Two premixed compression ignition modes for low octane gasoline are numerically investigated. The multiple premixed compression ignition (MPCI) mode is featured with a sequence of “spray- combustion- spray- combustion”, while the partially premixed compression ignition (PPCI) mode is a sequence of “spray- spray- combustion”. This paper compares the combustion process of the two modes using multi-dimensional CFD code, KIVA-3v, which can perform chemical reaction calculations for different fuels by a discrete multiple component (DMC) method. The fuel used for simulation consists of 58.5% i-C8H18 and 41.5% n-C7H16 in volume, and has the same RON and similar physical properties to straight-run naphtha used in the experiment. The engine operating condition is fixed at a 1600rpm and 0.7 MPa IMEP. The injection strategies for these two modes are different. All of the parameters in the simulation come from the single cylinder engine experiments.
The simulation results show good agreement with the experimental results. For the MPCI combustion, fuel injected at different stage burn at different time and region in the cylinder. Therefore, in-cylinder air can be better utilized than that of the PPCI mode. By analyzing the temperature distribution, it is found that the first combustion of the MPCI mode is premixed low temperature combustion, and the second combustion starting after the end of the second injection is close to premixed combustion. Due to this special combustion characteristic, low soot and NOx emissions are realized simultaneously.