An experimental investigation on pre-ignition phenomena: Emphasis on the role of turbulence

Abstract

Pre-ignition is an undesirable ignition event that affects chemical kinetic measurements in chemical reactors. Meanwhile, it appears randomly in engineering systems and is highly relevant to the soft knock or much stronger and detrimental super-knock in modern downsized engines. Currently its origins are still not fully understood. In this study, the role of turbulence in pre-ignition phenomena was experimentally investigated using a novel rapid compression machine. Different turbulent flow fields were achieved through calibrated orifice plates. Stoichiometric isooctane/air mixtures were tested under engine-relevant conditions in a target pressure range of 15–50 bar and a temperature range of 720–860 K. Useful insights into pre-ignition mechanism were obtained by combining instantaneous pressure acquisition with simultaneously recorded high-speed imaging. The experimental results demonstrate that owning to turbulent mixing with colder boundary layers, ignition timing is delayed when compared to ideal homogeneous compression ignition scenarios. However, pre-ignition phenomena can still be observed and become pronounced at lower target pressures with longer ignition delays. Moreover, pre-ignition formation can be characterized by single or multiple spherical flame kernels, distributed discretely inside core mixture or at near-wall regions. Different from the auto-ignition scenarios dominated by the chemical reactivity of test mixture, these pre-ignition flame kernels feature standard deflagration propagation. Finally, a dimensionless scaling analysis shows that pre-ignition formation is closely associated with turbulent length scale and laminar flame thickness.