UV-visible digital imaging of split injection in a gasoline direct injection engine

Ever tighter limits on pollutant emissions and the need to improve energy conversion efficiency have made the application of gasoline direct injection feasible for a much wider scale of spark ignition engines. Changing the way fuel is delivered to the engine has thus provided increased flexibility but also challenges, such as higher particulate emissions. Therefore, alternative injection control strategies need to be investigated in order to obtain optimum performance and reduced environmental impact. In this study, experiments were carried out on a single-cylinder gasoline direct injection optical engine fuelled with commercial gasoline in lean-burn conditions. The single-cylinder was equipped with the head of a commercial turbocharged engine with similar geometrical specifications (bore, stroke, compression ratio) and wall guided fuel injection. Optical accessibility was ensured through a conventional elongated hollow Bowditch piston and an optical crown, accommodating a fused-silica window. Experimental tests were performed at fixed engine speed and injection pressure, whereas the injection timing and the number of injections were adjusted to investigate their influence on combustion and emissions. The UV-visible digital imaging was applied in order to follow the combustion process, from ignition to the late combustion phase. All the optical data were correlated with thermodynamic analysis and measurements of exhaust emissions. Split injection strategies (i. e. two injections per cycle) with respect to single injection increased combustion efficiency and stability thanks to an improvement of fuel air mixing. As a consequence, significant reduction in soot formation and exhaust emission with acceptable penalty in terms of HC and NOx were measured.