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Prompt NO
In the flame reaction zone NO may be formed rapidly. The prompt NO is formed in the flame by reaction of intermediate chemical species of CN group with O and OH radicals. The hydrocarbon radicals CH, CH2, C, C2 etc. formed in the flame front react with molecular nitrogen to give intermediate species such as HCN and CN by the reactions (2.19) to (2.21). Large concentrations of HCN near the reaction zone in fuel rich flames have been observed and rapid formation of NO has been seen to be associated with rapid decay of HCN.
The contribution of prompt NO in the stoichiometric laminar flames is estimated to be about 5 to 10 percent only. In the engines as the combustion occurs at high pressures, the thickness of flame front is very small (~ 0.1 mm) and the residence time of chemical species in this zone is very short. Moreover, the burned gases produced by the charge elements that burn early during the combustion process are compressed to a much higher temperature than the temperatures attained immediately after combustion. The formation of thermal NO in the burned gases behind the flame front therefore, is much higher compared to any NO formation in the flame front. However, contribution of prompt NO may be significant under lean engine operation or engine operation with high dilution such as use of exhaust gas recirculation.
Fuel NO
Fuel NO is formed by combustion of fuels with chemically bound nitrogen. The fuel nitrogen produces at first intermediate nitrogen containing compounds and reactive radicals such as HCN, NH3, CN, NH etc. These species are subsequently oxidized to NO. Although petroleum crude may contain about 0.6 % nitrogen but gasoline has negligible nitrogen. Diesel fuels have higher nitrogen content than gasoline, but this too is usually less than 0.1% by mass. The fuel nitrogen therefore, does not make significant contribution to NO formation in automotive engines operating on gasoline, diesel, natural gas and alcohols etc.
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