Morphology and nano-structure of soot in diesel spray and in engine exhaust

- Soot was sampled in two locations in diesel spray flame and also at engine exhaust.
- Morphological and nano-structural analysis was done by TEM image processing.
- Soot particles become larger and more fractal along the spray axis.
- Engine-out particulates become smaller and less fractal compared to spray soot.
- Engine-out particulates become nano-structurally more ordered compared to spray soot.

This work investigates how morphology and nano-structure of the soot particles produced in a diesel spray flame evolve due to the rise in the temperature/pressure caused by the piston motion. The soot particles were sampled from the exhaust line of a diesel engine and were compared to the soot particles directly sampled from the spray flame of the same injector, in a constant volume chamber. Analysis of the high resolution transmission electron microscope (HRTEM) images acquired from the soot samples at two axial locations in the spray flame shows that the soot aggregates grow in size, become longer, more fractal and attain lower surface to volume ratio along the spray axis. However, the soot particles produced in the spray flame become smaller, shorter, less fractal, more compact and attain higher surface to volume ratio when emitted from the engine. This is discussed to be due to the increase in the oxidation rate by temperature rise caused by piston motion, and the consequent oxidation induced fragmentation of the soot aggregates. The effect of oxidation is also evident on the size of the soot primary particles, as the engine-out soot is found to have smaller primary particles compared to the soot initially produced in the spray flame. Nano-structural analysis shows that the crystallite size in the engine-out soot is increased compared to the soot originally produced in the spray flame. This suggests that among two possible effects of temperature on diesel soot crystallite size, being crystallite size increase due to enhanced graphitization from one hand, and crystallite size reduction due to oxidation enhancement from the other hand, the former effect is dominant and the soot particles become structurally more ordered under the effect of temperature rise caused by piston motion.