Numerical investigation on the combustion and emission characteristics of a heavy-duty natural gas-diesel dual-fuel engine

Abstract

Natural gas (NG)-diesel dual-fuel combustion is an effective approach to reduce soot and greenhouse gas emissions and mitigate the liquid fossil fuel crisis. In this work, a comprehensive numerical study on the combustion and emission characteristics of a NG-diesel dual-fuel engine operating at the high load condition was performed. Six significant parameters such as the start of injection (SOI) timing, exhaust gas recirculation (EGR), injection and intake pressures, nozzle number, and NG substitution ratio were investigated. A pathway to achieve highly efficient and clean combustion was proposed. It was demonstrated that at least an EGR rate of 40% should be employed to meet the NOx Euro VI regulation limit. Although a higher injection pressure enhanced the diesel-air mixing process and promoted engine efficiency, the highest achievable engine efficiencies were similar using different injection pressures. An elevated intake pressure with an earlier SOI timing promoted the oxidation process. Therefore, it resulted in a lower combustion loss and thus higher engine efficiency. Furthermore, the increase of nozzle number effectively promoted the air utilization rate and expedited the combustion heat release, which resulted in a higher engine efficiency but lower soot emission. But the growing trend of engine efficiency was limited and a nozzle number of 11 was found to be the optimal option. Finally, a peak engine efficiency of about 47.6% was achieved with a NG substitution ratio of 95% and an optimized SOI timing, and meanwhile, both the NOx and soot emissions were below the Euro VI emission regulation limits.