LES analysis for auto-ignition induced abnormal combustion based on a downsized SI engine

Engine knock and super-knock have become the main barriers to significantly improving engine thermal efficiency. To further study the nature of the abnormal combustion, this work quantitatively investigates engine knock and super-knock using a Large Eddy Simulation framework coupling detailed chemistry solver. Firstly, classical knocking cycles with different knocking intensities have been calculated through adjusting spark-ignition timing. It shows that knocking onset and intensity vary proportionally with the advance of spark-ignition timing, however, super-knock events are hot observed under the operation conditions. Then for a given spark-ignition timing, the blends of Primaty Reference Fuels are introduced in order to obtain different octane number of mixture, through which super-knock events With stronger knocking intensity are observed. The results show that as the decreases of octane number, knocking onset is significantly advanced due to the enhancement of low-temperature chemical reactivity. Consequently, more auto-ignition centers appear at hot exhaust valve side and even Cool intake valve side at very low octane number. But for the knocking intensity, it does not always show a proportional correlation with octane number during super-knock. Further auto-ignition scenarios Show that developing detonation wave can be induced by both multiple hot-spots auto-ignition and directly by single hot-spot auto ignition, with different reaction front curvatures. However, the later seems to produce much stronger. knocking intensity, especially when there are several developing detonation waves during super knock. Therefore, how to effectively regulate local auto-ignition initiation and development seems the key to the avoidance of abnormal combustion in modern engines. (C) 2017 Elsevier Ltd. All rights reserved.