2015 Volume 57 Issue 180 Pages 142-149
Homogeneous Charge Compression Ignition (HCCI) engines are being widely researched today, having attracted considerable interest for their low emissions and high efficiency. However, HCCI engines have a narrow range of stable operation owing to the occurrence of extremely rapid combustion at high loads and misfiring at low loads. Extremely rapid combustion at high loads is an especially large factor preventing expansion of the stable operating region. It is also known that abnormal combustion accompanied by in-cylinder pressure oscillations resembling those of knocking in a spark-ignition engine occurs in HCCI engines as well depending on the operating conditions. The purpose of this study was to identify the characteristics of pressure oscillations due to knocking in an HCCI combustion system. Using a two-stroke single-cylinder engine, pressure oscillations were investigated in detail on the basis of in-cylinder visualization/imaging of the combustion flame across the entire bore area, frequency analysis of the in-cylinder pressure waveform, and spectroscopic measurements. The results revealed that the maximum pressure rise rate, dP/dtmax, increased as the ignition timing advanced and the equivalence ratio increased and that in-cylinder pressure oscillations occurred under a condition of dP/dtmax higher than approximately 7 MPa/ms. The visualization results showed that HCCI combustion accompanied by in-cylinder pressure oscillations occurred in the latter stage of the combustion process due to rapid autoignition of the unburned end gas. Moreover, under a condition of a high equivalence ratio, a highly brilliant autoignited flame occurred over a wider area of the combustion chamber. The power spectrum of the in-cylinder pressure oscillations indicated that they possessed unique frequency components in high-frequency bands, in addition to the frequency components around 7 kHz.
