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

Abstract

International audienceUsing 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 predict

Engine hot spots: Modes of auto-ignition and reaction propagation

“…Following the earlier employment of the detonation peninsula [5][6][7][8][9][10], it has been used in a combined experimental engine and LES study to demonstrate how increasing spark advance, and the associated increasingly severe engine knock, cause the corresponding engine cycle loci to cross the threshold into the detonation peninsula [30].…”

Deflagrative, auto-ignitive, and detonative propagation regimes in engines

“…On the U/K diagram in [5], U is expressed in terms of both the Karlovitz stretch factor, K, which embodies the laminar burning velocity, ul, the integral length scale of the turbulence, l, the kinematic viscosity  , and also of the strain rate Markstein number, Masr, where…”

“…Other regimes of auto-ignitive combustion can also be indicated on the   / diagram, including those of benign controlled auto-ignition. Recently, Robert et al [5] have demonstrated how the diagram shows increasing the spark advance, increases the number of operational loci moving to within the peninsula, with increasingly severe engine knock.…”

Effect of Timing and Location of Hotspot on Super Knock during Pre-ignition

“…[40]. Formation of H 2 O 2 is observed at the flame front, which is a typical characteristics of hydrocarbon flames but the mass fraction of H 2 O 2 reveals that most of the end gas region has started reacting and undergoes decomposition phase, leading to knock as shown in Figure A3.…”