Potential of multiple injection strategies implementing the after shot and optimized with the design of experiments procedure to improve diesel engine emissions and performance

The potential of the after-injection versus engine-out emissions, combustion noise and brake specific fuel consumption has been evaluated for a Euro 5 diesel engine with a reduced compression ratio (16.3:1). The engine has been fueled with conventional diesel fuel. In particular, the effects of injection strategies that feature either pilot and after-injection shots, or double-pilot and single-after injection shots, have been assessed experimentally, in the presence of high EGR fractions. Calibrations with triple and quadruple injection schedules have been optimized by means of a design of experiments procedure. The performance of the thus calibrated propulsion system has been compared with data from a previously optimized double injection schedule, characterized by a retarded main injection timing in order to intensify the premixed combustion. The experimental data refer to different steady-state working conditions that are representative of passenger car engine applications over the European homologation cycle. In-cylinder analyses of the pressure, heat release rate, temperature and emissions have been performed in order to have a better understanding of the effects of the implemented injection strategies on engine performance. The substitution of the pilot-main injection schedule in the medium-high part-load zone of the NEDC region with a triple injection, featuring both pilot and after shots, has led to lower NOx and higher soot, while fuel consumption remains almost the same. In general, the EGR trade-off soot–NOx, bsfc–NOx, HC–NOx and CO–NOx curves do not change to any significant extent when an after shot is added to the pilot-main injection train. Reductions in the combustion noise, which depend on the changes in the pilot injection parameters that result from the design of experiments procedure, can also be obtained, as a consequence of the addition of the after-injection to the pilot-main injection schedule. Pilot–pilot-main-after strategies guarantee improved NOx engine-out emissions and bsfc–NOx EGR trade-off curve at medium loads and speeds, compared to both pilot–pilot-main and pilot-main-after strategies, and allow combustion noise to be diminished significantly.