Abstract: Lean burn is accepted as an effective approach to simultaneously improve spark ignition engine’s thermal efficiency and decrease exhaust emissions for natural gas engines. The cycle-by-cycle variation (CCV) is a main limiting factor in expending lean burn limit. Hydrogen addition is thought to be an ideal approach to tackle the problem. An experimental study aimed at examining the effects of hydrogen addition on cycle-by-cycle variation in an SI engine is conducted on a 6-cylinder throttle body injection turbocharged natural gas engine fueled with pure natural gas and 20% hydrogen enriched compressed nature gas(HCNG) in volume. The experiment is conducted under various conditions including different ignition timings and air fuel ratios. The in cylinder pressure data, combustion characteristics and emissions are analyzed. The results show that coefficient of variation (CoV) in both maximum pressure pmax and indicated mean effective pressure (IMEP) can be reduced by hydrogen addition and that positive effect will be more obvious as the engine is further leaned out. The duration of flame development and propagation as well as the CCVs in flame propagation duration can be simultaneously reduced by hydrogen addition. Hydrogen addition is also proved to be good to the control of emission of NOx and unburned HC.

Key words: Combustion duration, Cycle-by-cycle variations, Emissions, Hydrogen, Hydrogen enriched compressed nature gas(HCNG)