Wang, X. and Zhao, H., "Multi-Cycle Large Eddy Simulation (LES) of the Cycle-to-Cycle Variation (CCV) of Spark Ignition (SI) - Controlled Auto-Ignition (CAI) Hybrid Combustion in a Gasoline Engine," SAE Technical Paper 2017-01-2261, 2017.
The spark ignition (SI) - controlled auto-ignition (CAI) hybrid combustion, also known as spark-assisted compression ignition (SACI), is achieved by utilizing the temperature and pressure rise from the early flame propagation induced by the spark-ignition to trigger the auto-ignition of the remaining unburned mixture. This hybrid combustion concept can be used to effectively extend the operating range of gasoline CAI combustion and achieve smooth transitions between SI and CAI combustion mode in gasoline engines. However, the significant cycle-to-cycle variation (CCV) of the SI-CAI hybrid combustion hinders the practical application of the hybrid combustion. In order to understand the cause of its high CCVs, the SI-CAI hybrid combustion process in a gasoline engine was studied in this study by the large eddy simulations (LES). The turbulence is modelled by the sub-grid k model. The spark ignition and subsequent flame propagation were modelled by the ECFM-3Z LES model. A tabulated database of the gasoline auto-ignition chemistry was coupled with the CFD simulations to depict the subsequent auto-ignition process of the unburned mixture after the initiation of flame propagation. The LES simulation was validated and applied to analyze the hybrid combustion process in a single cylinder engine at 1500 rpm and 5.43 bar IMEP, which was characterized with a coefficient of variation (COV) of 11.81% in IMEP. The LES simulations of 15 consecutive cycles were performed and analyzed to evaluate the potential of LES simulations to predict the CCV of SI-CAI hybrid combustion. The analysis of the LES simulation results indicates that the average thermal and compositional parameters are not the main reason for the cycle-to-cycle variations of the SI-CAI hybrid combustion. The temperature and residual gas fraction (RGF) in the spark zone is also very stable among different cycles. In comparison, the average velocity in the whole cylinder reduces from 7.8 m/s in the strong combustion cycle (Cycle 11) to 6.4 m/s in the weakest combustion cycle (Cycle 14) with 21.9% reduction, and the average velocity in the spark zone reduces from 6.3 m/s to 3.8 m/s with 60.3% reduction. Therefore, the variations of the in-cylinder flow velocity, especially around the spark plug, could be the main reason for the large variations of the hybrid combustion observed in the experiments.