What is a Knock Sensor?

A automobile engine could have a million flaws.

In the past, it was up to users or professionals to fix potential automotive problems until they figured out what was causing the damage or failure. Modern cars, on the other hand, have become so intelligent that, thanks to sensors, they can detect issues as soon as they occur.

The entire vehicle is equipped with dozens of sensors that continuously monitor the vehicle's function and health. Some sensors measure oxygen, air, or temperature, while others, like knock sensors, maintain track of vibration and sound levels.

The knock sensor, on the other hand, serves a far broader purpose.

Ⅰ. What is Knock?

Engine knock, often known as knock, is the sound and reaction that occurs when a second, unintentional ignition or explosion occurs in the cylinder, apart from the normal regulated ignition of the spark plug. In other words, knocking is hazardous for car engines and will result in some negative consequences.

A flame front is created when a spark plug is ignited and passes through the rest of the cylinder area. The residual air and fuel mixture is pressurized by the movement of the flame front. When the pressure rises, the temperature rises as well. In rare situations, the temperature rises to the point where a second ignition takes place. A knock will occur when the second ignition produces a second flame front, and these two reactions collide.

The engine frequently makes a "pop" or "click" sound as it knocks. The sound usually gets louder when you use the throttle or accelerate.

Ⅱ. What is a Knock Sensor?

The knock sensor is a small "listening" device in or on the engine that detects abnormal vibrations and sounds coming from the engine block.

The knock sensor detects engine block vibration and sound, converts it to an electronic signal, and delivers it to the engine management unit (ECU). The computer in the automobile then evaluates the data and determines whether the ignition timing should be adjusted.

It may also illuminate the check engine light (CEL) or turn off a component of the engine to prevent further damage.

Knock sensor structure

According to their distinct architectures, automotive knock sensors are classified as inductive knock sensors or piezoelectric knock sensors. There are two types of piezoelectric knock sensors: piezoelectric resonance knock sensors and piezoelectric non-resonant knock sensors. There are three types of knock sensors: piezoelectric spark plug seat metal pad knock sensor, piezoelectric spark plug seat metal pad knock sensor, and piezoelectric spark plug seat metal pad knock sensor.

Inductive knock sensor

Induction coil, iron core, shell, and permanent magnet are the essential components of an inductive knock sensor. Figure 1 depicts its structure.

The electromagnetic induction principle is used by the inductive knock sensor to detect engine knock. The iron core of the engine is shaken, and the magnetic flux of the coil changes, resulting in an induced electromotive force. The signal voltage output by the sensor is the highest when the natural frequency of the sensor is the same as the vibration frequency when the engine knocks.

Piezo knock sensor

Piezoelectric resonance type knock sensor

Piezoelectric resonance type knock sensor structure diagram

The piezoelectric resonance type knock sensor is made up of several components, including a piezoelectric element, a vibrator, a base, and a shell. The figure above is a structural diagram.

The vibrator will cling to the piezoelectric element, and the vibrator will be fastened to the base. The vibrator's vibration pressure is detected by the piezoelectric element, which turns the vibration pressure into an electrical signal and sends it to the ECU. The signal produced is comparable to that of an inductive knock sensor. The resonant knock sensor vibrator must be used in conjunction with the engine because the vibration frequency when the engine knocks is the same as the natural frequency of the engine. When the engine knocks, however, the vibrator will resonate with the engine, and the voltage signal emitted by the piezoelectric element will be substantially higher, making measurement much easier.

Piezoelectric non-resonant knock sensor

Piezoelectric non-resonant knock sensor structure diagram

A sleeve, a piezoelectric element, an inertial weight, a plastic shell, and a wiring socket make up the piezoelectric non-resonant knock sensor. The figure above is a structural diagram.

By detecting acceleration signals, the piezoelectric non-resonant knock sensor detects knock. When the engine knocks, the inertial weight's alternating force and the vibration acceleration are proportionally added to the piezoelectric element, which converts the pressure signal into an electric signal and delivers it to the ECU. When there is knocking or not, the piezoelectric non-resonant knock sensor's output signal voltage does not change much. The filter detects whether there is banging frequency in the sensor output signal and determines whether knocking occurred. Because only the frequency range of the filter needs to be modified when this sort of sensor is used in different types of engines, it has a high degree of versatility.

Piezoelectric spark plug seat metal pad knock sensor

 Piezoelectric spark plug holder metal pad knock sensor 

The piezoelectric spark plug holder metal pad knock sensor has a construction that is similar to that of the piezoelectric non-resonant knock sensor. The figure above is a structural schematic.

This type of sensor places a piezoelectric device on the spark plug gasket, one for each cylinder, and senses knock information directly based on the combustion pressure of each cylinder, converting it to an electrical signal and sending it to the ECU.

Ⅲ. Where is the Knock Sensor?

The knock sensor is normally mounted on the outside of the engine block, but it can also be found under the intake manifold in some situations. The knock sensor is located in the engine block's midsection. As an example, consider a four-cylinder engine. It is fitted between cylinders 2 and 3, or between cylinders 1 and 2 and cylinders 3 and 4. Its purpose is to determine the degree of engine jitter and, when the engine knocks, to modify the ignition advance angle.

Ⅳ. How does the Knock Sensor Work?

Piezoelectric ceramics or piezoelectric components are used in most knock sensors. "Piezoelectric ceramics are smart materials that can turn mechanical effects (such as pressure, motion, or vibration) into electrical signals, and vice versa," according to Science Direct. Piezoelectric ceramics are widely employed in a number of fields due to electromechanical effects, such as motion sensors, timepieces, ultrasonic power sensors, stone crushers, ultrasonic cleaning, ultrasonic welding, active vibration absorbers, tweeters, atomic force microscope actuators, and so on."

Knock sensors are AC signal producers, but they're not like most other AC signal generators in cars. Magnetoelectric crankshaft and camshaft position sensors detect not only the speed and position of the rotating shaft but also vibration and mechanical pressure. They are usually piezoelectric devices, unlike stators and magneto-resistors. They're made of materials that can detect mechanical pressure or vibration (for example, an AC voltage can be generated when the engine knocks).

Engine damage is caused by engine knock caused by early ignition, poor exhaust gas recirculation, low-grade fuel, and other factors. The knock sensor sends a knock signal to the computer (some via the control module PCM), which allows the computer to modify the ignition timing to prevent future knocks. In the ignition timing feedback control loop, they operate as "oxygen sensors."

Knock sensors can be found in a variety of locations throughout the engine block or cylinder. When vibration or knocking occurs, a small voltage peak is produced; the stronger the knocking or vibration, the smaller the voltage peak. The higher the main peak of knock sensor manufacturing, the more knock sensors are produced. Knocking or knocking is indicated by a high frequency, and knock sensors are often built to measure frequencies in the 5 to 15-kilohertz range. The microprocessor in the control unit re-corrects the ignition timing to prevent further knocking when it detects these frequencies. The knock sensor is usually quite long-lasting. As a result, the sensor will only be harmed as a result of its own failure.

When the engine bangs, a pressure wave with a frequency of 1-10KHZ is produced. The pressure wave is transmitted to the cylinder block, creating vibration acceleration in the metal particles. The knock pressure is detected by the accelerometer knock sensor, which measures the vibration acceleration on the cylinder block's surface. Strength. One of the most common causes of banging is premature ignition time. Because the engine must produce maximum power, a knock sensor is added to allow the electronic control device to immediately modify the ignition time if the engine loses power without knocking.

Ⅴ. What’s the Cause of the Knock?

The knocking is caused by the flame not being entirely distributed after the combustion chamber gas has been ignited. Due to excessive temperature or pressure, the faraway unburned gas spontaneously ignites. When its flame collides with the conventional combustion flame, it creates a large amount of pressure, causing the engine to tap abnormally. The following are the main reasons for knocking: 1) The advance angle of the ignition is too large. Before the piston hits compression top dead center, ignition is frequently pre-ignited. If the ignition is started too early, the majority of the gasoline will burn during the piston's compression stroke, leaving only a tiny amount of fuel to be knocked under pressure. 2) The piston has carbon deposits on it. Long-term use causes a thick layer of coke to accumulate on the top of the piston, reducing the combustion chamber's volume and increasing the compression ratio, causing knocking. 3) The temperature of the engine is too high. The engine runs for an extended period of time, the water temperature is too high, and the water cooling cycle is ineffective, causing the engine to knock at high temperatures.4) The octane rating of the fuel is insufficient. Engines with a high compression ratio require high-octane fuel, while engines with a low compression ratio require low-octane gasoline. 5) There is an irregular air-fuel ratio. When the air-fuel ratio is larger than 14.7, the fuel-air mixture is leaner, leading the combustion temperature to rise and the engine temperature to rise as well, resulting in banging.

Ⅵ. Detection Method of the Knock Sensor

Take Passat Xinlingyu as an example, as shown in the picture.

Passat new Lingyu knock sensor detection

1) Detection of the knock sensor's circuit. 1 Remove the wiring harness's outer insulation, locate two shielded wires (black), and test the wiring harness's resistance between T3c/3-ground and T3a/3-ground at 200 ohms with a multimeter. The resistance is usually around 0.1 ohm. Otherwise, it's a sign that the harness isn't working properly. 2Disconnect the link between the knock sensor and the ECU and take measurements between T3c/1-T121/107, T3c/2-T121/99, T3a/1-T121/106, and T3a/2-T121/99. The harness resistance should be around 0.1 ohms under typical conditions; otherwise, the harness is defective.

2) Inspect the knock sensor for proper operation. 1Connect the oscilloscope's special tool to the knock sensor G61, start the engine at idle speed and record the knock sensor's signal waveform with the oscilloscope. If there is no knock signal, the knock sensor is likely destroyed. ② Measure the knock sensor G66 using the same manner.