Contd...

 K_NO is given by

and   mole fraction of N₂ at equilibrium,    x _[N2]e  ≈ 0.71

From the above,

(2.18)

Time  is in seconds, pressure, P is in atmospheres and temperature, T is in K.

Fig. 2.4   Dependence of initial rate of NO formation on temperature and fuel-air equivalence ratio (). The dashed line shows adiabatic flame temperature line  for different values of . The maximum NO formation rate  occurs at slightly leaner than stoichiometric mixture.

Fig. 2.4  shows the initial NO formation rate as a function of temperature for different fuel-air equivalence ratios. For a given value of   NO formation increases with temperature. And as is reduced more oxygen is available resulting in higher NO formation for a fixed reactant temperature. In the real combustion systems, the burned gas temperature also depends on the value of . On this figure, the adiabatic flame temperature for a hydrocarbon fuel-air mixture initially at 700 K and combustion at constant pressure at 15 atm for different fuel-air ratios is also shown.  The adiabatic constant pressure combustion of a charge element as an acceptable model for an internal combustion engine.  The initial NO formation rate is seen to be highest for a mixture slightly leaner than stoichiometric.