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Soot formation in high-pressure n-dodecane sprays is investigated under conditions relevant to heavy-duty diesel engines. Sprays are injected from a single-hole diesel injector belonging to the family of engine combustion network (ECN) Spray D injectors. Soot is quantified using a high-speed extinction imaging diagnostic with incident light wavelengths of 623 nm and 850 nm. Previously, soot measurements in a high-pressure spray using 406-nm and 520-nm incident light demonstrated a minimal wavelength dependence in the complex refractive index of soot (m), as demonstrated by a near unity ratio of the non-dimensional extinction coefficients (ke,406 nm/ke,520 nm). The present work, however, demonstrates a significant difference in m for measurements with infrared incident light. During the quasi-steady period of the spray combustion event, the experimentally determined ke ratio (ke,623 nm/ke,850 nm) is 1.42 ± 0.27.

For next-generation engines that operate using low-temperature gasoline combustion (LTGC) modes, a major issue remains poor combustion stability at low-loads. Negative valve overlap (NVO) enables enhanced main combustion control through modified valve timings to retain combustion residuals along with a small fuel injection that partially reacts during the recompression. While the thermal effects of NVO fueling on main combustion are well understood, the chemical effects of NVO reactions are less certain, especially oxygen-deficient reactions where fuel pyrolysis dominates. To better understand NVO period chemistry details, comprehensive speciation of engine samples collected at the end of the NVO cycle was performed by photoionization mass spectroscopy (PIMS) using synchrotron generated vacuum-ultraviolet light.