Dilution Boundary Expansion Mechanism of SI-CAI Hybrid Combustion Based on Micro Flame Ignition Strategy

In decade years, Spark Ignition-Controlled Auto Ignition (SI-CAI) hybrid combustion, also called Spark Assisted Compression Ignition (SACI) has shown its high-efficiency and low emissions advantages. However, high dilution causes the problem of unstable initial ignition and flame propagation, which leads to high cyclic variation of heat release and IMEP. The instability of SI-CAI hybrid combustion limits its dilution degree and its ability to improve the thermal efficiency. In order to solve instability problems and expand the dilution boundary of hybrid combustion, micro flame ignition (MFI) strategy is applied in gasoline hybrid combustion engines. Small amount of Dimethyl Ether (DME) chosen as the ignition fuel is injected into cylinder to form micro flame kernel, which can stabilize the ignition combustion process. Experimental results of the single cylinder engine showed that MFI strategy raises the dilution boundary from lambda 1.2 to 2.0 and thermal efficiency from 33.65% to 42.87% at 2000 rpm and IMEP 4 bar. Computational Fluid Dynamics (CFD) numerical simulation analysis indicate that the early injection case showed a 2-stage heat release process with one peak curves, like traditional SI-CAI hybrid combustion. With the delay of DME injection timing, the heat release process converted to 3-stage characteristic with two peaks. The early-injection DME can improve the fuel activity in cylinder and the flame propagation process, but it still need spark source to start the flame propagation process. The late-injection DME regarded as high-energy ignition kernel which can improve the initial ignition process and the accelerate initial heat release rate at diluted conditions. After the fusion of the flame propagation triggered by spark and local DME auto-ignition, the overall heat release feature still remained flame propagation until the presence of new auto-ignition points in the high concentration of PRF93.The dual injection strategy combined the advantages of early and late injection strategy though both of them had been weakened, and its feature was close to the late injection case. With the reasonable design of MFI, the different injection strategy could be applied to realize and control the hybrid combustion at high dilution conditions.