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

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.

KEYWORDS:

• Auto-ignition mode
• knocking intensity
• turbulence
• deflagration-to-detonation
• rapid compression machine

Acknowledgments

The authors would like to acknowledge the financial support provided by the National Natural Science Foundation of China (51706152, 52076149, 51825603) and Tianjin Natural Science Foundation (18JCQNJC07500).

Additional information

Funding

This work was supported by the National Natural Science Foundation of China [51706152,51825603,91641203]; Tianjin Natural Science Foundation [18JCQNJC07500].