Have you ever heard a weird knocking sound coming from your engine? That could be a sign of trouble, but luckily, your car has a built-in defense system—the knock sensor. This little device helps keep your engine running smoothly, improves fuel efficiency, and even boosts torque. So, how does it work? Let’s break it down.1-3
Image Credit: Standret/Shutterstock.com
So, What is Engine Knock, and Why is it a Concern in Modern Vehicles?
Engine knock, also known as pinging, happens when fuel ignites too quickly inside the engine’s cylinders, creating a knocking or pinging noise. If not properly managed, this can lead to long-term engine damage.
Engine knock is a major challenge in modern engine design, especially with trends like downsizing, turbocharging, and hybrid powertrains. Downsized engines are designed for better fuel efficiency, but their smaller cylinders experience higher mechanical and thermal stress, increasing the risk of knock. Turbocharging and supercharging add to the problem by raising in-cylinder temperatures and pressures, making knock even more likely.
Hybrid powertrains bring another layer of complexity. These engines operate under different driving modes, and during high-load conditions, the internal combustion engine runs at full capacity—generating more heat and pressure, which can lead to knock.
Knock is also a key research area for homogeneous charge compression ignition (HCCI) engines, which use autoignition instead of spark plugs. While this ignition method is controlled, HCCI engines can still experience knock under heavy loads.
If not addressed, engine knock can cause serious damage to engine components, reduce overall performance, and lower fuel efficiency. That’s why precise detection and prevention methods are critical to maintaining engine reliability and efficiency. This is where knock sensors come into play, providing real-time data that helps engines adjust ignition timing and prevent potential damage.4
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What is a Knock Sensor
A knock sensor is a small but essential part of your car’s engine management system. It listens for vibrations caused by knocking (also known as detonation) and helps adjust the engine’s timing to prevent damage. Knocking happens when fuel burns unevenly in the cylinders, which can lead to reduced performance and, over time, serious engine issues.
By detecting these vibrations and sending signals to the engine control unit (ECU), the knock sensor allows your car to make real-time adjustments, keeping everything running smoothly and efficiently.
The knock sensor is made up of a piezoelectric element. A working principle for piezoelectric elements involves the transmission of an electrical current in response to detecting a change in pressure or vibration by these elements. The piezoelectric element inside the knock sensor is tuned to detect the engine knock/detonation frequency. Figure 1 below illustrates the basic structure of a knock sensor.
As demonstrated in Figure 1, the knock sensor is made up of piezocrystals (piezoelectric elements), a shunt resistor, and a thread at one end of the sensor, which allows the device to be threaded into the block near the pistons.
Figure 1. The basic structure to a knock sensor. Image Credit: Schwaller A.E. (2005). Total Automotive Technology. USA, New York: Thomas Delmar Learning.
During combustion, a knock in the combustion chamber sends a vibration to the silicone rings attached to the piezoelectric crystals in the knock sensor (in the form of mechanical stress). The silicon ring is accelerated, forcing the knock sensor to generate an electrical voltage and a pressure wave through the cylinder block. Voltage output from the knock sensor will be high during a knock to the ignition system.
A typical voltage signal generated by the knock sensor can range between 300 millivolts to approximately 500 millivolts; however, this will depend purely on the intensity of the knock during combustion. The following video animates the operation of the knock sensor in a vehicle system:
Knock Sensor Operation
Types of Knock Sensors
Vibration-based knock sensors fall into three main categories based on their sensing mechanisms:1,5
- Inductive resonant sensors
- Piezoelectric resonant sensors
- Piezoelectric non-resonant sensors
Both piezoelectric and inductive resonant sensors use a vibration plate tuned to resonate at the frequency of engine knock. When knock occurs, the plate vibrates, and the sensor converts this vibration into an electrical signal using either the piezoelectric effect (which generates voltage in response to mechanical stress) or electromagnetic induction.
Most automotive engines rely on piezoelectric knock sensors due to their accuracy in detecting knock frequencies, which typically range from 5 to 30 kHz.1,5
Regardless of the type, knock sensors all tend to operate in harsh environments and must function reliably in temperatures ranging from -40 °C to 150 °C. Extreme temperatures can affect the properties of the piezoelectric material, potentially altering its sensitivity and accuracy.5
Piezoelectric Knock Sensors: Structure and Function
A piezoelectric knock sensor consists of:
- Piezoelectric crystals (piezocrystals) that generate voltage when compressed
- A shunt resistor to stabilize the signal
- A threaded mount for attaching the sensor near the pistons
During combustion, if knock occurs, vibrations from the combustion chamber travel through the engine block. These vibrations put mechanical stress on the silicone rings attached to the piezoelectric crystals inside the sensor. This stress causes the sensor to generate an electrical voltage, signaling knock detection to the ignition system.
The voltage output from a knock sensor typically ranges from 300 to 500 millivolts, depending on the intensity of the knock event.1,5
Resonant vs. Non-Resonant Sensors
- Resonant sensors are designed to detect knock at a specific frequency. They use a spring-mass system that resonates at a set frequency based on the engine’s design. These sensors provide a strong signal but are limited in adaptability since knock frequencies can vary with engine speed.
- Non-resonant sensors detect knock over a wider frequency range (5 kHz - 15 kHz). Instead of relying on mechanical tuning, they use electronic signal processing to identify knock events. This makes them more versatile and better suited for engines with varying knock frequencies.
The resonant frequency of a piezoelectric sensor depends on its material composition, shape, and thickness. In general, thinner piezoelectric elements have higher resonant frequencies.1,5
New Developments
With engine technology evolving rapidly, knock detection methods are also improving. Researchers and manufacturers are continuously refining sensor accuracy and efficiency to meet modern engine demands. Some of the most exciting advancements include:
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Low-cost in-cylinder pressure sensors: These offer a 0.5–0.7 % increase in engine efficiency by enabling more precise spark timing. This improvement results from more optimal spark timing, leading to higher pressures and better thermal efficiency. While the pressure-based method is more accurate than conventional vibration sensors, it is more expensive and requires access to the combustion chamber, which could raise concerns about durability and cooling issues for the engine.4
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Piezoelectric accelerometers: A new type of knock sensor with a stronger signal that eliminates the need for external amplification, reducing costs while maintaining accuracy. Initial tests showed promise without external amplification devices, and future research aims to explore the full potential of these cost-effective sensors for precise knock detection in prototype ICEs.4
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Machine learning-based detection: Researchers have developed AI-driven systems that analyze data from multiple engine sensors. While promising, current systems detect only about 81 % of knocking cycles, indicating room for improvement.
Leading Companies in the Automotive Knock Sensor Market
The automotive knock sensor market is driven by several major players leading innovation and growth. Companies such as Continental AG, Denso Corporation, Robert Bosch GmbH, Sensata Technologies, and Analog Devices, Inc. are at the forefront, leveraging advanced technologies and expanding their product offerings to maintain a competitive edge. These companies are investing heavily in research and development (R&D) to create cutting-edge solutions that address evolving customer demands and regulatory requirements.
One of the key advancements in the industry comes from Robert Bosch GmbH, which has developed multi-frequency knock sensor technology. This innovation allows for more precise knock detection across different engine conditions, improving adaptability and ensuring accurate performance in various environments. Similarly, Analog Devices Inc. has introduced a next-generation sensor system equipped with advanced signal processing algorithms. This system enhances the precision of knock detection, contributing to better engine performance and reliability.
Sensata Technologies has taken a strategic approach by forming partnerships with leading automakers to integrate high-performance knock sensor modules into upcoming vehicle models. By tailoring their sensors to specific vehicle designs, Sensata ensures optimized engine efficiency and durability. Meanwhile, Continental AG has launched a real-time monitoring knock sensor, improving precision and overall engine performance. Denso Corporation has also made significant strides by developing a highly sensitive, compact knock sensor designed to enhance fuel efficiency and reduce emissions. Its smaller size makes it particularly well-suited for modern, space-constrained engines.
To stay competitive, these companies are focusing on several key strategies. They are working to improve sensor sensitivity and durability, ensuring long-term reliability. Additionally, they are advancing digital integration capabilities, allowing for seamless compatibility with modern vehicle systems. The development of compact and efficient designs is also a priority, particularly as engine sizes continue to shrink. Furthermore, optimizing fuel efficiency and emissions reduction remains crucial in meeting increasingly strict environmental regulations. Strategic partnerships with automakers are another critical strategy, enabling manufacturers to customize sensors for specific vehicle models.
As the automotive industry moves toward electric and hybrid vehicles, innovation in knock sensor technology will remain essential. These key players are expected to continue driving advancements that enhance engine efficiency, reliability, and compliance with environmental standards, ensuring they stay ahead in the evolving market.
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References and Further Reading
- Knock Sensors [Online] Available at https://cecas.clemson.edu/cvel/auto/sensors/knock.html (Accessed on 27 February 2025)
- Korenciak, D., Gutten, M., Adamec, J., Glowacz, A., Cichy, A. (2018). Analysis Of Engine Knock Sensor. Komunikácie, 20(1). DOI: 10.26552/com.c.2018.1.37-41, https://komunikacie.uniza.sk/artkey/csl-201801-0006_analysis-of-engine-knock-sensor.php
- Hussein, H. (2023). Optimal Knock Sensor Spatial Locations of In-line Combustion Engines: A Case Study of Scania Engines. https://www.diva-portal.org/smash/record.jsf?dswid=-8254&pid=diva2%3A1795810
- Mittal, V. (2024). A Review of Recent Advancements in Knock Detection in Spark Ignition Engines. Signals, 5(1), 165-180. DOI: 10.3390/signals5010009, https://www.mdpi.com/2624-6120/5/1/9
- Fernandez, B. O., Aguiar, P. R., Alexandre, F. A., Viera, M. A. A., Bianchi, E. C. (2020). Study of knock sensors as low-cost alternatives to acoustic emission sensors. IEEE Sensors Journal, 20(11), 6038-6045. DOI: 10.1109/JSEN.2020.2972778, https://ieeexplore.ieee.org/abstract/document/8988273
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