Knock Indexes Threshold Setting Methodology

Gasoline engines can be affected, under certain operating conditions, by knocking combustions: this is still a factor limiting engines performance, and an accurate control is required for those engines working near the knock limit, in order to avoid permanent damage. HCCI engines also need a sophisticated combustion monitoring methodology, especially for high BMEP operating conditions. Many methodologies can be found in the literature to recognize potentially dangerous combustions, based on the analysis of the in-cylinder pressure signal. The signal is usually filtered and processed, in order to obtain an index that is then be compared to the knock threshold level. Thresholds setting is a challenging task, since usually indexes are not intrinsically related to the damages caused by abnormal combustions events. Furthermore, their values strongly depend on the engine operating conditions (speed and load), and thresholds must therefore vary with respect to speed and load. The knock phenomenon is associated to a steep increase in the combustion speed: some of the indexes proposed in the literature are based on the evaluation of the Rate of Heat Release (ROHR) by means of the in-cylinder pressure signal. The filtering operation, in this case, is crucial: in this paper a novel methodology for ROHR filtering is proposed, consisting in the application of a zero-dimensional model based on Wiebe functions. The observation of reconstructed Heat Release traces leads to determine whether the combustion will lead to knock or not. The methodology allows defining a knock index which is essentially dependent on the knocking-combustion rate, and therefore operating conditions-independent. This means that it can be used as a knock intensity reference for other indexes, making it possible to associate to the given index for the given operating condition a proper threshold level.