# A Review of Recent Advancements in Knock Detection in Spark Ignition Engines
## 2 Background
### Knock in Modern Vehicles  
Figure 3 plots the number of scholarly publications related to engine knock that have been published by the Society of Automotive Engineers (SAE) from 2000 to 2022 [11]. Several general trends are readily apparent: First, over this time, there was a total of 118,000 publications from SAE; 6300 of them were related to knock, equating to approximately 5.4 percent of publications. While this value might seem small, it is important to note that there are many other automotive research fields. Second, the number of publications steadily increased from 2000 to 2017. These publications were focused primarily on better modeling of the autoignition process, characterizing the signals to enhance knock detection, and identifying new additives to mitigate knock. From 2012 to 2017, an issue termed mega-knock became a key area of research. Mega-knock is an intense knock event that particularly occurs with high-ethanol-content fuels [12]. From 2017 to 2022, the number of articles started to decrease, in part as the automotive industry began focusing more on electrification, moving away from traditional combustion engines. However, it is important to note that in 2022, 5.1 percent of the articles published by SAE were still related to knock, indicating that it is still an issue that is being actively researched. Much of the current research is looking at knock mitigation for bio-fuels or for new engine operating regimes that are common for hybrid operations.  
Figure 3: Scholarly publications in the Society of Automotive Engineers (SAE)’s journals and conference proceedings from 2000 to 2022 related to engine knock.  
Figure 2: (**a**) Schematic of an experimental setup recording different knock signals including in-cylinder pressure, block vibrations, and audible noise. (**b**) Raw pressure signals from the different instrumentations.  
Knock remains a pertinent concern in contemporary engine design due to its implications across diverse technological advancements, notably in the context of engine downsizing, boosting, and hybrid powertrains. While downsizing involves reducing the engine displacement to enhance fuel efficiency without compromising power output, it increases the risk of knock due to increased thermal and mechanical loads in the cylinder of smaller engines [13]. Simultaneously, an increasing number of SI engines are turbocharging or supercharging, which increases the in-cylinder pressures and temperatures, hence increasing the propensity of the engine to knock.  
Moreover, the integration of hybrid powertrains introduces new complexities in engine operation, impacting their knock propensity. Hybrid engines combine internal combustion engines with electric propulsion, allowing the vehicle to operate in different modes. Typically, under high-load conditions, hybrid vehicles will use the engine, sometimes in conjunction with the motor. Under such conditions, the engine is operating at full load, resulting in the high temperatures and pressures associated with knock [14].  
Knock has also been a topic of research for Homogeneous Charge Compression Ignition (HCCI) engines [15]. HCCI engines operate at higher pressures and temperatures than SI engines; as such, rather than using a spark plug, the content of the engine autoignites. This autoignition event is a controlled event, which is different from knock, which is an abnormal event that can lead to engine damage. However, HCCI engines can still experience knock at high loads. HCCI engines typically use an SI scheme over a certain operating range and an HCCI scheme over a specific operating range. Therefore, for an HCCI engine, it is increasingly imperative that knock can be detected and predicted. Accurate knock detection and prediction are vital for HCCI engines to optimize combustion timing, prevent abnormal combustion events, and enhance the overall engine efficiency while ensuring that the engine operates within safe and reliable parameters [16].