Effect of the fuel-injection strategy on flame-front evolution in an optical wall-guided DISI engine with gasoline and butanol fueling

This work investigates the effect of n-butanol on combustion processes in a direct injection spark ignition (DISI) engine through the analysis of flame front propagation. Specific attention is given to the sensitivity of n-butanol when changing injection mode in terms of timing and number of injections. Tests were carried out on an optically accessible single-cylinder DISI engine fueled 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. A conventional elongated hollow piston is used and an optical crown, accommodating a fused-silica window, is screwed onto it. The injector is side mounted and features six holes oriented so that the spray is directed toward the piston crown. During the experimental activity, injection pressure was maintained at 100 bar for all conditions; homogeneous charge conditions performed through single- and split-injection strategy (i.e., two injections per cycle) were compared for investigating their influence on combustion and emissions. Cycle-resolved visualization was applied in order to follow the combustion process, from ignition to the completion of flame front propagation. Macroscopic parameters (inflamed area, shape factor) and microscopic data (curvature distributions) were evaluated through image processing. All the optical data were correlated with conventional measurements of thermodynamic analysis and exhaust emissions. The effect of split injection was found to be relatively negligible for gasoline, while significant differences were recorded when switching from single to double injection for butanol. This points to a different mixture formation process, mainly related to the evaporative properties of butanol.