Improvement of performance and emission of future internal combustion engine for passenger cars is mandatory during the transition period toward their substitution with electric propulsion systems. In middle time, direct injection spark ignition (DISI) engines could offer a good compromise between fuel economy and exhaust emissions. However, abnormal combustion and particularly knock and super-knock are some of the most important obstacles to the improvement of SI engines efficiency. Although knock has been studied for many years and its basic characteristics are clear, phenomena involved in its occurrence are very complex and are still worth of investigation. In particular, the definition of an absolute knock intensity and the precise determination of the knock onset are arduous and many indexes and methodologies has been proposed.
In this work, most used methods for knock onset detection from in- cylinder pressure signal have been considered. Moreover, knock intensity has been evaluated by means of two common indexes. High speed imaging has been carried out in the combustion chamber of a high performance DISI engine provided with an optical window in the piston top. In this way, it has been possible to follow the entire combustion evolution almost until the cylinder borders. Particularly, the knock onset has been determined with a resolution of about 0.1 crank angle degrees. Two engine conditions characterized by high and medium knock intensities were explored.
In this way, it has been possible to evaluate the real resolution of different knock metrics for intensity and onset detection also depending on knock severity.