Analysis of the Correlation between Flow and Combustion Characteristics in Spark-Ignited Engine 2021-01-0463
As global emission standards are becoming more stringent, it is necessary to increase thermal efficiency through the high compression ratio in spark-ignited engines. Various studies are being conducted to mitigate knocking caused by an increased compression ratio, which requires an understanding of the combustion phenomena inside the combustion chamber. In particular, the in-cylinder flow is a major factor affecting the entire combustion process from the generation to the propagation of flames. In the field of spark-ignited engine research, where interest in the concept of lean combustion and the expansion of the EGR supply is increasing, flow analysis is essential to ensure a rapid flame propagation speed and stable combustion process. In this study, the flow around the spark plug was measured by the Laser Doppler Velocimetry system, and the correlation with combustion in spark-ignited engines was analyzed. The turbulence intensity at the spark plug was calculated by measuring the flow velocity by using a probe adaptor installed in place of the spark plugs under motoring conditions. To measure the flow characteristics of various engine conditions, LDV measurement experiments were conducted with different engine speeds, intake pressures and valve timings. In addition, the design of the intake port was changed to analyze the flow characteristics according to the tumble ratio. Compared to other engine variables, the engine speed and intake valve timing have greater influence on the flow around the spark plug. In engines with different tumble ratios, the profile of the flow velocity is different. To identify the relationship between the flow characteristics and the combustion process, combustion under various conditions was compared to the flow measured with the LDV system in the same engines. Under part load conditions, there was a lack of correlation between the turbulence velocity near the spark plug and the burn duration, however, the initial flame propagation speed showed a high correlation.