Growth and Combustion of Soot Particulates in the Exhaust of Diesel Engines

Abstract

In order to clarify particulate growth and combustion mechanisms in the exhaust pipe, the size and aggregation of soot particles emitted from a diesel engine were investigated using a transmission electron microscope and their chemical composition analyzed by gas chromatography. When the gas temperature was lowered below 400 degrees C in the exhaust pipe, the particle size distribution increased from 50 to 100 angstroms. With the lower temperatures downstream in the exhaust pipe, there was also an increase of polynuclear aromatic hydrocarbons in the soot. This growth of particles is considered the result of condensation and polymerization of hydrocarbons, water, and other components in the exhaust gas. When two hydrocarbons, propane and acetylene, were added to the exhaust, soot particle growth rate and smoke density increased. When the exhaust was heated to above 600 degrees C, soot particles became smaller or burned off. An exhaust gas oxygen concentration above 6% was necessary for this effect. The average diameter of soot particles in the combustion chamber was in the 230 to 280 angstrom range.