In-cylinder spectroscopic measurements of knocking combustion in a SI engine fuelled with butanol–gasoline blend

•UV–visible natural emission spectroscopy was applied in an optical SI engine fuelled with gasoline and butanol–gasoline blend.•A different local air and fuel distribution in the combustion chamber was evaluated for gasoline and butanol–gasoline blend.•The flame kernel was detected early for butanol–gasoline blend than gasoline.•Spectral evidence of soot precursors allowed to confirm the influence of fuel injection mode on the fuel deposits burning.•The emission of OH radicals at 325 nm may be considered a good knocking optical indicator.

Recent studies have shown that biobutanol can play a significant role in a sustainable, non-petroleum-based, industrial system. About the use in spark-ignition engines, butanol blended with conventional hydrocarbon fuels can increase fuel octane rating and power for a given engine displacement and compression ratio. Several reference studies measured performance, fuel consumption and exhaust emissions for spark-ignition engines fuelled with butanol–gasoline blends. Very few experiments have been performed on in-cylinder butanol–gasoline combustion process. In this paper, an experimental investigation was carried out in a port fuel-injection, spark-ignition engine with an external boosting device. A blend of 20% of n-butanol in volume with commercial gasoline was used for the experiments. The optical engine was equipped with the head of commercial SI turbocharged engine with the same geometrical specifications as the research engine. The engine worked under stoichiometric mixture at 2000 rpm, medium boosting and wide open throttle. Knocking condition was realized advancing the spark timing. In cylinder UV-visible natural emission spectroscopy was applied to follow the formation and the evolution of the principal compounds and radical species that characterize the normal and abnormal combustion process from the spark ignition until the exhaust. Particular interest was devoted to OH evolution.