The device and principle of operation of the knock sensor
Auto Terms,  Car device,  Engine unit,  Vehicle electrical equipment

The device and principle of operation of the knock sensor

A modern car is equipped with a large number of electronic devices, with the help of which the control unit controls the operation of various car systems. One such important device that allows you to determine when the engine begins to suffer from knock is the corresponding sensor.

Consider its purpose, principle of operation, device and how to identify its malfunctions. But first, let's figure out the detonation effect in the motor - what it is and why it occurs.

What is detonation and its consequences?

Detonation is when a portion of the air / fuel mixture farther from the spark plug electrodes ignites on its own. Because of this, the flame spreads unevenly throughout the chamber and there is a sharp push on the piston. Often this process can be recognized by a ringing metal knock. Many motorists in this case say that it is "knocking fingers."

Under normal conditions, a mixture of air and fuel compressed in the cylinder, when a spark is formed, begins to ignite evenly. Combustion in this case occurs at a speed of 30m / sec. The detonation effect is uncontrollable and chaotic. At the same time, the MTC burns out much faster. In some cases, this value can reach up to 2 thousand m / s.

The device and principle of operation of the knock sensor
1) Spark plug; 2) Combustion chamber; A) Normal fuel combustion; C) Knocking combustion of gasoline.

Such an excessive load negatively affects the condition of most parts of the crank mechanism (read about the device of this mechanism separately), on valves, hydrocompensator each of them, etc. An engine overhaul in some models can cost as much as half an identical used car.

Detonation can disable the power unit after 6 thousand kilometers, and even earlier in some cars. This malfunction will depend on:

  • Fuel quality. Most often, this effect occurs in gasoline engines when using inappropriate gasoline. If the octane number of the fuel does not meet the requirements (usually uninformed motorists buy cheaper fuel, which has an RON lower than that specified), indicated by the ICE manufacturer, then the probability of detonation is high. The octane number of the fuel is described in detail. in another review... But in short, the higher this value, the lower the likelihood of the effect under consideration.
  • Power unit designs. To improve the efficiency of the internal combustion engine, engineers are making adjustments to the geometry of various engine elements. In the process of modernization, the compression ratio may change (it is described here), the geometry of the combustion chamber, the location of the plugs, the geometry of the piston crown and other parameters.
  • The condition of the motor (for example, carbon deposits on the actuators of the cylinder-piston group, worn o-rings, or increased compression after a recent modernization) and its operating conditions.
  • States spark plugs(on how to determine their malfunction, read here).

Why do you need a knock sensor?

As you can see, the impact of the detonation effect in the motor is too great and dangerous for the condition of the motor to be ignored. To determine whether a micro-explosion occurs in a cylinder or not, a modern engine will have an appropriate sensor that reacts to such bursts and disturbances in the operation of the internal combustion engine (this is a shaped microphone that converts physical vibrations into electrical impulses). Since the electronics provides a finer tuning of the power unit, only the injection motor is equipped with a knock sensor.

The device and principle of operation of the knock sensor

When detonation occurs in the engine, a load jump is formed not only on the KShM, but on the cylinder walls and valves. To prevent these parts from failing, it is necessary to adjust the optimal combustion of the fuel-air mixture. To achieve this, it is important to fulfill at least two conditions: select the right fuel and correctly set the ignition timing. If these two conditions have been met, then the power of the power unit and its efficiency will reach the maximum parameter.

The problem is that at different modes of operation of the motor, it is required to slightly change its setting. This becomes possible due to the presence of electronic sensors, including detonation. Consider his device.

Knock sensor device

In today's automotive aftermarket, there is a wide variety of sensors for detecting engine knock. The classic sensor consists of:

  • A housing that is bolted to the outside of the cylinder block. In the classic design, the sensor looks like a small silent block (rubber sleeve with a metal cage). Some types of sensors are made in the form of a bolt, inside which all the sensitive elements of the device are located.
  • Contact washers located inside the housing.
  • Piezoelectric sensing element.
  • Electrical connector.
  • Inertial substance.
  • Belleville springs.
The device and principle of operation of the knock sensor
1. Contact washers; 2. Inertial mass; 3. Housing; 4. Belleville spring; 5. Bolt of fastening; 6. Piezoceramic sensing element; 7. Electrical connector; 8. Block of cylinders; 9. Cooling jacket with antifreeze.

The sensor itself in an in-line 4-cylinder engine is usually installed between the 2nd and 3rd cylinders. In this case, checking the engine operating mode is more effective. Thanks to this, the operation of the unit is leveled not due to malfunctions in one pot, but as much as possible in all cylinders. In motors with a different design, for example, the V-shaped version, the device will be located in a place where it is more likely to detect the formation of detonation.

How does a knock sensor work?

The operation of the knock sensor is reduced to the fact that the control unit can adjust the UOZ, providing a controlled combustion of the VTS. When a detonation occurs in the motor, a strong vibration is generated in it. The sensor detects load surges due to uncontrolled ignition and converts them into electronic pulses. Further, these signals are sent to the ECU.

Depending on the information coming from other sensors, different algorithms are activated in the microprocessor. Electronics changes the operating mode of the actuators that are part of the fuel and exhaust systems, ignition of a car, and in some engines sets the phase shifter in motion (the description of the operation of the variable valve timing mechanism is here). Due to this, the combustion mode of the VTS changes, and the operation of the motor adapts to the changed conditions.

The device and principle of operation of the knock sensor

So, the sensor installed on the cylinder block works according to the following principle. When an uncontrolled combustion of VTS occurs in the cylinder, the piezoelectric sensing element reacts to vibrations and generates a voltage. The stronger the vibration frequency in the motor, the higher this indicator.

The sensor is connected to the control unit using wires. The ECU is set to a certain voltage value. When the signal exceeds the programmed value, the microprocessor sends a signal to the ignition system to change the SPL. In this case, the correction is made in the direction of decreasing the angle.

As you can see, the function of the sensor is to convert the vibrations into an electrical impulse. In addition to the fact that the control unit activates the algorithms for changing the ignition timing, the electronics also corrects the composition of the mixture of gasoline and air. As soon as the oscillation threshold exceeds the permissible value, the electronics correcting algorithm will be triggered.

The device and principle of operation of the knock sensor

In addition to protecting against load surges, the sensor helps the control unit to tune the power unit for the most efficient combustion of the BTC. This parameter will affect engine power, fuel consumption, the state of the exhaust system, and especially the catalyst (about why it is needed in the car, it is described separately).

What determines the appearance of detonation

So, detonation can appear as a result of improper actions of the car owner, and for natural reasons that do not depend on a person. In the first case, the driver may mistakenly pour inappropriate gasoline into the tank (for what to do in this case, read here), it is bad to monitor the condition of the engine (for example, deliberately increase the interval of scheduled maintenance of the engine).

The second reason for the occurrence of uncontrolled fuel combustion is the natural process of the engine. When it reaches higher revs, the ignition starts firing later than the piston reaches its maximum effective position in the cylinder. For this reason, in different operating modes of the unit, either earlier or later ignition is required.

The device and principle of operation of the knock sensor

Do not confuse cylinder detonation with natural engine vibrations. Despite the presence balancing elements in the crankshaft, ICE still creates certain vibrations. For this reason, so that the sensor does not register these vibrations as detonation, it is configured to trigger when a certain range of resonance or vibrations is reached. In many cases, the noise range at which the sensor will begin to signal is between 30 and 75 Hz.

So, if the driver is attentive to the state of the power unit (serves it on time), does not overload it and fills in the appropriate gasoline, this does not mean that detonation will never occur. For this reason, the corresponding signal on the dashboard should not be ignored.

Types of sensors

All modifications of detonation sensors are divided into two types:

  1. Broadband. This is the most common device modification. They will work according to the principle indicated earlier. They are usually made in the form of a rubber round element with a hole in the center. Through this part, the sensor is screwed to the cylinder block with a bolt.The device and principle of operation of the knock sensor
  2. Resonant. This modification is similar in design to an oil pressure sensor. Often they are made in the form of a threaded union with faces for mounting with a wrench. Unlike the previous modification, which detects vibrations, resonance sensors pick up the frequency of microexplosions. These devices are made for specific types of motors, since the frequency of microexplosions and their strength depends on the size of the cylinders and pistons.The device and principle of operation of the knock sensor

Signs and Causes of Knock Sensor Malfunction

A faulty DD can be identified by the following features:

  1. In normal operation, the engine should run as smoothly as possible without jolting. Detonation is usually audible by the characteristic metallic sound while the engine is running. However, this symptom is indirect, and a professional can determine a similar problem by sound. Therefore, if the engine starts to shake or it works in jerks, then it is worth checking the knock sensor.
  2. The next indirect sign of a faulty sensor is a decrease in power characteristics - poor response to the gas pedal, unnatural crankshaft speed (for example, very high at idle). This can happen due to the fact that the sensor transmits incorrect data to the control unit, so the ECU unnecessarily changes the ignition timing, destabilizing the operation of the engine. Such a malfunction will not allow to accelerate correctly.
  3. In some cases, due to a breakdown of the DD, the electronics cannot adequately set the UOZ. If the engine has had time to cool down, for example, during overnight parking, it will be difficult to cold start. This can be observed not only in winter, but also in the warm season.
  4. There is an increase in gasoline consumption and at the same time all car systems are working properly, and the driver continues to use the same driving style (even with serviceable equipment, an aggressive style will always be accompanied by an increase in fuel consumption).
  5. The check engine light came on on the dashboard. In this case, the electronics detects the absence of a signal from the DD and issues an error. This also happens when the sensor readings are unnatural.

It is worth considering that none of the listed symptoms is a 100% guarantee of sensor failure. They can be evidence of other vehicle malfunctions. They can only be accurately recognized during diagnosis. On some vehicles, the self-diagnosis process can be activated. You can read how to do this. here.

The device and principle of operation of the knock sensor

If we talk about the causes of sensor malfunctions, then the following can be distinguished:

  • The physical contact of the sensor body with the cylinder block is broken. Experience shows that this is the most common reason. This usually occurs due to a violation of the tightening torque of the stud or fixing bolt. Since the motor still vibrates during operation, and due to inaccurate operation, the seat can be contaminated with grease, these factors lead to the fact that the fixation of the device is weakened. When the tightening torque decreases, jumps from microexplosions are worse received on the sensor, and over time it ceases to respond to them and generate electrical impulses, defining detonation as a natural vibration. To eliminate such a malfunction, you need to unscrew the fasteners, remove the oil contamination (if any) and just tighten the fastener. At some unscrupulous service stations, instead of telling the truth about such a problem, the craftsmen inform the car owner about the failure of the sensor. An inattentive customer can spend money on a non-existent new sensor, and the technician will simply tighten the mount.
  • Violation of the integrity of the wiring. This category includes a large number of different faults. For example, due to improper or poor fixing of the electrical line, the wire cores may break over time or the insulating layer will fray on them. This could result in a short circuit or open circuit. It is often possible to find the destruction of the wiring by visual inspection. If necessary, you just need to replace the chip with wires or connect the DD and ECU contacts using other wires.
  • Broken sensor. By itself, this element has a simple device in which there is little to break. But if it breaks down, which happens extremely rarely, then it is replaced, since it cannot be repaired.
  • Errors in the control unit. In fact, this is not a breakdown of the sensor, but sometimes, as a result of failures, the microprocessor incorrectly captures data from the device. To identify this problem, you should carry out computer diagnostics... By the error code, it will be possible to find out what interferes with the correct operation of the unit.

What do knock sensor malfunctions affect?

Since DD affects the determination of the UOZ and the formation of the air-fuel mixture, its breakdown primarily affects the dynamics of the vehicle and fuel consumption. In addition, due to the fact that the BTC burns incorrectly, the exhaust will contain more unburned gasoline. In this case, it will burn out in the exhaust tract, which will lead to breakdowns of its elements, for example, a catalyst.

If you take an old engine, which uses a carburetor and a contact ignition system, then to set the optimal UOZ, it is enough to turn the distributor cover (for this, several notches are made on it, by which you can determine which ignition is set). Since the injection engine is equipped with electronics, and the distribution of electrical impulses is carried out using signals from the corresponding sensors and commands from the microprocessor, the presence of a knock sensor in such a car is mandatory.

The device and principle of operation of the knock sensor

Otherwise, how will the control unit be able to determine at what moment to give an impulse for the formation of a spark in a particular cylinder? Moreover, he will not be able to adjust the operation of the ignition system to the desired mode. Car manufacturers have foreseen a similar problem, so they program the control unit for late ignition in advance. For this reason, even if the signal from the sensor is not received, the internal combustion engine will work, but only in one mode.

This will have a significant impact on fuel consumption and vehicle dynamics. The second especially concerns those situations when it will be necessary to increase the load on the motor. Instead of picking up speed after pressing the gas pedal hard, the internal combustion engine will "choke". The driver will spend much more time to reach a certain speed.

What happens if you turn off the knock sensor completely?

Some motorists think that to prevent detonation in the engine, it is enough to use high-quality gasoline and timely carry out scheduled maintenance of the car. For this reason, it seems that under normal conditions there is no urgent need for a knock sensor.

The device and principle of operation of the knock sensor

In fact, this is not the case, because by default, in the absence of a corresponding signal, the electronics automatically sets the late ignition. Disabling DD will not turn off the engine immediately and you can continue to drive the car for some time. But it is not recommended to do this on an ongoing basis, and not only because of the increased consumption, but because of the following possible consequences:

  1. Can pierce the cylinder head gasket (how to change it correctly, it is described here);
  2. Parts of the cylinder-piston group will wear out faster;
  3. The cylinder head may crack (read about it separately);
  4. May burn out valves;
  5. One or more may be deformed. connecting rods.

Not all of these consequences will necessarily be observed in every case. It all depends on the parameters of the motor and the degree of detonation formation. There may be several reasons for such malfunctions, and one of them is that the control unit will not try to troubleshoot the ignition system.

How to determine a malfunction of a knock sensor

If there is a suspicion of a faulty knock sensor, then it can be checked, even without dismantling. Here is a simple sequence of such a procedure:

  • We start the engine and set it at the level of 2 thousand revolutions;
  • Using a small object, we simulate the formation of detonation - do not hit hard a couple of times near the sensor itself on the cylinder block. It is not worth making efforts at this moment, since cast iron can crack from impact, since its walls are already affected during the operation of the internal combustion engine;
  • With a working sensor, the revolutions will decrease;
  • If the DD is faulty, then the rpm will remain unchanged. In this case, additional verification using a different method is required.

Ideal car diagnostics - using an oscilloscope (you can read more about its types here). After checking, the diagram will most accurately show whether the DD is working or not. But to test the sensor's performance at home, you can use a multimeter. It must be set in resistance and constant voltage measurement modes. If the wiring of the device is intact, then we measure the resistance.

The device and principle of operation of the knock sensor

In a working sensor, the indicator of this parameter will be within 500 kΩ (for VAZ models, this parameter tends to infinity). If there is no malfunction, and the motor icon continues to glow on the tidy, then the problem may not be in the sensor itself, but in the motor or gearbox. There is a high probability that the instability of the unit operation is perceived by the DD as a detonation.

Also, for self-diagnosis of malfunctions of the knock sensor, you can use an electronic scanner that connects to the car's service connector. An example of such equipment is Scan Tool Pro. This unit is synchronized with a smartphone or computer via Bluetooth or Wi-Fi. In addition to finding errors in the sensor itself, this scanner will help identify most common control unit errors and reset them.

Here are the errors that the control unit fixes, like DD malfunctions, relate to other breakdowns:

Error code:Explanation:Cause and solution:
R0325Open circuit in the electrical circuitYou need to check the integrity of the wiring. Visual inspection is not always sufficient. Wire strands may break, but remain isolated and periodically short-circuit / open. Most often, this error occurs with oxidized contacts. Much less often, such a signal can indicate slippage. timing belt a couple of teeth.
R0326,0327Low signal from the sensorSuch an error may indicate oxidized contacts, through which the signal from the DD to the ECU is poorly received. You should also check the tightening torque of the fastening bolt (it is quite possible that the tightening torque is loose).
R0328High sensor signalA similar error can occur if the high voltage wires are in close proximity to the sensor wiring. When the explosive line breaks through, a voltage surge may occur in the sensor wiring, which the control unit will determine as a detonation or a malfunction of the DD. The same error can occur if the timing belt is not tensioned enough and slipped a couple of teeth. How to properly tension the timing gear drive is described here.

Most knock sensor problems are very similar to late ignition symptoms. The reason is that, as we have already noticed, in the absence of a signal, the ECU automatically switches to emergency mode and instructs the ignition system to generate a late spark.

Additionally, we suggest watching a short video on how to choose a new knock sensor and check it:

Knock sensor: signs of malfunction, how to check what it is for

Questions and answers:

What is the knock sensor used for? This sensor detects detonation in the power unit (mainly manifested in gasoline engines with low-octane gasoline). It is installed on the cylinder block.

How to diagnose a knock sensor? Better to use a multimeter (DC mode - constant voltage - range less than 200 mV). A screwdriver is pushed into the ring and easily pressed against the walls. The voltage should vary between 20-30 mV.

What is a knock sensor? This is a kind of hearing aid that allows you to listen to how the motor works. It catches sound waves (when the mixture does not ignite evenly, but explodes), and reacts to them.

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