The Impact of Operating Conditions on Post-Injection Efficacy; a Study Using Design-of-Experiments

Post-injection strategies prove to be a valuable option for reducing soot emission, but experimental results often differ from publication to publication. These discrepancies are likely caused by the selected operating conditions and engine hardware in separate studies. Efforts to optimize not only engine-out soot, but simultaneously fuel economy and emissions of nitrogen oxides (NOx) complicate the understanding of post-injection effects even more. Still, the large amount of published work on the topic is gradually forming a consensus. In the current work, a Design-of-Experiments (DoE) procedure and regression analysis are used to investigate the influence of various operating conditions on post-injection scheduling and efficacy. The study targets emission reductions of soot and NOx, as well as fuel economy improvements. Experiments are conducted on a heavy-duty compression ignition engine at three load-speed combinations. Regression analysis shows that the eventual decrease in engine-out soot heavily depends on the air-excess ratio. This observation supports the suggestion that enhanced late-cycle mixing of fuel and oxidizer is an important contributor to observed soot reductions. Furthermore, simultaneous reductions in emissions of NOx and fuel consumption with little or no impact on soot are obtained for particular injection scheduling at low load. At higher engine speed and load, soot reductions are preserved, although shifting the NOx-soot trade-off is more difficult to establish. It was found that rate of exhaust gas recirculation (EGR), timing of the main injection event and fuel pressure generally need careful adjustment to make optimal use of a post-injection scheme. Finally, several points of attention for post-injection scheduling and selecting appropriate operating parameter settings are listed.