Knock Sensor: An Electronic Guide to It

The knock sensor is an essential component in the engine electronic control system. Its function is to detect whether the engine has knocked and send the signal to the engine ECU. There are two common knock sensors, one is a magnetostrictive knock sensor, and the other is a piezoelectric knock sensor.

Knock sensor location

The knock sensor is usually mounted directly on the outside of the engine block, but in some cases, it is located below the intake manifold.

Knock Sensor Types and Working Principle

Knock sensor types: magnetostrictive knock sensor and piezoelectric knock sensor.

The shape and structure of the magnetostrictive knock sensor are shown in the figure below. There are permanent magnets, ferromagnetic iron cores excited by the permanent magnets, and coils around the iron cores. The composition of the magnetostrictive knock sensor is shown below. How does a knock sensor work? Its working principle is: when the cylinder block of the engine vibrates, the sensor resonates with the engine at about 7kHz, the magnetic permeability of the ferromagnetic material iron core changes, and the magnetic flux density of the permanent magnet passing through the iron core also changes, thereby generating an induced electromotive force in the windings around the iron core, and inputting this electrical signal to the ECU.

1. winding    2. iron core   3. shell   4. permanent magnet

Figure: shape and structure of the magnetostrictive knock sensor 

1. soft magnetic sleeve 2. terminal 3. spring 4. housing 5. permanent magnet 6. winding 7. magnetostrictive rod 8. electrical insulator

Figure: composition of the magnetostrictive knock sensor

The block diagram of the piezoelectric knock sensor is shown below. Piezoelectric knock sensors use the piezoelectric effect of crystalline or ceramic polycrystal to work, and some also use the piezoelectric resistance effect of doped silicon. The shell of the sensor is equipped with piezoelectric elements, weights, and lead wires. Its working principle is: when the cylinder block of the engine vibrates and the vibration is transmitted to the sensor housing, the relative movement occurs between the housing and the counterweight, and the pressure on the piezoelectric element sandwiched between the two changes, resulting in a voltage. The ECU detects this voltage and judges the knocking intensity according to its value.

1. lead 2. weight 3. piezoelectric element

Figure: block diagram of the piezoelectric knock sensor

What does a Knock Sensor do?

Knock sensor purposes:

1. Installed on the cylinder block to detect the deflagration condition of the engine, and provide it to the ECU to adjust the ignition advance angle according to the signal. The main component is a piezoelectric ceramic crystal, and the screw makes an inertia weight block press the piezoelectric ceramic crystal piece;

2. When knocking occurs, the knocking pressure wave compresses and deforms the piezoelectric ceramic crystal sheet through the inertial counterweight, and generates a much larger voltage signal than the non-deflagration time. In order to avoid misjudgment caused by interference, a judgment interval is set according to the ignition time;

3. The knock sensor can detect knock by three methods: cylinder pressure, engine block vibration, and combustion noise. When the engine knocks, the sensor will generate a corresponding electrical signal and send it to the electronic controller, so that the electronic controller can eliminate the engine deflagration by delaying the ignition;

4. There are mainly magnetostrictive and piezoelectric. The operating temperature of the magnetostrictive knock sensor is -40°C 125°C, and the frequency range is 510kHz; the piezoelectric knock sensor has a sensitivity of 200mV\/g at the center frequency of 5.417kHz and has good linearity during amplitude of 0.1g-10g.

What are the Correct Steps to Replace a Car’s Knock Sensor?

1. The knock sensor is one of the main factors reflecting the ignition advance angle. The knock sensor is installed on the cylinder block or cylinder head. Its function is to convert the mechanical vibration transmitted to the cylinder block or cylinder head during knocking into a voltage signal and send it to the ECU. The ECU will judge whether the engine knocks and the knock strength according to the voltage signal. The detected signal is input to the ECU, and the ECU adjusts the ignition advance angle according to the feedback signal of the knock sensor to keep the ignition advance angle in the best position to avoid engine detonation, improve the working performance of the engine, and prolong the working life of the engine.

2. When the engineer adjusts the knock sensor, he writes the vibration pattern of the knock into the electronic control unit. If the knock sensor detects a vibration pattern, the electronic control unit will determine engine knock and then retard the spark advance. Today’s advanced knock sensors can even determine which cylinder is knocking and retard the cylinder’s spark advance angle individually. The knock sensor is installed in the middle of the engine cylinder block. For example, a four-cylinder engine is installed between cylinders 2 and 3, or one between cylinders 1 and 2 and one between cylinders 3 and 4. It is used to measure engine jitter and adjust the ignition advance angle when the engine knocks, usually a piezo.

3. When the engine vibrates, the ceramic inside is squeezed to generate an electrical signal. Because this electrical signal is very weak, the connecting wires of ordinary knock sensors are wrapped with shielded wires. A knock sensor is an AC signal generator, but it is very different from most other automotive AC signal generators. In addition to detecting the speed and position of rotating shafts like magneto-electric crankshaft and camshaft position sensors, they also detect vibration or mechanical stress. Unlike stators and magnetoresistive, they are usually piezoelectric devices. They are made of special materials that can sense mechanical stress or vibration (for example, when an engine starts to knock, it generates an AC voltage.

4. The knock sensor is an accessory installed in the middle of the cylinder of the car engine. If the car is a four-cylinder engine, then the second and third cylinders are the car’s knock sensors. When the car knocks, the engine jitter can be judged by adjusting the ignition advance angle. There are many kinds of knock sensors, the most common of which is the telescope sensor, which is mainly composed of a magnetic core, a permanent magnet, and an induction coil. When the engine vibrates, an electrical signal is generated that drives the car’s engine to respond.

Knock Sensor Fail Symptoms

The parts of the knock sensor can be damaged easily

Magnetostrictive knock sensor: coil

Piezoelectric sensor: oscillator or piezoelectric element

Bad knock sensor symptoms

In this part, Easybom will discuss “what happens when a knock sensor goes bad“.

The engine has no obvious response, the fuel consumption increases and the engine shakes slightly.

1. When the engine is working, it will cause engine knocking due to the influence of excessive ignition timing (ignition advance angle), engine load, temperature, and fuel quality. When knocking occurs, due to the combustion of gas before the piston moves to the top dead center, the light one will generate noise and reduce the power of the engine, and the heavy one will damage the mechanical parts of the engine.

2. If the knock sensor is damaged, the engine control unit (ECU) cannot monitor the degree of knocking. When the engine control unit detects knocking in the engine through the knock sensor, it will reduce the ignition advance angle, thus erroneously judging that no knocking has occurred. , in this case, the engine control unit will increase the ignition advance angle, causing the engine to work rough and shake due to knocking.

3. After the knock sensor is broken, it will automatically delay the ignition time to prevent the engine from knocking. Therefore, the engine will be in a working state of retarded ignition.

4. The fuel consumption of the engine will increase, the combustion energy rate will decrease, and the engine power will decrease.

The Reasons for the Damage to the Knock Sensor 

Next, we will talk about “what causes a knock sensor to go bad“.

1. Excessive wear between the piston and the cylinder leads to excessive clearance;  

2. The direction of the piston is reversed or the piston is deformed; 

3. The cylinder wall is poorly lubricated; 

4. The connecting rod is bent or twisted and deformed; 

5. Lubricating oil 

6. The carbon deposition in the combustion chamber is serious; 

7. The fuel injection advance angle is too large; 

8. The combustion speed of the combustible mixture is too fast, causing the pressure in the cylinder to be too high; 

9. The cylinder gasket is too thin or the top surface of the cylinder block is worn too much, causing the diesel engine compression ratio to increase.

Michael Caine

Michael Caine is the Owner of Amir Articles and also the founder of ANO Digital (Most Powerful Online Content Creator Company), from the USA, studied MBA in 2012, love to play games and write content in different categories.