Effect of hydrogen enrichment on the flame propagation, emissions formation and energy balance of the natural gas spark ignition engine

•The initial spark kernel formation, growth and development were presented in the NG SI engine.•The relationship between exhaust emissions and mediate species was analyzed in the NG SI engine.•The methane oxidation process, intermediates species and reaction pathways were analyzed.•The energy balance of the NG SI engine fuelled with/without hydrogen were compared. Technically, using hydrogen in the natural gas (NG) will increase the combustion rate of the spark ignition (SI) engine, particularly at the lean burn condition, which also benefits to improve the thermal efficiency and combustion efficiency. The aim of this study was to investigate the effect of hydrogen addition on the flame propagation, emissions formation, energy balance and the relationships between exhaust emissions and mediate species during the flame propagation in the natural gas (NG) SI engine. The 1D and 3D simulation models of the NG SI engine were built and validated against with experimental data. The results indicated that the initial spark kernel volume of the NG SI engine with hydrogen addition was larger than that of without hydrogen enriched. The NO concentration increased with the hydrogen due to its higher peak combustion temperature. The NO emission mainly generated inside the flame front, where located in the high temperature combustion zone. In addition, the CO formation rate of the NG SI engine with the hydrogen addition was faster and oxidized more completely. With adding hydrogen in natural gas, the amount of H and OH were significantly increased due to the chemical amplifiers, thereby enhancing the radical pool. The HC and soot emissions of the NG SI engine were much lower with the hydrogen addition. Furthermore, the energy efficiency and fuel economy of the NG SI engine were improved with the hydrogen addition, particularly at the lean mixture conditions.