Influences of natural gas energy fraction on combustion and emission characteristics of a diesel pilot ignition natural gas engine based on a reduced chemical kinetic model

Abstract

In this research, a reduced chemical kinetic mechanism was coupled in computational fluid dynamics (CFD) model to study the detailed influence mechanism of natural gas (NG) energy fraction on the diesel pilot ignition (DPI) NG engines. Based on the CFD model calibrated by experimental data, the impacts of NG energy fraction on the combustion and emission generation processes in DPI NG engine were investigated. The results show that, the start of combustion (SOC) in DPI NG engine is advanced as the NG energy fraction declines and the maximum advance comes up to 4.1°CA at 1000 rpm and 100% load. The deep analysis reveals that the positive influence of R342 on the temperature is the main reason for the shorter ignition delay. The 10–90% combustion duration increases first as the NG energy fraction changes from 50% to 60%, and then decreases if the NG energy fraction further increases. Furthermore, the combustion mode is changed from premixed combustion to non-premixed combustion with the NG energy fraction decreasing. For the emissions, the NOx emission drops with the decline of NG energy fraction, and the maximum decrease of NOx emission comes up to 0.96 mg/kJ at 1000 rpm and 100% load. The CO emission increases first and then decreases as the NG energy fraction rises from 50% to 90%.