A single-cylinder high-speed direct-injected diesel engine with Bowditch-style optical access was used to make direct measurements of liquid spray penetration length and soot incandescence through the use of high-speed imaging. For the case of spray imaging, Mie scattering was used. In addition, preliminary single-shot images of OH chemiluminescence were acquired to assess diesel flame lift-off characteristics, using an intensified camera and optical filters to isolate light in the UV portion of the spectrum.
Due to the fact that the engine used in this study was recently reconstructed, it was first necessary to baseline the engine and determine its combustion behavior.
Liquid spray penetration length was measured at varying swirl ratios, engine speeds, injection pressures, injection timings and intake pressures. The results show that high swirl velocities cause axial spray penetration to decrease. The time behavior of spray penetration was found to be largely independent of engine speed, provided that swirl velocities were held constant. It was found that in a regime close to the start of injection, all sprays with a common injection pressure collapsed together regardless of ambient conditions. Injection timings were only varied in a narrow range near TDC, and no significant effects were seen. Higher injection pressures greatly increased spray penetration. In addition, higher injection pressures were found to reduce the SOC-to-SOI delay. Spray penetration was found to decrease with increasing intake pressure due to higher ambient densities at SOI.
The soot incandescence data were acquired at varying swirl ratios, engine speeds, injection pressures, injection timings, intake pressures and loads. Very little incandescence was seen during the premixed burn. In general, the incandescence intensity was found to decrease with increasing swirl, with very high swirl velocities sometimes causing combustion instability. The temporal location of soot incandescence was found to be more sensitive to wall-clock time rather than engine time (crank angle). Thus, a decrease in engine speed was found to shift the peak soot incandescence to an earlier crank angle. Less soot incandescence was seen at higher injection pressures than lower ones. However, more incandescence was seen with increasing intake pressure. Increases in injection timing advance in a narrow window about TDC caused more soot incandescence to be seen. Increases in load through increases in injection duration caused a substantial increase in soot incandescence. During quasi-steady lifted combustion, the axial location of the soot plume of each spray jet was not found to vary significantly with time.
Due to an equipment failure, only preliminary OH chemiluminescence images for one operating condition were acquired. These images, coupled with the with the soot incandescence images from the same condition showed that the flame lift-off location flashes back to the injector nozzle orifice at EOL.