# A Review of Recent Advancements in Knock Detection in Spark Ignition Engines
## 4 Discussion  
Figure 8 displays a high-level summary of the advances in different signal processing approaches for the detection of knock in SI engines. Each of the signal processing approaches has a different goal. The use of pressure signals is to help develop better tools for fundamental knock research, since in-cylinder pressure measurements are unlikely to be used on commercial vehicles. Meanwhile, the research into vibration signals is aimed towards creating cheaper and more accurate knock sensors for commercial vehicles, especially as vehicles start to transition to operating in downsized, turbocharged, hybrid configurations with alternative fuels. The research into audio signals and the other approaches are intended for the long-term replacement of the traditional vibration-based knock sensor.  
The research trends generally demonstrate that the assessment of knock through the spectral analysis of pressure traces remains the gold standard for knock detection. Indeed, almost every study used pressure signals as the \"ground truth\" in their studies to determine if the cycle is knocking. The advancements in the pressure sensors and processing yielded only marginally better results from what can currently be measured.  
Meanwhile, the research into vibration signals indicates that there are improvements that can be made on commercial knock sensors. Indeed, as engines enter hybrid configurations and become increasingly turbocharged, knocking frequencies will likely be over a broader frequency range than what a conventional knock sensor may be able to detect. Many of the studies in this area focused on new processing techniques, most of which would take advantage of the increased computational power that is available in hybrid vehicles. The studies showed improvements and have the potential to be integrated into vehicles in the short term to decrease the cost and increase the reliability of knock sensors.  
Figure 8: Summary of the different studies being conducted to understand knock signals, the overarching goals of the studies, and the focus of the studies.  
The other signal processing approaches for the detection of knock are still relatively immature. While they can support in a lab setting to fundamentally understand knock, they are unlikely to be integrated into vehicles in the short term. However, over the long term, many of these approaches show promise for replacing the traditional knock sensor, especially through the processing of signals that are already collected by the engine's processing unit, such as the engine speed or exhaust gas temperature, or by using equipment that is already included in the engine, such as the spark plug.  
This review article also provided insights into the expected future trends of knock research. Since knock will remain a problem in automotive design, there will be continued research into how to detect knock more accurately in an engine, especially at borderline knock conditions. Studies will continue to leverage new advances in signal processing and data analytics and apply them to the different knock signals. In particular, research will likely focus on how to detect knock from other signals that are collected by the engine, in order to avoid needing a separate knock sensor. For example, measuring small perturbations in engine speed, exhaust gas temperature, and the voltage across the spark plug gap, coupled with other signals that are collected by the engine (e.g., intake conditions, emissions, coolant temperatures) could provide a new method that is more accurate and cheaper than conventional knock sensors and less intrusive than pressure transducers.