture in the combustion chamber ignites spontaneously before the spark plug fires. This premature ignition can lead to a variety of issues, including reduced engine performance, increased emissions, and potential damage to the engine.

The knock sensor is typically mounted on the engine block or cylinder head and is designed to detect the vibrations and sounds associated with engine knock. When a knock is detected, the sensor sends a signal to the engine control unit (ECU) or engine control module (ECM). In response, the ECU may adjust the ignition timing or fuel injection to prevent further knocking and optimize engine performance.

So the definition of the knock sensor would be like this, Engine knock sensor is the type of piezoelectric sensor or vibration sensor that detects the vibration coming out from the engine block due to the engine knocking and converts the signal into the electronic media, and this signal to the ECU.

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Engine Knock Sensor Working Principle


The engine knock sensor works on the Piezoelectric principle. This element is installed into the sensor. This Piezoelectric element helps to convert the vibration coming out from the engine into the electric current. Then this electric signal sends its frequency to the Engine Control Unit.

The knock sensor is typically mounted on the engine block or cylinder head, near the combustion chamber. When the air-fuel mixture in the combustion chamber ignites spontaneously before the spark plug fires, it creates shock waves or vibrations. This is referred to as an engine knock.

The knock sensor picks up these vibrations and converts them into an electrical signal. This signal is then sent to the engine control unit (ECU) or engine control module (ECM). Upon receiving the signal from the knock sensor, the ECU takes corrective action to prevent further knocking. This may involve adjusting the ignition timing, fuel injection, or other parameters to optimize combustion and prevent engine damage.

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Engine Knock Sensor Circuit


The knock sensor is a piezoelectric sensor that generates an electrical voltage in response to engine vibrations caused by knocking or detonation. The knock sensor is connected to the engine control unit (ECU) or engine control module (ECM) through a wiring harness. This harness carries the electrical signals between the sensor and the control unit.

The electrical signal generated by the knock sensor is typically a small voltage fluctuation. The signal may be in the form of an AC (alternating current) voltage or a digital signal, depending on the sensor type and the engine control system. This signal from the knock sensor may be weak, so the circuit may include an amplifier to strengthen the signal.

The processed knock sensor signal is then fed into the ECU or ECM. The control unit interprets the signal to determine if an engine knock is occurring. The ECU employs a knock detection algorithm that analyzes the knock sensor signal over time. If abnormal vibrations indicative of knocking are detected, the ECU activates the knock control strategy.

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Function of Engine Knock Sensor

One of the primary functions of the knock sensor is to protect the engine from damage caused by detonation or knocking. Engine knock can lead to increased temperatures and pressures in the combustion chamber, potentially resulting in piston damage, cylinder wall scoring, and other forms of engine wear.

By preventing engine knock and optimizing combustion, the knock sensor indirectly contributes to fuel efficiency. When the engine operates smoothly without knocking, it can achieve better fuel combustion, leading to improved fuel economy.

In scenarios where the quality of fuel varies, the knock sensor allows the engine to adapt to different fuel compositions. It helps the engine management system make real-time adjustments to accommodate variations in fuel octane levels, ensuring optimal performance and protection against knocking.

This sensor also helps to minimize unburned fuel and harmful emissions. This is particularly important in meeting environmental regulations and emission standards.

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Types of Engine Knock Sensor

Depending upon the design of the vehicle the engine knock sensor can be different. There are two types of knock sensors used in an engine-

  1. Resonant Knock Sensor
  2. Non-Resonant Knock Sensor

1. Resonant Knock Sensor

his type of knock sensor uses the piezoelectric effect. Piezoelectric materials generate an electric charge in response to mechanical stress or vibrations. It consists consists of a piezoelectric crystal or ceramic element. When an engine knock occurs, causing vibrations, the crystal generates a voltage signal.

The voltage signal is then converted into an electrical signal, which is sent to the engine control unit (ECU). These types of knock sensors are known for their sensitivity and ability to detect even slight vibrations associated with the early stages of engine knock.

2. Non-Resonant Knock Sensor

This type of knock sensor measures changes in electrical resistance due to mechanical strain caused by engine knock. The sensor typically contains a strain-sensitive element, such as a strain gauge, bonded to the engine structure. When knocking occurs, the strain gauge experiences deformation, altering its electrical resistance.

The change in resistance is converted into an electrical signal, which is then transmitted to the engine control unit. These types of knock sensors are known for their durability and reliability. They are less susceptible to external vibrations and can operate effectively in harsh environments.

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Advantages of Engine Knock Sensor

The major advantages of knock sensors are-

  1. By detecting the early signs of abnormal combustion, the sensor allows the engine control unit (ECU) to make timely adjustments to prevent potential damage.
  2. This sensor ensures that the combustion process occurs under ideal conditions, maximizing power output and efficiency.
  3. Efficient combustion, facilitated by the knock sensor, helps minimize unburned fuel and reduces harmful emissions.
  4. The knock sensor serves as a diagnostic tool for the engine control system.
  5. The knock sensor contributes to the overall longevity and durability of the engine.


In summary, the engine knock sensor is a critical component that enhances engine durability, performance, and efficiency while contributing to reduced emissions. Its real-time monitoring and feedback to the engine control system make it an essential feature in modern vehicles.

Frequently Asked Questions(FAQ’S)

What are the consequences of an engine knock?

Engine knock can lead to increased temperatures and pressures in the combustion chamber, potentially causing damage to pistons, cylinder walls, and other engine components. It can also result in reduced engine performance and increased emissions.

Can a knock sensor adapt to different fuel qualities?

Yes, the knock sensor enables the engine to adapt to varying fuel qualities by allowing the engine control system to make real-time adjustments. This adaptability ensures consistent performance and protection against knocking under different fuel compositions.

Can a malfunctioning knock sensor impact engine performance?

Yes, a malfunctioning knock sensor can lead to degraded engine performance, reduced fuel efficiency, and increased emissions. It may also prevent the engine control system from effectively detecting and addressing knocking events.

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