Effects of hydrogen addition on cycle-by-cycle variations in a lean burn natural gas spark-ignition engine

An experimental study aimed at examining the effects of hydrogen addition on cycle-by-cycle variation (CCV) in an SI engine was conducted on a 6-cylinder throttle body injection natural gas (NG) engine. Two types of fuels, CNG and 80/20 (in volume) CNG/hydrogen mixtures, were used for comparison purposes. The results showed that coefficient of variation (CoV) in both maximum pressure (P_max) and indicated mean effective pressure (IMEP) could be reduced by hydrogen addition and that positive effect would be more obvious as the engine was further leaned out. The duration of flame development and propagation as well as the CCVs in flame propagation duration could be simultaneously reduced by hydrogen addition, all of which are beneficial to improve combustion stability and thought to be reasons for the reduction in CoV_imep. Stability of combustion phasing that is crucial to the effectiveness of ignition timing optimization was also improved by hydrogen. Hydrogen addition was also proved to be good to the control of emission of NO_x and unburned HC. Hence it is concluded that hydrogen addition is an effective and applicable approach to keep down CCVs in lean burn spark-ignition engines.