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Contd...
Depending upon the burned gas temperature, the contribution of different reactions to kinetics of NO formation also changes and hence the deviations may be observed from the simple stoichiometric adiabatic flame temperature model. However, the dominant contribution to NO formation still results from the spray regions that burn in near stoichiometric conditions.
NO is formed at varying rates in the spray depending upon the local equivalence ratio and temperature. As the combustion progresses, the already burned gases keep on mixing with colder air and fuel vapour, changing its composition and temperature and hence the NO formation chemistry. Temperature of the reacting gases also changes due to compression and expansion.
Sudden cooling of the burned gases may result due to mixing with cooler air and consequently resulting in freezing of NO kinetics and the NO decomposition reactions. Thus, cooling of burned gases by mixing with cooler air and fuel-air mixture in diesel engines causes more rapid freezing of NO kinetics, which results in NO concentration frozen at higher levels compared to those in the SI engines.
Kinetic models based on the extended Zeldovich mechanism discussed earlier are widely used for calculations of engine-out NO emissions from the DI diesel engines. At high pressures typical of diesel combustion with high residual gas dilution (EGR), the Zeldovich mechanism alone may not predict adequately NO formation. Additional reactions involving N2O in formation of NO as given in reactions 2.24 – 2.25 have been proposed.
The significance of N2O mechanism for NO formation in the real engines however, is yet to be established.
Summary of NO Formation in CI Engines
- In the CI engines, most NO is formed in the burned gases resulting from near stoichiometric combustion
- Kinetically formed NO is frozen at higher levels compared to SI engines as sudden cooling of the burned gases may be caused due to mixing with cooler air or cylinder charge, thereby freezing the NO decomposition reactions
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