Study Of Knock Characteristics And Suppression Strategies Based On Spark Assisted Compression Ignition (SACI)

Spark assisted compression combustion(SACI)is considered as an effective means to solve the load problem and combustion phase controlled problem of gasoline compression combustion mode.At the same time,compared with the traditional spark ignition(SI)combustion mode,SACI improves the thermal efficiency and becomes a highly practical and efficient combustion technology for gasoline engines.However,SACI knock limits the further improvement of load and thermal efficiency.Therefore,this work studied the basic combustion and knocking characteristics of SACI based on a SI engine with high compression ratio.In addition,the EGR strategy,the injection strategy,the Miller cycle strategy and the combined strategies were studied to suppress SACI knock and promote more efficient combustion,providing great significance for the development and application of SACI engine.Firstly,the basic combustion characteristics and control methods of SACI were studied.The results indicated that the flame propagation ratio and auto-ignition ratio in SACI combustion process can be well controlled through changing the ignition timing and i-EGR.When the ignition is significantly advanced,the SACI knock will occurred.When the ignition is too delayed,the combustion mode will transition from SACI to SI.Additionally,i-EGR control SACI combustion process by controlling the initial temperature and changing the gas composition in the combustion chamber.Compared with the SI mode,the auto-ignition stage of SACI stable combustion condition is controllable.However,too large auto-ignition ratio in combustion process will cause the SACI knock,with an excessive peak of heat release rateand strong pressure oscillation.In addition,the influence of EGR,split injection,Miller cycle and combined strategies on SACI knock and combustion characteristics was studied in this work to realize SACI knock suppression.The results showed that: The e-EGR reduces the in-cylinder temperature and increases the heat capacity of mixture,thus reducing the proportion of compression ignition and realizing the suppression of SACI knock.The effects of split injection on knock suppression are affected by the initial in-cylinder temperature and the level of fuel stratification.When initial in-cylinder temperature is high with large i-EGR ratio,the knock suppression effect first increases and then decreases as the retarded second injection timing.When the initial in-cylinder temperature is low with little i-EGR,the knock suppression effect gradually increases as the retarded second injection timing.In addition,the coupling strategy that combines e-EGR and split injection demonstrates the best power performance and fuel economy,with BMEP increasing by approximately 0.10 MPa and BSFC decreasing by approximately 26.78 g/kW·h relative to the baseline case.When the Miller cycle strategy is adopted,the negative valve overlap can be adjusted to ensure the dynamic performance and the significant impact on knock suppression of SACI mode.With the increase level of Miller cycle,combustion mode will transition from SACI to SI,and BMEP will increase firstly and then decrease.When Miller cycle and split injection are combined,the BMEP increased by approximately 0.08 MPa and BSFC decreased by approximately 22.08 g/kW·h relative to the baseline case.