Shahlari, A. and Ghandhi, J., "A Comparison of Engine Knock Metrics," SAE Technical Paper 2012-32-0007, 2012, doi:10.4271/2012-32-0007.
The objective of this work is to compare and contrast standard metrics that are used to quantify engine knock phenomena. Several methods found in the literature were investigated, and the fundamental bases for the methods were elucidated. A new knock onset metric was developed and compared to the commonly used threshold value exceeded metric. The standard knock intensity metrics were categorized based on the underlying signal (pressure, heat release), domain of analysis (time or frequency), and the calculation method (single-valued or integral). Each category was evaluated based on the fundamental advantages and disadvantages of that category. Single-value methods were found to be inadequate because they are prone to biases due to wave interference and beating. Heat release-based metrics were found to be redundant and be based on flawed assumptions. The time- or frequency-domain integration of the pressure oscillation energy were found to be the most reliable indicator of knock intensity.