Knock metrics experimental evaluation for pre-chamber SI engines

The present study analyzes the occurrence of pressure oscillations in engines operating with pre-chambers at stoichiometric conditions, aiming to assess whether the increase in the amplitude of pressure oscillations detected by means of cylinder pressure measurements is related to knock, or it is a result of mass transfer from the pre-chamber to the main chamber. Initially, a literature review of the phenomenon is conducted and several knock indicators are used to evaluate experimental data obtained with a commercial four-cylinder SI engine, equipped with pre-chamber ignition system prototype, at 1500 rpm and 4 bar of IMEP. Among the knock indicators analyzed the excitations of resonance modes, the angle and time-based maximum pressure rise rate, maximum amplitude of pressure oscillations, ringing intensity, and adapted knock index can be cited. Furthermore, a study of the predominant pre-chamber ignition mechanism is carried out, distinguishing torch and jet ignition, which is also correlated with knock occurrence. Therefore, the present study shows the importance of conducting an individual knock analysis for each pre-chamber configuration and presents adequate tools for such a study, in order to guarantee engine integrity while exploring the benefits of the system, without limitations of inappropriate knock thresholds.