Summary of Soot Formation Process

Sequence of the different events taking place during soot formation process is summarized in Fig. 2.25.

• Pyrolysis of fuel molecules produces soot precursors.
• The precursors condense, polymerize and form soot nuclei which coagulate and also grow as a result of continuing condensation of precursor molecules.  Thus, the nuclei grow to primary soot particles.
• Soot primary particles then, form chainlike structures.
• The soot particle surface growth finally ceases as the precursors are depleted or expansion in the cylinder cools the gases terminating formation of the precursors.

In the diesel engines, the flame temperature being high around 2800 K, entire soot formation and oxidation processes take place over a few milliseconds during engine combustion.
During expansion and exhaust strokes and in the exhaust system heavy hydrocarbons from unburned fuel and oil condense and are adsorbed on soot particles. Water, sulphates and other trace inorganic compounds such as ash formed by engine oil additives and fuel contaminants, wear particles etc. are also adsorbed on the particulate matter.