Role of Piston Bowl Shape to Enhance Late-Cycle Soot Oxidation in Low-Swirl Diesel Combustion

Late-cycle soot oxidation in heavy-duty (HD) diesel engine low-swirl combustion was investigated using single-cylinder engine and spray chamber experiments together with engine combustion simulations. The in-cylinder flow during interactions between adjacent flames (flame-flame events) was shown to have a large impact on late-cycle combustion. To modify the flame-flame, a new piston bowl shape with a protrusion (wave) was designed to guide the near-wall flow. This design significantly reduced soot emissions and increased engine thermodynamic efficiency. The wave’s main effect was to enhance late-cycle mixing, as demonstrated by apparent rate of heat release after the termination of fuel injection. Combustion simulations showed that the increased mixing is driven by enhanced flow re-circulation, which produces a radial mixing zone (RMZ). The leading edge of the RMZ extends toward the center of the piston bowl, where unused ambient gas is available, promoting oxidation. The wave also enhances mixing in the trailing edge of the RMZ when it detaches from the wall, accelerating the burn-out of the RMZ. This flame interaction effect was isolated and studied further using a new optical setup in a spray chamber with a 2-hole nozzle fuel injector. A conceptual model relating piston bowl geometry to soot oxidation efficiency was developed to explain late-cycle soot oxidation in low-swirl HD engines.