1. What Happens Inside a Gasoline Engine When It Knocks?
Inside a gasoline engine, spark plugs ignite a mix of fuel and air. The flame spreads across the combustion chamber. However, sometimes the unburned fuel at the edge ignites all by itself instantly. This creates a large pressure wave that shakes the chamber walls [1]. This event is called knocking.
Drivers hear this as a metallic "knock-knock" sound. But inside the knock sensor, there are no parts that collect air sounds like a microphone does. A patent describes a non-resonant sensor made of a tube-shaped piezoelectric ceramic with a weight on top [3]. The element and weight shake together with the engine body. They send out a signal that looks like a wave matching those vibrations.
So, the sensor is not hearing sound through the air. It is feeling the shaking of the metal itself. The next question is: how does it choose only the knocking vibration from all the other shakes?
2. How Does a Knock Sensor Tell Knocking Vibrations Apart From Other Vibrations?
The main idea is to look at the speed of the vibration, which scientists call frequency. Knocking vibrations are thought to stand out in a specific range of frequencies. The sensor sends its signal through a band-pass filter before it reaches the computer (ECU). This filter lets only the knocking frequencies pass through so the computer can decide if knocking is happening [3]. It is a bit like choosing one radio station while ignoring others, but you do not turn a dial to match it.
The exact frequency depends on the engine. One patent suggests that knocking vibrations are noticeable in a band of 7–9 kHz (kilohertz) [5]. In an experiment with a single-cylinder engine, researchers at MIT found peaks in cylinder pressure vibration just above 6 kHz and between 15–22 kHz. The peak near 6 kHz also matched the knocking sound heard by an outside microphone [6].
These numbers look different because one is an example value and the other comes from a specific test. Since engines vary, there is no single fixed number for every car.
3. After Knocking Is Detected, How Does the Computer Fix It?
After detecting knocking, the computer changes when the spark plug fires. Nissan explains that the computer automatically adjusts the ignition timing to prevent knocking [1]. Honda’s guide for a motorcycle (CBR600RR) gives more detail: when potential knocking is detected, the sensor triggers a delay in ignition timing. Then, it gradually moves the timing back earlier to reach ideal combustion [2]. Note that this is an example from a motorcycle; not all cars work exactly the same way.
Delaying the spark makes knocking less likely to happen. However, if the spark stays delayed, the engine cannot make full use of its power [2]. So, the system gradually advances the timing again. It searches for the limit where knocking almost happens but does not.
A patent filed by Nippon Denso (now Denso) in 1981 describes this clearly. It picks up engine vibration with a piezoelectric element, filters out the knocking frequency, and judges if it is knocking based on strength. If knocking is found, it delays ignition; if not, it advances it. The amount of delay changes depending on how strong or frequent the knocking is [4]. This date refers to when the patent was filed, not when the product was sold.
4. Why Is Knocking Sometimes Hard for the Computer to Identify?
The frequency of knocking is not always steady. The resonance frequency changes during the expansion stroke because the volume and temperature inside the chamber change [7]. Also, at high engine speeds, it becomes harder to separate the knocking signal from noise made by other parts.
A 1995 paper proposed analyzing signals in both time and frequency to improve the ratio of signal to noise, making it easier to find knocking vibrations [7]. How the sensor is attached also matters. A patent warns that if the mounting is poor, extra vibrations near the knocking frequency can cause false judgments [3].
To make the knock sensor work well, the sensor, filter, and judgment method must be designed together as a system.
5. How Did People Measure Engine Knocking Before Electronic Sensors?
People measured knocking long before electronic sensors existed. In the 1920s, Thomas Midgley and Charles Kettering are said to have created an indicator that measured pressure inside the engine cylinder around 1922. The Henry Ford Museum notes that records were made on light-sensitive paper [9].
Another tool was the "bouncing pin." A loosely placed pin would shake during normal burning but bounce violently during knocking. Observers used this to measure how strong the knock was. It was a standard instrument until 1948, though sources do not confirm who invented it [10].
For cars driving on the road, Toyota is noted as using knock control in the Crown (2.0L OHC turbo) in autumn 1980, according to a 1986 paper. That paper says they continued developing resonant and non-resonant ceramic sensors. A new resonant sensor in the 1986 Supra greatly widened the detectable frequency range [8]. This is just one example of adoption, not proof that it was the first knock sensor in the world.
6. How Can You Feel Vibration Detection With a Table at Home?
You can try a small version of this idea at your desk. Place your palm on the table and ask a family member to tap lightly from far away. Even if the sound is quiet, you feel the vibration in your hand. If they change how hard or fast they tap, the feeling changes too. This shows how knock sensors pick up body vibrations rather than air sounds.
You can also try "selecting" signals. If other movements like foot tapping mix with the table taps, can your hand count only the "tap-tap" rhythm? Your hand might not be able to separate them perfectly. An electric filter selects components in a set frequency band, which is what this manual test tries to mimic.
Do this gently. Do not hit hard or throw things; just use light taps so you do not break the table. If you want to read more about the structure, searching for the patent numbers listed in the sources can help you find the technical descriptions.