Time-resolved OH-PLIF measurements of early flame propagation in an optically-accessible SI IC engine operated in the lean condition
Lean combustion has been considered as one of the critical technologies to decrease the heat loss of spark-ignition engines. However, under the lean operation, the low laminar flame speed provides a longer combustion duration, and it becomes an obstacle to improving thermal efficiency. In the recent engines, the high turbulence generation is applied to enhance the turbulent flame speed and to reduce the combustion period. Although the interaction of the turbulent characteristics and the flame structure has been a hot topic in the combustion research field, only a few studies observed the flame structure in the actual engine. This study discusses the flame propagation in the actual engine and elucidates the relationships between the crank angle history of the heat release, the flame propagation speed, and flame surface structure. For the purpose, a high-tumble optically-accessible SI IC engine operating at lean (excess air ratio (λ) of 2.0) and stoichiometric conditions were diagnosed by time-resolved OH-PLIF measurements. The engine was operated 2000 rpm with a port fuel injection of isooctane. The cross-sectional imaging of OH distribution was achieved at 12 kHz resulting in a temporal resolution of 1? crank angle at 2000 rpm. This allows the investigation of the temporal evolution of the ignition and flame propagation process. The results show that the OH radical is generated without any heat release by the spark discharge during approximately 15 ? crank angle after ignition timing in the ultra-lean condition, and then subsequent flame propagation occurs. In the flame propagation process, fractal analysis shows that fractal dimension has around the same values of the previous researches, even though this measurement was conducted in λ=2.0 condition. This suggests that the conventional theory can be used in the ultra-lean condition in the SI IC engine.