Optical Experiments on Auto-Ignition Modes in a Turbulent Field under Engine-Relevant Conditions

Abstract

ABSTRACT Thermal efficiency improvement of downsized spark-ignition engines is limited by the knocking issues, which are closely associated with auto-ignition modes. Previous studies are mainly conducted under quiescent or standard flow conditions, but the role of turbulence-chemistry interactions is not fully considered, motivating further fundamental investigations on auto-ignition modes. In this study, an optical experiment using a rapid compression machine that allows for turbulence effect was carried out, and the difference of auto-ignition modes between standard flow and turbulence conditions was comparatively investigated under engine-relevant conditions. The results show that under given target thermodynamic conditions, the overall temperature decrease caused by turbulence and cold-wall interactions reduces the reactivity of end-gas mixtures, inhibiting the transition of auto-ignition modes into developing detonation. Compared with standard flow scenarios, the propagation speed of auto-ignition reaction waves is much lower under turbulence conditions. Meanwhile, multiple secondary auto-ignition kernels occur sequentially in the unburned region, and normal combustion characteristics are observed under low thermodynamic conditions. However, further increases in thermodynamic conditions result in an intensive secondary auto-ignition, manifesting deflagration-to-detonation transition and thereby strong pressure oscillations. Besides, the correlations between knocking intensity, unburned mass fraction, and energy density indicate that the thermo-chemical-turbulent mechanism dominates the auto-ignition processes. The current studies shall give insights into the auto-ignition and turbulent combustion as well as knocking suppression under engine-relevant conditions.