Study on reducing carbon dioxide and harmful emissions of diesel-ignited natural gas engine

Natural gas (NG) is a clean and low-carbon fuel and the NG engine is one of the main measures used by the public transportation industry to achieve carbon peak targets. However, NG engines have problems, such as ignition difficulty and low thermal efficiency. The use of diesel to ignite NG is an effective solution to these problems; however, this solution will increase CO₂ emissions compared with NG engines. In this study, the potential of reducing CO₂ emissions from a diesel-pilot-ignited (DPI) NG engine by optimizing the injection parameters (injection pressure and injection timing) under different loads is studied through experiments. Furthermore, the formation mechanism of CO₂ in combination with chemical kinetics is analyzed. The results show that combustion in the DPI mode presents an obvious two-stage heat release and its CO₂ emission is 17.15% lower than that of pure diesel combustion (PDC). Under high-load conditions, as the diesel injection pressure increase, the THC and NO_X emissions emissions decrease by 67.53% and 84.32%. As the diesel injection timing (DIT) advances, the NO_X emissions increase from 1.84 to 22.96 g/kW·h respectively. According to the analysis of the chemical kinetic mechanism, the formation of CO2 in DPI mode is primarily through the reaction of CO + O₂ = CO₂ + O, whereas in conventional diesel combustion (CDC) mode, CO₂ is formed through the reaction of CO + OH = CO₂ + H. Within the range of -18 to -5°CA ATDC DIT, increasing the diesel injection pressure (DIP) or advancing the DIT can improve the thermal efficiency of DPI NG engines and reduce CO₂ emissions.