Numerical Simulation and Experimental Research on PPC Combustion and Emissions Characteristics of Diesel Engine

The partially premixed combustion (PPC) was realized in a 6-cylinder heavy turbocharged diesel engine with single injection and EGR strategies. Combined with chemical reaction mechanism, the combustion and emission processes of this mode was analyzed using level-set turbulent flame propagation model. Based on the experiments and simulations, the impacts of PPC on the heat release, NOx and PM emissions were further investigated. The results show that both the early-injection and late-injection strategies have a two-stage heat release and significantly higher premixed combustion proportion than the conventional combustion mode, however, mixing controlled combustion was also observed in the late-injection mode. The NOx and PM emission was reduced simultaneously in PPC. However, in the late-injection strategy, this was achieved at the expense of increased HC emission. The dynamic process of the core components in PPC was much different from those in conventional diesel spray combustion. Ketohydroperoxide was found to play an important role in the first stage ignition of PPC. The accumulated H₂O₂ in the first stage ignition leads to a high heat release peak in the second stage ignition. NOx was heavily observed in the downstream of the spray where OH was formed, while soot was formed in the fuel-rich mixture near the combustion chamber wall. The high HC emission in the late-injection PPC was primarily formed in the fuel-lean mixture where the second stage ignition was not achieved rapidly.