Analysis of combustion cyclic variation in a lean burn spark-ignited engine using large eddy simulation

Abstract

The low-temperature combustion technique improves engine efficiency by reducing the thermal and exhaust losses. The representative method of low-temperature combustion technique is lean burn. However, lean burn has been limited because it increases the cycle-to-cycle variation (CCV) owing to poor initial ignition stability. To overcome the low initial ignition stability, increasing the spark energy intensity and strengthening the tumble motion have been proposed. Therefore, lean burn engines with ignition stability are being rapidly developed recently. In this study, the result of the LES simulation of a lean burn engine showed the importance of ignition delay increased more than that of a conventional gasoline engine. With the importance of ignition delay in lean burn condition, the sources of the ignition delay were analyzed. The primary ignition delay sources were the flow velocity near the spark plug and the subgrid-scale turbulent kinetic energy. Next, the parameters affecting the 5% mass fraction burned (MFB) duration were analyzed by comparing five characteristic cycles. Also, factors that decreased the correlation coefficient were analyzed by comparing with the outlying cycle. As a result of analyzing the cycle with a significant change in MFB 5 and MFB 50 ranking, the burn duration was found to be affected by curl TKE, SGS TKE, large-scale flow motion, and air–fuel ratio. Finally, methods to reduce CCV were proposed. The first method was to increase tumble X+ and tumble X- using pistons that enhance the wall flow. Consequently, the coefficient of variance (COV) during the ignition delay was reduced by 27%. The second method involved changing the injector target to enhance tumble Y. The enhanced tumble Y reduced the COV of burn duration by 11%.