The objective was to study factors influencing soot formation, such as gas entrainment and lift-off position, and to find correlations with engine-out particulate matter (PM) levels. The effects of nozzle hole size, injection pressure, inlet oxygen concentration, and ambient density at TDC were studied. The position of the lift-off region was determined from OH chemiluminescence images of the flame. The liquid penetration length was measured with Mie scattering to ensure that the Raman measurement was performed in the gaseous part of the spray.
The local Φ value was successfully measured inside a fuel jet. A surprisingly low correlation coefficient between engine-out PM and the local Φ in the reaction zone were observed. This indicates that the soot forming rate is not the dominant factor behind the trends in engine-out PM. High correlation coefficients between PM and portion of heat release after end of injection (EOI), engine-out UHC and engine-out CO all suggests that the soot oxidation process may be a more important factor.