1. Core Functional Characteristics
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Strong Specificity:
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Its core task is not to measure ordinary engine vibrations, but toaccurately identify and extract high-frequency vibration signals generated by knock. During normal engine operation, the vibration frequency is relatively low, while knocking produces specific high-frequency vibrations (typically in the range of 5kHz to 15kHz or higher, depending on engine design and RPM).
High Signal-to-Noise Ratio:
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This is one of its most important characteristics. The built-in circuit of the sensor or its collaboration with the ECU (Engine Control Unit) can effectivelyfilter out low-frequency noise interference caused by mechanical movement, road bumps, etc., only amplifying and transmitting high-frequency signals related to knocking, ensuring that the ECU can make accurate judgments.
Resonant Frequency Tuning:
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Many knock sensors are designed to have a specificmechanical resonant frequency (for example, commonly around 10kHz). This frequency is set within the typical frequency range of engine knocking. When knocking occurs, the sensor generates a voltage signal far exceeding other frequencies at its resonant frequency, greatly enhancing detection sensitivity.
2. Technical Performance Characteristics
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Wide Frequency Response Range:
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Although it is optimized for knocking frequencies, its effective working frequency range is usually very wide, capable of covering all knocking frequencies that may occur under different RPMs and loads of the engine.
High Sensitivity:
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It can detect extremely weak knocking signals. Even slight knocking can cause long-term damage to the engine, so the sensor must be sensitive enough. Its sensitivity is usually expressed in mV/g.
High-Temperature Resistance:
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Installed directly on the engine block or cylinder head, the working environment temperature is extremely high (usually needing to withstand temperatures from -40°C to +150°C or even higher), so its materials and structure must be able to operate stably at high temperatures for a long time.
Electromagnetic Interference Resistance:
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The engine compartment is filled with various strong electromagnetic interference sources (such as ignition coils, generators, etc.). The sensor itself and its shielded cables need to have good electromagnetic interference resistance to ensure the purity of signal transmission.
3. Structural and Application Characteristics
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Piezoelectric Principle as the Mainstream:
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The vast majority of knock sensors usepiezoelectric ceramics as the sensitive element. When the knocking vibration acts on the piezoelectric element, it generates a charge signal proportional to the vibration acceleration. This principle responds quickly, has a robust structure, and a long lifespan.
Robust and Durable:
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Typically, they are packaged in a fully sealed metal shell, with excellentoil, water, and corrosion resistance to adapt to the harsh environment of the engine compartment.
Installation Torque Sensitivity:
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The installationtorque of the sensor is crucial. It must be tightened according to the manufacturer’s specified torque value (usually a small precise value, such as 20 N·m). Excessive torque can lead to changes in its frequency response characteristics and reduced sensitivity; too little torque may result in poor contact, preventing effective vibration transmission.
Key Role in Engine Control:
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It is not a passive monitoring device, but rather the “ear” of anactive control system. The ECU dynamically adjusts the ignition timing based on the signals it receives, achieving “closed-loop control of ignition timing.” Once knocking is detected, the ECU immediately retards the ignition to eliminate knocking, then gradually advances the ignition, keeping the engine operating at the optimal efficiency state on the “knocking edge.”