Study on combustion behaviors and cycle-by-cycle variations in a turbocharged lean burn natural gas S.I. engine with hydrogen enrichment

Lean burn is widely accepted as an effective approach to simultaneously improve spark-ignition engine's thermal efficiency and decrease exhaust emissions. But although lean burn has a lot of advantages it is also associated with several difficulties including slower flame propagation speed and increased cycle-by-cycle variations. Hydrogen addition is thought to be an ideal approach to tackle these problems. 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, spark timings, manifold absolute pressures and engine speeds. It is also found that hydrogen addition contributes 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 spark timings, various equivalence ratios, MAPs, engine speeds 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 relatively high level over a much wider range of equivalence ratios. When fixed at MBT spark timing, NOx emission also can be reduced by hydrogen addition.