The Effect of Intake Boost Pressure on MK (Modulated Kinetics) Combustion

In previous work, an RSM optimization was performed to demonstrate the emission reduction capability of the combined effects of high injection pressure, boost pressure, and cooled EGR on a HSDI diesel engine equipped with a common rail injection system. The RSM optimization led optimum operating parameters to low-temperature and premixed combustion characteristics, i.e., the MK combustion region, resulting in simultaneous reductions in NO_x and PM emissions without sacrificing BSFC. However, further retardation of injection timing and increase of EGR rate from the optimum point resulted in significant deterioration of the engine stability with misfire. In the present work, further RSM optimization was conducted to investigate the effect of intake boost pressure on MK combustion. It was found that the increase of intake boost pressure shortened ignition delay, which was not favorable for MK combustion. However it allowed the use of heavier EGR and later injection timings, which intensified the characteristics of MK combustion, i.e., lower-temperature and more thoroughly premixed combustion characteristics. Compared to the previous optimum point, the NO_x emission level of the new optimum point was improved by 48%, and it was possible to improve PM emissions further by 71%, while retaining BSFC at the same level. At the new optimum point NO_x and PM emissions were 0.54 and 0.092 g/kW-hr, respectively, which met the EPA Tier II 2004 automotive diesel mandates.