In this study, experiments were carried out in an optical single-cylinder Direct Injection Spark Ignition engine fuelled with n-butanol and gasoline, alternatively. The engine is equipped with the head of a commercial turbocharged engine with similar geometrical specifications (bore, stroke, compression ratio). The head has four valves and a centrally located spark device with surface charge ignition. A conventional elongated hollow Bowditch piston is used and an optical crown, accommodating fused-silica window, is screwed onto it. The injector is side mounted and features 6 holes oriented to guide the jets towards the piston crown.
During the experimental activity, the injection pressure was maintained at 100 bar for all conditions; the injection timing and the number of injections were adjusted to investigate their influence on combustion and emissions. Different optical diagnostics were applied in order to follow the combustion process, from ignition to the completion of flame front propagation: cycle resolved visualization, chemiluminescence and natural emission flame spectroscopy in the UV-Vis range.
The optical data was correlated with conventional measurements of thermodynamic analysis and exhaust emissions. The effect of split injection (i.e. two injections per cycle) was found to be relatively negligible for gasoline, while for the alternative fuel significant differences were recorded when switching from single to double injection. This points to a different mixture formation process, mainly related to the evaporative properties of n-butanol.
