LES study of deflagration to detonation mechanisms in a downsized spark ignition engine

Anthony Robert (1, 2) , Stéphane Richard (1) , Olivier Colin (1) , Thierry Poinsot (3)

1 IFPEN - IFP Energies nouvelles
2 PSA - PSA Peugeot - Citroën
3 IMFT - Institut de mécanique des fluides de Toulouse

Using 15 LES cycles of a high load / low speed spark ignition engine operating point, two different fresh gases autoignition regimes called knock and super-knock are analyzed. A direct " a posteriori " analysis of pressure waves and autoignition heat release observed in LES is proposed. It reveals that low to moderate knock intensity, corresponding to late spark timings (ST) is characterized by one or several random autoignition (AI) spots which consume the surrounding fresh gases without coupling with the AI heat release. On the contrary , the highest knock intensities correspond to what is usually called super-knock, a very intense knock observed under pre-ignition conditions or for very early ST, as done in this study. LES shows that the pressure waves generated by one or a couple of AI spots are strong enough to induce locally a strong fresh gases temperature increase leading itself to a substantial decrease of the AI delay. This allows to generate a coupling between the pressure wave and the AI reaction rate which reinforce each other, leading to maximum pressures and propagation speeds close to those of a detonation. These results therefore strongly support the hypothesis proposed in the literature that super-knock is characterized by a deflagration to detonation transition (DDT). An " a priori " analysis is also performed thanks to the use of a local detonation indicator based on Bradley's DDT diagram. It is shown that this tool not only predicts

Mots clés: LES; Spark ignition; Engine; Knock; Super-knock; Deflagration to detonation transition DDT; Large eddy simulation; SI engines; Fuels; Auto ignition

Domaines: Sciences de l'ingénieur [physics]; Autre

Anthony Robert, Stéphane Richard, Olivier Colin, Thierry Poinsot. LES study of deflagration to detonation mechanisms in a downsized spark ignition engine. Combustion and Flame, 2015, 162 (7), pp.2788-2807. ⟨10.1016/j.combustflame.2015.04.010⟩. ⟨hal-01176439⟩