Importance of Spray-Bowl Interaction in a DI Diesel Engine Operating under PCCI Combustion Mode

In the Premixed-Charge Compression Ignition (PCCI) combustion mode, fuel is injected fairly early before top-dead-center (TDC) of compression compared to the conventional near-TDC injection combustion mode. Early fuel injection into a low temperature in-cylinder environment results in long ignition delay and high peak heat release rate. Since the onset of ignition occurs after the end of injection, importance of spray and bowl induced flow field and mixing is not so obvious. In the present work, computational analysis is used to investigate the effects of spray-bowl interactions on PCCI combustion and emissions at a light-load (4Bar BMEP) operation of a medium-duty, direct injection diesel engine. Multidimensional CFD code KIVA-3V coupled with detailed chemical kinetics is used to perform combustion simulations. The model is first validated with the available engine measurements and then applied to correlate in-cylinder details of flow-field, temperature, and soot with measured trends in exhaust soot emissions. The simulations are performed for two different piston bowl shapes, a range of injector spray cone angles (100º-158º), and a range of start of injection (SOI) timings (15º BTDC to 35º BTDC). The model predictions suggest that the optimum combination of spray cone angle and SOI to produce lower exhaust emissions of soot is dependent on the level of reentrancy and radius of curvature of the piston bowl near the bowl-lip. Interaction of spray plume, piston bowl, and squish flows generate a complex flow field structure near the piston bowl wall. This flow structure plays an important role in post heat-release mixing and soot oxidation.