This paper presents an experimental work aimed at investigating the effects of hydrogen addition on the combustion behaviors and cycle-by-cycle variations in a turbocharged lean burn natural gas SI engine. The experiments were conducted over a wide range of hydrogen enhancement levels, equivalence ratios and spark timings.
The results show that the phase of heat curve advances with the increase of hydrogen fraction reflecting hydrogen’s ability to speed up combustion and furthermore this effect becomes more obvious at leaner conditions. It is also found that hydrogen addition contribute much more to reducing flame development duration, whose reduction has greatly positive effects on keeping down cycle-by-cycle variations than to reducing flame propagation duration. Based on these combustion results, the effects of hydrogen addition on cycle-by-cycle variations at various equivalence ratios and spark timings were also examined and it is concluded that cycle-by-cycle variations, which is of high importance in lean burn SI engines can indeed be lowered by hydrogen addition.
Engine thermal efficiency and exhaust emissions were also investigated during the experiments. It is found that after hydrogen addition the engine’s thermal efficiency could keep at relative high level over a much wider range of equivalence ratios compared to engine fueled by pure natural gas. However, NOx emissions were increased by hydrogen addition. But this is not thought to be a problem since in practice, the ability to burn effectively leaner overall mixtures after hydrogen addition will be an asset in keeping oxides of nitrogen very low.