Optimization of the direct injection natural gas engine under different combustion modes

The objective of this study is to optimize the performance of a direct injection natural gas engine by using the newly developed many-objectives optimization genetic algorithm NSGA-III. A total number of 11 parameters are optimized simultaneously with respected to four optimization objectives, including the ISFC, NO, CH4 and Soot emissions. The main findings are as follows: More than half of the calculated cases in the Pareto front can meet the EURO V emission regulation. In addition, a larger EGR rate is needed in order to achieve a lower NO emission. The optimized natural gas spray angles are around 80 degrees and 70 degrees; the diesel spray angles are around 60 degrees, 70 degrees and 75 degrees; the EGR rates are around 28% and 14%; the swirl ratios are around 1.6 and 0.5; the compression ratios are around 16.5 and 18.5; the azimuth angles are around -3 degrees and 5 degrees. The combustion modes of the optimized cases can be divided into three categories based on the difference between the natural gas and diesel injection timings. The high-temperature combustion regions in the NG PPCI (Partial Premixed Compression Ignition) combustion mode with open combustion chamber are widely distributed, which result in an obviously higher NO emission than the other combustion modes. However, its Soot emission is the lowest. For diesel PPCI and HPDI (High Pressure Direct Injection) combustion mode, the natural gas is mainly consumed through diffusion combustion. Therefore, the Soot emissions are relatively higher. However, as the ignition timing is postponed, their NO emissions are relatively lower.