Experimental and computational study on the effects of exhaust gas recirculation on thermodynamics, combustion and emission characteristics of a diesel pilot ignition natural gas engine

This study investigates the impact of diesel pilot ignition (DPI) natural gas (NG) engines on combustion and emission characteristics across various exhaust gas recirculation (EGR) volumes. It utilizes a three-dimensional computational fluid dynamics (CFD) model coupled with a simplified chemical kinetic model. Experimental results guide simulation calculations under three distinct operational conditions. Visual analysis of the calculation outcomes presents the combustion process and emission characteristics in the engine, elucidating the influence mechanism in relation to EGR volume rates. The simulation results show that, with the rise of EGR rates, the peak in-cylinder pressure decreases by 15 bar, the heat release rate (HRR) shoots up later, and the maximum difference of CA50 is less than 2.7 °CA. The variation trends of CA90, 50–90% and 10–90% combustion durations exhibit similarity. When the rate of EGR volume is below 20%, the CA90, 50–90%, and 10–90% combustion durations lengthen as the rate of EGR volume increases. When the rate of EGR volume exceeds 20%, NOx emissions remain at a low level, staying below 500 ppm. Concurrently, as the rate of EGR volume increases from 5% to 30%, there is a corresponding rise in unburned methane emissions, with the maximum surge observed from 343 ppm to 21 021 ppm. Additionally, CO emissions increase as the rate of EGR volume increases, reaching 989 ppm in case 3. While in case 2, there is an initial ascent to 1381 ppm, followed by a decline to 1148 ppm, and ultimately, a subsequent rise.