Strict regulations on fuel economy are driving modern gasoline engines to adopt more advanced technologies to improve thermal efficiency. Some of the technologies, for example ultra-high compression ratio and spark assisted compression ignition, will elevate the thermodynamic condition near top dead center (TDC) to a considerably high level, even up to or beyond the negative temperature coefficient (NTC) region. This will definitely increase knock tendency when the combustion is not well controlled. Previous knock-related research mainly focused on temperature ranges in/below the NTC region, while the knock combustion beyond the NTC region has rarely been studied. To understand the knock behavior beyond the NTC region, in this study the flame propagation process and end-gas auto-ignition of iso-octane under wide thermodynamic conditions across the NTC region were optically studied using dual-camera photography. The results showed that the flame propagation speed increased with increasing initial temperature and decreasing initial pressure, exhibiting no NTC characteristic. With the intervention of flame propagation, the residence time of the end-gas was shortened as the initial thermodynamic conditions were promoted, indicating no NTC behavior in the overall ignition delay time of the end-gas. Two kinds of detonation initiation processes were identified. In the cases strongly affected by low temperature chemistry (LTC), the auto-ignition showed a two-stage characteristic during which a widespread but relatively weak auto-ignition (first-stage) was observed prior to the final detonation initiation. In contrast, when the LTC was absent, the detonation was initiated directly in a single auto-ignition event. Lower initial energy densities were needed to initiate detonation in the cases less affected by LTC. Thermodynamic analyses based on Bradley's ε-ξ diagram showed that, for the LTC-affected cases, the pressure rise which resulted from the widespread weak first-stage auto-ignition had vital impacts on the final detonation initiation by shifting the ε-ξ location into or away from the detonation region. Finally, thermal diffusivity was demonstrated to be capable of distinguishing detonation from other combustion modes as detonation tended to occur with lower thermal diffusivities of the mixture.

严格的燃油经济性法规正在推动现代汽油发动机采用更先进的技术来提高热效率。一些技术，例如超高压缩比和火花辅助压缩点火，会将上止点 (TDC) 附近的热力学条件提升到相当高的水平，甚至达到或超过负温度系数 (NTC) 区域。当燃烧没有得到很好的控制时，这肯定会增加爆震倾向。以前的爆震相关研究主要集中在 NTC 区域内/以下的温度范围，而 NTC 区域以外的爆震燃烧研究很少。要了解 NTC 区域以外的爆震行为，在这项研究中，使用双相机摄影对异辛烷在整个 NTC 区域的宽热力学条件下的火焰传播过程和尾气自燃进行了光学研究。结果表明，火焰传播速度随着初始温度的升高和初始压力的降低而增加，不表现出NTC特性。在火焰传播的干预下，随着初始热力学条件的提高，尾气的停留时间缩短，表明尾气的整体点火延迟时间没有 NTC 行为。确定了两种起爆过程。在受低温化学 (LTC) 强烈影响的情况下，自燃表现出两阶段特征，在此期间，在最终起爆之前观察到广泛但相对较弱的自燃（第一阶段）。相比之下，当 LTC 不存在时，引爆是在单个自燃事件中直接引发的。在受 LTC 影响较小的情况下，需要较低的初始能量密度来引发爆炸。基于 Bradley 的热力学分析ε - ξ图表明，对于受 LTC 影响的情况，通过将ε - ξ位置移入或远离引爆，广泛的弱第一级自燃引起的压力升高对最终起爆产生重要影响地区。最后，证明了热扩散率能够将爆震与其他燃烧模式区分开来，因为爆震往往在混合物的热扩散率较低的情况下发生。