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Best Guide to Engine Knock Sensor

engine knock sensor

The engine knock sensor, also known as a detonation sensor or knock probe, is a device used to detect engine knock.

What is engine knock sensor

An engine knock sensor, also referred to as a detonation sensor or knock probe, is a device that can detect engine knock. Engine knock, also known as detonation, is an abnormal combustion process within the engine that can degrade engine performance and even cause damage. The sensor operates by detecting the vibration frequency of the engine to determine if knock is occurring. When knock is detected, the sensor sends a signal to the Engine Control Unit (ECU), which then adjusts parameters such as ignition timing and fuel injection quantity to eliminate the knock.

knock sensor

Knock sensor purpose

Engine knock, caused by factors such as low-octane fuel and premature ignition, can lead to engine damage. The role of the knock sensor is to detect whether engine knock is occurring during operation and transmit the knock signal to the engine's ECU in the form of an electrical signal. The ECU, based on this signal, issues instructions to control the ignition coil's primary circuit, adjusting the ignition timing to prevent knock.

When the engine experiences knock, it generates pressure waves with a frequency of 1-10KHZ. These pressure waves are transmitted to the cylinder body, causing its metal particles to vibrate at an accelerated rate. Accelerometer knock sensors detect the strength of the knock pressure by measuring the vibration acceleration on the cylinder body surface. Premature ignition timing is a major cause of engine knock. To ensure the engine can deliver maximum power without losing power or causing knock, a knock sensor is installed, allowing the electronic control unit to automatically adjust the ignition timing.

Knock Sensor Location

The knock sensor is typically located on the engine block or cylinder head, and there can be one or multiple installed. It is usually screwed into the block and has a wiring harness connected to it. The knock sensor needs to be in direct contact with the engine to accurately detect any knocking or pinging. The exact location may vary depending on the make and model of the vehicle, but it needs to be in a position where it can accurately detect engine knock.

knock sensor location

For a four-cylinder engine, it is common to have:

  • (1) One installed between the 2nd and 3rd cylinders

  • (2) One installed between the 1st and 2nd cylinders, and another between the 3rd and 4th cylinders, for a total of two sensors.

When vibration or knocking occurs, it generates a small voltage peak. The larger the knock or vibration, the larger the peak produced by the knock sensor. A certain high frequency indicates knock or vibration, and knock sensors are usually designed to measure frequencies in the range of 5 to 15 kilohertz. When the control unit receives these frequencies, the computer corrects the ignition timing to prevent further knocking. Knock sensors are usually very durable, so the sensor will only be damaged due to its own failure.

Type of Knock Sensor

Knock sensors can be categorized into resonant piezoelectric knock sensors, non-resonant piezoelectric knock sensors, and magnetostrictive knock sensors.

Magnetostrictive knock sensor

Knock sensors come in many varieties, with the earliest application being the magnetostrictive knock sensor. It mainly consists of a magnetic core, a permanent magnet, and an induction coil, the structure diagram is as follows. When the engine vibrates, the core vibrates and deflects, and an induced electromotive force is generated in the coil, which outputs a voltage signal.

Magnetostrictive knock sensor

When engine knock occurs, it creates pressure waves that cause the engine block to vibrate. These vibrations are transferred to the magnetostrictive knock sensor. The sensor contains a magnetostrictive material that changes its shape in response to these vibrations. This change in shape alters the magnetic field within the sensor, which can be detected and measured. The sensor then converts this magnetic signal into an electrical signal that can be interpreted by the engine control unit (ECU).

Piezoelectric Knock Sensor

A Piezoelectric Knock Sensor operates using the piezoelectric effect of materials like crystals or polycrystalline ceramics, and some also utilize the piezoresistive effect of doped silicon.

What are Piezoelectric materials?

Piezoelectric materials can be natural or man-made. Natural piezoelectric materials include quartz, Rochelle salt, and certain types of ceramics. Man-made piezoelectric materials include various types of ceramics and polymers.

What is the piezoelectric effect?

Piezoelectric materials generate electricity when subjected to strain such as compression or collision, a phenomenon known as the positive piezoelectric effect. The piezoelectric effect is reversible, meaning that materials exhibiting the piezoelectric effect can also deform in response to an applied electric field.

Piezoelectric Resonant Knock Sensor

The most commonly used type of piezoelectric knock sensor is the resonant type, which is typically mounted on the upper part of the engine. It uses the piezoelectric effect to convert the mechanical vibrations produced during knocking into a signal voltage. When the vibration frequency produced during knocking (around 6000Hz) matches the natural frequency of the piezoelectric sensor, resonance occurs. At this point, the sensor outputs a high knock signal voltage to the ECU, which promptly corrects the ignition timing to prevent knocking.

The resonant piezoelectric knock sensor is composed of piezoelectric elements, an oscillator base, a shell, etc. (the structure is shown in the figure below). When detonation occurs, the oscillator resonates with the engine, and the signal voltage output by the piezoelectric element also significantly increases, making it easy to measure.

Piezoelectric Resonant Knock Sensor Structure

Piezoelectric Resonant Knock Sensor Structure Diagram

When the crystal is subjected to external force, a voltage is generated on both surfaces of the crystal. The magnitude of the voltage is directly proportional to the magnitude of the external force; when the external force is removed, the crystal is not charged.

When the engine vibrates, it triggers the sensor. The piezoelectric element deforms due to the vibration and outputs a signal voltage.

Non-resonant Piezoelectric Knock Sensor

Compared with the resonant detonation sensor, the non-resonant detonation sensor does not have an oscillating piece inside, but sets a counterweight, which is pressed on the piezoelectric element with a certain pre-tightening pressure. When the engine detonates, the counterweight applies an alternating force proportional to the vibration acceleration to the piezoelectric element, and the pressure element then converts this pressure signal into an electrical signal and sends it to the ECU.

Non-resonant Piezoelectric Knock Sensor Structure

Non-resonant Piezoelectric Knock Sensor Structure Diagram

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1. Where is the knock sensor located?

The knock sensor is typically located on the engine block or cylinder head. Its placement allows it to detect the vibrations caused by engine knock effectively.

2. What do knock sensors do?

Knock sensors detect abnormal vibrations, or "knock," in the engine. These vibrations can be caused by the air-fuel mixture in the cylinders detonating in more than once place at a time. When the knock sensor detects these vibrations, it sends a signal to the engine control unit (ECU), which then adjusts the timing of the engine's ignition to prevent further knock.

3. What happens when a knock sensor goes bad?

When a knock sensor goes bad, it may fail to detect engine knock, or it may send false signals to the ECU. This can lead to a variety of problems, including reduced engine performance, increased fuel consumption, and potentially serious engine damage. The check engine light may also come on, and the vehicle may fail an emissions test.

4. How many knock sensors does a car have?

The number of knock sensors in a car can vary depending on the vehicle's make and model. Some vehicles have one knock sensor, while others may have two or more. Vehicles with V6 or V8 engines, for example, often have two knock sensors, one for each bank of cylinders.

5. Can I drive with a bad knock sensor?

While it's technically possible to drive with a bad knock sensor, it's not recommended. A faulty knock sensor can lead to serious engine damage if engine knock occurs and the ECU isn't notified to adjust the ignition timing. This could lead to costly repairs in the long run.

6. Can a knock sensor cause a no start?

A faulty knock sensor typically won't prevent the engine from starting. However, if the knock sensor is sending false signals to the ECU, it could cause the engine to run poorly or even stall after it starts. In rare cases, a severely damaged knock sensor could potentially cause a no-start condition, but there are usually other underlying issues in such cases.

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