Research On The Mechanism Of Auto-ignition And Knock Under The Combustion Environment With High Temperature/Pressure And High Degree Of Constant Volume

Improving engine thermal efficiency helps in reducing fossil fuel consumption,which is crucial for achieving carbon neutrality in the transportation industry.However,further improvements in engine thermal efficiency are severely limited by engine knock whose essence has been proven to be uncontrolled auto-ignition of the mixture.Despite extensive research,so far,the auto-ignition behavior and its underlying initiation mechanism during the knock process have not yet been fully revealed,and the knockcontrolling methods require further investigation.In this context,this paper mainly focuses on the auto-ignition and knock behaviors of the mixture under high temperature,high pressure,and high degree of constant volume combustion conditions.The fundamental auto-ignition behavior,knock occurrence mechanism,and knock-controlling methods were comprehensively investigated using a rapid compression machine.In terms of mixture auto-ignition behavior,a fully-visible combustion system with dual optical windows was constructed based on a rapid compression machine to uncover the controlling factors of auto-ignition for primary reference fuels from various perspectives.It was found that low fuel activity and high turbulence intensity can promote weak ignition tendency while the ignition delay time can be deemed as the pivotal factor determining ignition mode.Under vortex-minimized conditions,auto-ignition displays a typical "near-wall initiation,off-wall propagation" characteristic and such a feature is universal in a wide range of thermodynamic test conditions spanning the negative temperature coefficient region and multi-stage ignition of primary reference fuels.Under vortex-existing conditions,with the interaction of the local flow field and fuel activity,the burned mass fraction at the instant of auto-ignition is higher than that under vortexminimized conditions and the burnt and unburnt regions are distributed in an enclosed layout,resulting in the reduction in auto-ignition energy release and thus lower knock intensity.Regarding the knock mechanism,a three-dimensional and high-resolution spatiotemporal analysis of the entire conventional/super knock process of iso-octane was conducted.A reverse trend in knock intensity was observed when the energy density was lowered across the negative temperature coefficient region.Two types of iso-octane detonation initiation modes were identified,and the degree of low-temperature chemistry involvement in auto-ignition process was found to be the key factor in determining the detonation mode.When the low-temperature chemical participation intensity is high,the detonation process includes two stages consisting of large-area auto-ignition and the final detonation initiation.Conversely,only a single-stage characteristic of direct detonation initiation can be observed otherwise.The energy density required for single-stage detonation initiation is much lower than that for two-stage detonation initiation.Under the two-stage detonation mode,the thermodynamic uplift caused by the first-stage autoignition is the key to determining the final detonation.The thermal diffusion rate can be utilized to differentiate the auto-ignition mode,where a lower thermal diffusion rate is favorable for the formation of temperature gradients suitable for detonation initiation.In terms of knock control,the relationship between knock intensity,burning velocity,and the burned mass fraction was comprehensively revealed.With the increasing burning velocity,the knock intensity first decreases and then increases while the burned mass fraction exhibits the opposite trend.Intermediate burning rates were found to obtain a favorable balance between high degree of constant volume combustion and noticeable knock suppression effects across a wide thermodynamic condition.The feasibility of achieving high degree of constant volume combustion and non-knock combustion using jet ignition was demonstrated on a rapid compression machine,and the basic organizational principle for efficient jet combustion was proposed.Jet configurations that separate the unburned region from the wall by enclosing it with jet streams and with intermediate jet-flow cross-section,intermediate jet hole number,and multi-porous jet beams should be used to achieve optimal knock suppression effect while obtaining high burning velocity.