Pressure oscillation often occurs in high-load homogeneous charge compression ignition (HCCI) combustion, which is a challenge in the development of HCCI engines for automobiles. This work proposes a novel method of reducing the pressure oscillation in HCCI combustion at high loads. The proposed technique injects air into homogeneous mixtures before compression, thereby giving local fuel concentration gradient. The fuel concentration gradient is expected to suppress a rapid pressure rise, resulting in reduced pressure oscillation. High-load HCCI combustion was simulated via a rapid compression machine with a high compression ratio. Varying the period from air injection to compression, that is, the waiting time, controlled the magnitude of fuel concentration gradient. The pressure oscillation was quantified and evaluated via the knock intensity (KI) and the averaged pressure rise rate. For the short waiting time; in other words, when the local fuel concentration gradient was large, the KI was very lower than that for no air injection. The KI, however, increased with the waiting time to approach that for no air injection. The oscillation modes were also different with and without air injection according to a modal analysis. The in-cylinder temperature distribution was visualized via the infrared radiometry to better understand the effect of air injection. For no air injection, the temperature in the cylinder uniformly increased, and the whole mixtures were ignited instantaneously. With air injection and for the short waiting time, on the other hand, hot spots developed on the rim of the injected air where the specific heat ratio was higher and then gradually spread throughout the chamber. Therefore, retarded auto-ignition and subsequently slow spread would limit a rapid pressure rise, resulting in reduced pressure oscillation in HCCI combustion. In conclusion, the proposed technique is effective for reducing the pressure oscillation in high-load HCCI combustion only for the short waiting time.

在高负荷均质压燃（HCCI）燃烧中经常发生压力振荡，这是汽车用HCCI发动机开发的一个挑战。这项工作提出了一种减少高负荷下 HCCI 燃烧中压力波动的新方法。所提出的技术在压缩前将空气注入均质混合物中，从而产生局部燃料浓度梯度。燃料浓度梯度有望抑制压力的快速上升，从而减少压力振荡。通过具有高压缩比的快速压缩机模拟高负荷 HCCI 燃烧。改变从空气喷射到压缩的时间，即等待时间，控制了燃料浓度梯度的大小。通过爆震强度 (KI) 和平均压力上升率对压力振荡进行量化和评估。等待时间短；换言之，当局部燃油浓度梯度较大时，KI 比不喷气时非常低。然而，KI 会随着等待时间而增加，以接近没有空气注入的情况。根据模态分析，有和没有空气注入的振荡模式也不同。通过红外辐射计可视化缸内温度分布，以更好地了解空气喷射的影响。不喷气时，气缸内温度均匀升高，整个混合气瞬间点燃。另一方面，通过空气注入和较短的等待时间，在比热比较高的喷射空气边缘形成热点，然后逐渐扩散到整个腔室。因此，延迟的自动点火和随后的缓慢扩散将限制压力的快速上升，从而导致 HCCI 燃烧中的压力波动减小。总之，所提出的技术仅在较短的等待时间内有效地减少了高负荷 HCCI 燃烧中的压力振荡。