A response surface optimization was performed to demonstrate the emission reduction capability of the combined effects of high injection pressure, boost pressure, multiple injections, and cooled EGR on a High Speed Direct Injection (HSDI) diesel engine equipped with a common rail injection system.
The optimizations were conducted separately for a single injection case and a split injection case at the same engine load and speed conditions, which was 1757 rev/min, 45% load. Four factors were considered for the single injection optimization, which included EGR rate, SOI, boost pressure, and injection pressure. For the split injection case, two more factors (the percentage of fuel in the first injection and the dwell between injections) were added, thus a total of six factors were considered. In addition, the effect of intake boost pressure on MK combustion and the swirl effects were investigated.
At the single injection optimum point, the NOx and PM emission levels met the EPA tier II 2004 mandates. On the other hand, at the split injection optimum point, the NOx emission still exceeded the EPA mandates by 29%, but the PM emission was consistent with the mandates. The CO and THC levels for both single and split injection optimum points were also within the EPA automotive diesel mandates. Thus, the single injection optimum point was better than the split injection optimum point in terms of its NOx and PM emissions at this particular engine operating condition. However, the split injection optimum point had better fuel economy.
Through the present optimization process, it was shown that RSM optimization is an effective and powerful tool for realizing the full advantages of the combined effects of combustion control techniques. In addition, observation of the optimization process provides a more thorough understanding of HSDI diesel combustion since the observed trends can be explained by monitoring the effects of the operating parameters. For example, the single injection optimization process led naturally to the Modulated Kinetics (MK) combustion regime, with simultaneous NOx and PM reductions by means of high EGR and retarded injection timing.