What is the Function of a Knock Sensor?
3 Answers
The functions of a knock sensor are: 1. To measure engine vibration and adjust the ignition timing when engine knocking occurs; 2. It serves as an AC signal generator to detect the speed and position of the rotating shaft. A knock sensor consists of: a magnetic core, permanent magnet, and induction coil. The working principle of a knock sensor is: when vibration or knocking occurs, it generates a voltage peak. The greater the knocking or vibration, the higher the peak voltage produced by the knock sensor. A certain high frequency indicates knocking or vibration. The knock sensor can measure frequencies in the range of 5 to 15 kHz. When the control unit receives these frequencies, the computer recalibrates the ignition timing to prevent further knocking.
The knock sensor is like the engine's little ears, specifically responsible for listening for any pinging or knocking sounds in the cylinders. When we suddenly floor the accelerator or use low-quality gasoline while driving, the fuel in the cylinders may ignite prematurely. This uncontrolled combustion is called knocking, and it sounds like banging on a tin can. As soon as the sensor detects this noise, it immediately sends a signal to the engine control unit (ECU), which promptly adjusts the ignition timing to restore smooth engine operation. Without this little device, knocking can crack piston rings and melt spark plugs, leading to costly repairs. I've noticed that in some older cars, if the check engine light comes on and acceleration feels sluggish, nine times out of ten it's because the sensor is clogged with carbon buildup or the wiring harness is loose.
Remember that old Touareg I fixed last time with the constantly illuminated malfunction light? The knock sensor failure caused the engine to always operate at the knock threshold. This thing is actually a precision vibration detector, tightly screwed onto the engine block, specifically designed to capture knock frequencies between 5-8kHz. It ignores ordinary vibrations, but when the engine starts knocking with that characteristic 'tat-tat-tat' sound, its internal piezoelectric crystal sends signals to the ECU like a telegraph. Upon receiving these signals, the ECU retards ignition timing by 0.5-2 degrees, acting like a sedative for the angry engine. Some owners try to save money by using 92-octane fuel in high-altitude areas, resulting in frequent sensor intervention that actually consumes more fuel than using 95-octane would.
