SOOT AND PARTICULATE EMISSIONS

• Soot is a carbonaceous particulate matter and is produced during combustion of the rich  fuel - air mixtures.
• Appearance of black smoke emissions in the exhaust indicates high concentration of soot in the exhaust gases.
• Soot is mostly produced in the diffusion combustion systems, but overly rich premixed combustion also produces soot.
• As the spark ignition engines generally operate close to stoichiometric air-fuel ratio, soot emissions from these engines are not significant. With the use of unleaded gasoline, lead particulates from the SI engines have been eliminated.

Here, we will discuss particulate emissions only from the diesel engines as these are of major health concern and are more difficult to control. Soot emissions have been associated with respiratory problems and are thought to be carcinogenic in nature. The particle size is important as the particles smaller than 2.5 μ can reach lungs along with the inhaled air and cause health problems. The particles smaller than 2.5 μ constitute more than 90 percent mass of the total particulate matter in the diesel exhaust.
The fuel composition also is an important factor in soot production and emissions. For diffusion combustion soot-forming tendency is generally in the following order;

Composition and Structure of Diesel Particulates

US Environmental Protection Agency (USEPA) defines  the particulate matter as any substance other than water that is collected by filtration of the diluted exhaust gases at or below 325 K (125 F).
Composition of particulate matter collected on a filter is schematically shown on Fig. 2.20. Various components adsorbed on the surface of spherical soot particles are shown.