Module 2:Genesis and Mechanism of Formation of Engine Emissions
  Lecture 12:Mechanisms of Formation of Soot and PM
 


Diesel Smoke

Visible black smoke emissions from diesel engine result on account of high concentration of soot in the exhaust gas. All the diesel engine design and operating variables that affect soot formation and oxidation also influence black smoke intensity. Initially, smoke emission standards for production diesel vehicles were in force to control black smoke.

  • Smoke emissions increase with increase in engine load due to overall richer fuel-air ratios and hence, the rated engine power was specified based on the maximum permitted smoke density to curb black smoke emissions during engine operation. The rated power was also known as ‘smoke limited power’.
  • Poor control of fuel injection rate during acceleration also increases smoke.
  • Use of EGR reduces combustion temperatures and oxygen concentration in the burned gases. EGR also reduces oxidation of soot and hence overall effect of EGR is to increase smoke.
  • Smoke emissions can be reduced by accelerating combustion. Higher combustion rates are obtained by increasing fuel air mixing through use of high swirl rates, by increasing injection rate and improving fuel atomization. Advancing injection timing increases combustion temperatures and allows more time for oxidation of soot thereby reducing smoke emissions.

Smoke is measured by measurement of light absorbed (opacity) in a defined specific  length of column of exhaust gas. The smokemeters employing this principle are known as  light extinction type of smokemeter such as Hartridge or AVL smokemeters. Smoke has also been measured by filtering a fixed volume of exhaust gases through a filter paper and the smoke stain thus formed is evaluated on a grayness scale by a light reflectance meter (Bosch smokemeter). 
As the diesel particulate matter (PM) mainly consists of soot and the adsorbed unburned hydrocarbons (SOF) on soot core, the PM content has been related to exhaust soot content and HC concentration as below;
(2.37)

Soot content and HC concentration are in mg/m3. Soot content can be roughly estimated from smoke measurement from correlations developed by SAE (SAE Handbook), where soot content has been correlated with Bosch and Hartridge smoke units and opacity measured by full flow  type smoke meters.