喷油策略对分层火焰引燃混合燃烧影响的研究

摘要

通过一台单缸汽油机,采用进气道喷射(PFI)和缸内直喷(DI)实现燃油分层,结合火花点火实现分层火焰引燃(SFI)混合燃烧,探索了点火时刻、直喷比例和直喷时刻对SFI混合燃烧特性的影响.结果表明:点火时刻直接影响前期火焰传播放热过程,进而实现对后期放热过程的有效控制,而直喷比例和直喷时刻则直接影响缸内燃油分布,进而影响SFI燃烧特性.点火时刻提前,使火焰传播阶段的平均放热速率增加,自燃始点提前;直喷比例增加,可以提升前期火焰传播阶段的平均放热速率,但是后期自燃阶段的平均放热速率呈先增加后减小趋势;直喷时刻为上止点前60°CA时,火焰传播阶段和自燃阶段的平均放热速率都达到最高,但此时的传热损失也最大.通过优化直喷比例和直喷时刻,可以实现燃油经济性的提升,在转速为1 500 r/min、平均指示压力(IMEP)约为0.7 MPa工况下,优化后的SFI混合燃烧相比于纯均质混合气稀释燃烧,指示热效率可以提升7.4％.%The SFI(stratified flame ignition) hybrid combustion was achieved in a single cylinder gasoline engine through port fuel injection (PFI) and direct injection (DI) combined with spark ignition.Then,the effect of spark timing,mass ratio of fuel directly injected (DI_Ratio)and start of DI timing (SOI)was investigated in the engine.The results indicate that spark timing directly influences the early exothermic and flame propagation process.As a result,it can be used to control the following auto-ignition and heat release process effectively.On the other hand,DI_Ratio and SOI influence SFI hybrid combustion characteristics through altering the in-cylinder fuel distribution.As the spark timing is advanced,the heat release rate in the early stage of flame propagation increases,and then the start of autoignition is advanced.As the DI_Ratio is increased,the average heat release rate in the early stage of flame propagation increases gradually,while the average heat release rate in the latter auto-ignition process increases first but then it decreases.When the SO1 is 60° CA before the top dead center (BTDC),the average heat release rate in both the early flame propagation process and the latter auto-ignition process reaches its maximum value,while the heat loss reaches the highest value as well.The fuel consumption can be reduced by optimizing the DI_Ratio and SOI.The net indicated efficiency of optimized SFI hybrid combustion is found to be 7.4％ higher than homogeneous charge diluted combustion at the speed of 1 500 r/min and the IMEP of 0.7 MPa.