Effects of piston shapes and swirl ratios on combustion and emissions of a micro diesel pilot-ignition natural gas engine

Abstract

To improve the combustion and emission performance of a micro diesel pilot-ignition (DPI) natural gas (NG) engine, four groups of piston shapes were designed and the effects of piston shapes and swirl ratios on the combustion process were discussed by computational fluid dynamics (CFD) coupled with the simplified chemical kinetic mechanism. Based on a calibrated CFD model, the effects of piston shape and swirl ratio on the combustion and emission characteristics of a DPI NG engine were investigated in detail, and some new findings were obtained. Both P_max and the peak heat release rate (HRR) increase first and then fall with the swirl ratio increasing from 0.5 to 3.0 in the four piston shapes. Additionally, the HRR has only one peak as the swirl ratio ≤1.0 but two peaks if the swirl ratio ≥1.5. When the piston shape changes from P0 to P3, P_max decreases gradually and the position of P_max advances from 14.7°CA to 11.3°CA. The ignition delay becomes shorter as the swirl ratio increases from 0.5 to 3.0. Additionally, it is observed that the ignition delay is the longest in P3, which has a larger opening size and shallower grooves, even at the same swirl ratio. Moreover, P3 leads to a decrease in NO_x emissions, with a maximum reduction of 1446 ppm. Except for P3, when the swirl ratio increases, both CH₄ and CO emissions show an increasing trend with a maximum increase of 298 ppm and 277 ppm, respectively. All these are helpful for the design and optimization of the combustion system of a DPI NG engine to meet stringent emission standards.