# A Review of Recent Advancements in Knock Detection in Spark Ignition Engines
## 2 Background
### Signals for Measuring Knock  
Since knock is a compilation of a chemical event triggering pressure oscillations inside the combustion chamber, inducing engine block vibrations and resulting in an audible sound, there are a number of different signals that can be measured to detect and study knock. These different signals were first measured by [PERSON] in 1928, who measured the pressure oscillations associated with a knocking engine and found that they relate to the resonance frequencies of the engine cavity [5]. Today, the same signals are used, albeit with more advanced equipment. The three most common signals for detecting knock are measuring the in-cylinder pressure, detecting block vibrations, and audible detection [6]. Each of these three methods, with their advantages and disadvantages, are given in Table 1.  
For engine research, the primary method to assess knock is through in-cylinder pressure transducers, which precisely track the initial pressure changes and oscillations due to fuel autoignition [7]. However, pressure transducers are expensive and require being placed with direct access to the inside of the engine cylinder. Moreover, fast-response pressure transducers are required, given that the frequencies associated with knock are between 6 kHz and 30 kHz, further driving up the cost [8]. Also, these expensive pressure transducers are exposed to the engine conditions, so they must be able to withstand significant heat. An alternative approach for evaluating knock involves using accelerometers that are affixed to the engine block. These devices capture block vibrations that are linked with knock and are relatively cost-effective. While a number of different types of accelerometers are available, most knock sensors in modern automobiles are piezoelectric devices that convert engine vibrations into electrical signals. Typically, knock sensors tend to implement a bandpass filter to focus on a set of frequencies between 3 and 25 kHz, which are primarily associated with knock [9]. Prior to the inclusion of knock sensors, audible noise was the most common metric for assessing if an engine is knocking. Indeed, knock is by definition a noise, and the primary goal of avoiding knock is to avoid an unpleasant sound that would annoy the vehicle users. In older vehicles, prior to the inclusion of knock sensors, the engines would be calibrated based off engineers listening for knock. While this was somewhat arbitrary, there is a sharp sound at 6 kHz which is easy for most people to detect even at low knocking levels [10]. Figure 2 illustrates the unprocessed signals obtained from two pressure transducers, a high-speed accelerometer and a microphone, during a cycle bordering on knock in an engine. Positioned on the engine block's side, the accelerometer and the microphone, situated 1 cm from the block, registered data. Autoignition occurs around 15\({}^{\circ}\) after the top dead center (ATDC), causing in-cylinder pressure oscillations. The accelerometer's vibration measurement initiates very soon after the onset of pressure oscillations, recorded by the transducers in the cylinder. Subsequently, the microphone detects noise after the accelerometer commences vibration measurement. Furthermore, the signals shown in Figure 2 illustrate the progression to audible knock. Autoignition induces gas pressure fluctuations within the cylinder, which are promptly captured by the pressure transducers. These gas oscillations within the cylinder engender vibrations throughout the engine block. The accelerometer captures these vibrations, characterized by a slower buildup and decay compared to the rapid pressure oscillations. Consequently, these vibrations from the engine block emit an audible noise, which is then detected by the microphone.