Knocking combustion experiments were conducted in this study using a test engine that allowed the entire bore area to be visualized. The purpose was to make clear the detailed characteristics of knocking combustion that occurs accompanied by cylinder pressure oscillations when a Homogeneous Charge Compression Ignition (HCCI) engine is operated at high loads. Knocking combustion was intentionally induced by varying the main combustion period and engine speed. Under such conditions, knocking in HCCI combustion was investigated in detail on the basis of cylinder pressure analysis, high-speed photography of the combustion flame and spectroscopic measurement of flame light emissions. The results revealed that locally occurring autoignition took place rapidly at multiple locations in the cylinder when knocking combustion occurred. In that process, the unburned end gas subsequently underwent even more rapid autoignition, giving rise to cylinder pressure oscillations. In addition, when the engine speed and main combustion period were varied, it was found that the intensity of the cylinder pressure oscillations, i.e., knocking intensity PKI, correlated strongly with the maximum pressure rise rate per unit time dP/dtmax [MPa/msec]. The frequency characteristics of the knocking were the same even when the engine speed was varied. The primary vibration mode was the (1,0) mode in which pressure waves traverse the cylinder in the bore direction. Under operating conditions causing strong pressure oscillations, pressure oscillations at higher frequencies were observed.
