Study of cycle-by-cycle variations of direct-injection combustion fueled with natural gas/hydrogen blends using a constant volume bomb

Cycle-by-cycle variations of direct-injection combustion fueled with natural gas/hydrogen blends were experimentally studied using a constant volume bomb. Flame propagating photographs and pressure history were recorded at various hydrogen volumetric fractions (from 0% to 40%) under the overall equivalence ratios of 0.6 and 0.8, respectively. The effect of fuel-air mixture stratification and hydrogen addition on the cycle-by-cycle variations was analyzed via flame development photographs and pressure-derived combustion parameters. The results showed that the cycle-by-cycle variations were initiated at the early stage of flame development. The flame kernel is more concentrated to the spark electrode and flame pattern has less irregularity with hydrogen addition. Direct-injection natural gas combustion can achieve the stable lean combustion along with low cycle-by-cycle variations due to the mixture stratification. Cycle-by-cycle variations decreased with the increase of hydrogen addition and this trend is more obvious at ultra-lean burn condition. Hydrogen addition weakens the effect from the turbulent flow on flame propagating process. There exists interdependency between the early combustion stage and the subsequent combustion process for direct-injection combustion.