NO₂ Formation

Nitrogen dioxide emissions from the spark-ignition engines are negligibly small and constitute less than 2% of the total NO_x emissions. NO emissions range from a few hundred ppm to several thousands of ppm while, the maximum NO₂ emissions are around 60 to 70 ppm only compared to 3000- 4000 ppm of NO at full load conditions. In diesel engines however, the NO₂ emissions account generally for 10 to 30 percent of the total NO_x emissions. In diesel engines, NO₂ concentration may be in the range of 200 to 400 ppm compared to NO concentrations typically in the range of 1500 to 3000 rpm. NO₂ is rapidly formed in the combustion zone by reaction of NO with HOO^- radical.

(2.26)

Later in the post combustion gases NO₂ on reaction with atomic O^- is converted back to NO and O₂.

(2.27)

However, if the high temperature burned gases due to presence of high turbulence mix rapidly with colder air or air-fuel mixture, the reactions that decompose NO₂ back to NO and oxygen are frozen and  relatively higher concentrations of NO₂ result.  The relative concentration of NO₂ with engine power typically observed in diesel engines is shown in Fig.2.10.  An increase in relative concentration of NO₂ with decrease in engine load as seen in Fig 2.10 supports this mechanism. As the engine load decreases the air to fuel ratio increases and the probability of high temperature gases coming suddenly in contact of cooler air/ charge in the engine cylinder also increases. Hence, there is a higher probability of freezing of NO₂ decomposition reactions in diesel engines at lighter loads.