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.