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Knocking is an abnormal combustion phenomenon caused by the spontaneous ignition of the mixture at the end of the combustion chamber in a gasoline engine. Knocking not only produces a loud knocking sound within the cylinder, but it also subjects mechanical components such as pistons, connecting rods, and crankshafts to excessive stress, leading to mechanical damage and engine overheating. As a result, **the service life of the engine is reduced; therefore, knocking in engines needs to be controlled. The most widely used method at present is to detect engine vibration with a knock sensor in order to determine whether knocking is occurring. Knock sensors are divided into two main categories: resonant and non-resonant. The resonant type is further divided into magnetostrictive and piezoelectric types, while the non-resonant type includes only the piezoelectric type. Since resonant sensors generate a relatively high voltage during engine knock, it is possible to determine whether knock has occurred without the need for a filter ； Non-resonant knock sensors require filters to detect the knock signals. The vast majority of modern cars use resonant piezoelectric knock sensors, which detect the occurrence of knocking by taking advantage of the resonance phenomenon that occurs when the vibration frequency generated by engine knocking matches the natural frequency of the sensor itself. Its output signal is a voltage, and the magnitude of this voltage indicates the intensity of detonation. Let’s take a look at how to measure the signal waveform of a knock sensor using an oscilloscope. First, use a BNC to banana plug cable to connect channel 1 of the oscilloscope; connect the black connector of the banana plug to the alligator clip and the red connector to the probe tip. Connect the alligator clip to the negative terminal of the battery to ground it. The red positive pin is connected to the sensor plug. Since the knock sensor responds very quickly, it is necessary to set an appropriate time base for the oscilloscope. For example, if it is set to 10ms, the total time for one screen is 140 ms. The voltage range should be set to −5 to +5 volts. Therefore, the vertical scale can be set to 1V/div. Select edge trigger for the triggering mode, and then set the trigger level to around 640 mV. Finally, click to trigger Single SEQ once. Next, go to the car and turn on the ignition switch; do not start the engine. Use some metal objects to tap on the area near the knock sensor. After tapping the engine block, a waveform is immediately displayed on the oscilloscope; the greater the force of the tap, the larger the vibration amplitude, and the higher the voltage value of the waveform as well. The waveform shown below is an example of engine knock: engine knock caused by factors such as premature ignition, poor exhaust gas recirculation, and low-octane fuel can lead to engine damage. The knock sensor provides a knock signal to the computer (in some cases via the ignition control module), allowing the computer to adjust the ignition timing again in order to prevent further knocking. They actually act as the “oxygen sensor” in the ignition timing feedback control loop. Knock sensors are installed at various locations on the engine block or cylinders. When vibration or cylinder knocking occurs, it generates a small voltage peak; the greater the knocking or vibration, the higher the peak value generated by the knock sensor. A very high frequency indicates detonation or knocking; detonation sensors are typically designed to measure frequencies in the range of 5–15 kHz. When the control unit receives these frequencies, the computer adjusts the ignition timing again to prevent further knocking. Knock sensors are usually very durable, so in most cases they are damaged only due to their own failure. Some oscilloscopes designed for automotive repair come with built-in software for automotive testing; by simply selecting the appropriate option, the vertical scaling, time base, trigger settings of the oscilloscope are adjusted automatically. Of course, the results are the same as those obtained when making adjustments manually.