Abstract Uncontrollable autoignition with engine knock has been the main obstacle for high thermal efficiency and low CO2 emissions in downsized engines. Therefore, it is necessary to study the autoignition and knocking characteristics and reveal the similarities/differences between different combustion modes. In this study, synchronization measurement was performed through simultaneous pressure acquisition and high-speed natural flame acquisition, and knocking experiments were comparatively conducted under spark-ignition (SI) and compression-ignition (CI) conditions in a high-strength optical engine. The CI experimental results show that early ignited fuel injection leads to advanced combustion phase thus concentrated heat release. The advanced combustion phase can mitigate cyclic variation at low energy density while induce knocking combustion at high energy density. The stochastic analysis shows that CI knocking intensity mainly depends on the combustion rate whereas SI knock intensity is more random because of the stochastic end-gas autoignition. Visualized combustion images show that compared to SI combustion, the burning rate of CI combustion is much higher due to the multipoint autoignition. At low energy density, there is no pressure oscillation of CI and SI and the main reason is the low value of peak heat release rate (HRR) regardless of the autoignition. Whereas at high energy density, there are obvious two-stage HRRs for the CI knocking combustion and the high second peak HRR results in the engine knock. Further flame comparison shows that the AI flame speeds in CI and SI modes are similar but much higher than traditional SI flame speed.

中文翻译：

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CI与SI燃烧模式下双燃料发动机自燃和爆震特性的光学研究

摘要 发动机爆震引起的不可控自燃一直是小型发动机实现高热效率和低CO2排放的主要障碍。因此，有必要研究自燃和爆震特性，揭示不同燃烧模式之间的异同。本研究通过同步压力采集和高速自然火焰采集进行同步测量，并在高强度光学发动机的火花点火（SI）和压缩点火（CI）条件下对比进行爆震实验。CI 实验结果表明，提前点火燃料喷射导致提前燃烧阶段，从而集中放热。提前燃烧阶段可以减轻低能量密度下的循环变化，同时在高能量密度下诱发爆震燃烧。随机分析表明，CI 爆震强度主要取决于燃烧率，而 SI 爆震强度由于随机尾气自燃而更加随机。可视化的燃烧图像表明，与 SI 燃烧相比，CI 燃烧的燃烧速率要高得多，因为它是多点自燃。在低能量密度下，CI 和 SI 没有压力振荡，主要原因是峰值放热率 (HRR) 值低，而与自燃无关。而在高能量密度下，CI爆震燃烧存在明显的两级HRR，高第二峰值HRR导致发动机爆震。