Trends in Soot Oxidation Rates
Typical variation in soot oxidation rates in a diesel engine using NS-C model are shown in Fig. 2.24. The oxidation rate depends on temperature, pressure and mixing rate. The soot oxidation rate initially increases to a peak as the concentration of O2 rises due to mixing with air. The oxidation rate subsequently decreases as the gas temperature falls and more than offsets the effect of increasing O2 concentration in the soot pocket. The mass of soot burned in the late mixing pocket is much lower (only about 40%) than for the early mixing pocket. The high mass of soot burned in the early formed soot pocket is due to high oxidation rate resulting from high temperatures rather than the higher residence time.
Figure 2.24. |
Trends in soot oxidation rate in a diesel engine computed by NS-C model for an early (at tdc) and a late formed (40 ºCA atdc) soot pocket, mixing rate with air was kept constant. |
The net soot emissions depend upon the rates at which soot is produced and oxidized. Temperature plays an important role in net soot emissions. Temperature on one hand increases soot oxidation while on the other hand it increases fragmentation of fuel molecules that may lead to higher soot formation. The effects of several interacting factors thus govern the net soot emissions.
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