Potential of double pilot injection strategies optimized with the design of experiments procedure to improve diesel engine emissions and performance

The potential of pilot–pilot–main triple injection strategies versus engine-out emissions, combustion noise and brake specific fuel consumption has been assessed experimentally on a Euro 5 diesel engine with a reduced compression ratio (16.3:1). The engine has been fueled with conventional diesel fuel. The experimental tests on the engine have been carried out in a dynamometer cell under different steady state working conditions, that are representative of passenger car engine applications over the European homologation cycle. Furthermore, in-cylinder analyses of the pressure, heat-release rate, temperature and emissions have been performed in order to obtain more detailed knowledge on the cause-and-effect-relationships between the implemented injection strategies and the results of the experimental tests. The implemented double-pilot injection engine calibrations have been optimized by means of the design of experiments procedure. The plotted data of the engine performance and emissions have been compared with data from the original double-injection schedule, characterized by a retarded main injection timing, in order to intensify the premixed combustion phase. The benefits and the disadvantages of the PCCI concept are preliminarily discussed, on the basis of the experimental pilot–main injection strategy results. The substitution of the pilot–main injection schedule with the triple injection, for light engine loads and low engine speeds, has led to higher mean combustion pressures, lower heat release rates, shorter ignition delays and lower brake specific fuel consumption. Above all, a significant improvement in engine noise and in both CO and HC engine-out emissions has been achieved and the NOx emission have been limited by the application of high EGR rates. When medium engine loads and speeds are analyzed, the considered double-pilot injection strategy allows the NOx emissions to be reduced, compared to the baseline pilot–main injection schedule. However, the combustion noise does not improve and the soot deteriorates, even though the soot penalties are not relevant.