Effects of miller cycle strategies on combustion characteristics and knock resistance in a spark assisted compression ignition (SACI) engine

Spark assisted compression ignition (SACI) is a proven strategy to extend the operation load range and control the combustion phase of homogeneous charge compression ignition (HCCI) with high thermal efficiency. However, the occurrence of abnormal combustion, such as engine knock, limits SACI to operate in a wider load range. In this paper, the effects of Miller cycle on knock suppression effect and combustion characteristics are experimentally studied using a single-cylinder gasoline engine. Two types of Miller cycle tests under different boundary conditions are conducted: one is Miller cycle with constant negative valve overlap (NVO), and the other is Miller cycle with variable NVO while maintaining intake charge. The results show that different Miller cycle strategies can be used to adjust the ratio of fuel consumed by flame propagation and auto-ignition, and it is effective to optimize the combustion process in SACI to suppress engine knock while maintaining high efficiency. The results show that thermal efficiency of the optimized condition increased by 2.75% compared to the baseline. Besides, split injection strategy is also studied and the results show that a proper second injection timing (180-100 CAD BTDC) could moderate engine knock. •Combustion characteristics and knock suppression mechanism in SACI are studied.•Miller cycle could suppress engine knock by optimizing combustion process.•The thermal efficiency of Miller cycle with constant intake volume is improved.•Coupling Miller cycle and split injection could help mitigating engine knock.