NO Formation in CI Engines

In the compression ignition engines, rapid combustion of fuel and air that are mixed during delay period occurs This rapid combustion phase is termed as ‘premixed combustion’ and is followed by mixing controlled diffusion combustion process. The diffusion combustion rates are controlled by the .rate at which fuel and air mix and hence the name ‘mixing controlled combustion’.  Fuel-air equivalence ratio varies widely from very rich at the core of spray to very lean at the spray boundaries and, the formation of emissions is governed by the local air-fuel ratio besides other factors like temperature and pressure.  In the premixed combustion phase, mixture formed within the flammable limits burns spontaneously. On the other hand, in the mixing controlled combustion phase, it is believed that combustion occurs in those regions of spray where equivalence ratio is close to stoichiometric.
In the classical spray combustion models, formation of NO starts in the burned gases produced on combustion of close to stoichiometric and lean flammable mixtures during premixed combustion phase.  New combustion research on turbocharged/supercharged engines suggests that most NO is formed in mixing controlled diffusion combustion at spray boundaries and in the post combustion high temperature gases. The diffusion combustion takes place at near stoichiometric conditions. In the supercharged/turbocharged engines the delay period is rather short and overall a significantly smaller fraction of fuel burns in premixed phase. In the modern turbocharged, high-pressure direct injection engines with retarded injection timing, more than half of NO_x in the cycle is produced in the post combustion gases after peak pressure. In the naturally aspirated engines with long ignition delays and sufficient time available for premixing of fuel and air, the contribution of premixed combustion to NO formation is considered to be substantial.
The hypothesis that most of NO_x in diesel engines is formed in the burned gases produced by combustion at near stoichiometric conditions has been demonstrated by the following results.  NO formation index (EINO_x) in diesel engines has been  correlated with the stoichiometric adiabatic flame temperature, using the following Arrhenius type expression

(2.23)

 =  Adiabatic flame  temperature for stoichiometric mixture, K
E = overall activation energy, J/gmol
R = universal gas constant  J/gmol.K

was evaluated at tdc motoring conditions using polytropic compression process. with polytropic index, n = 1.33. The use of diluents like nitrogen, exhaust gas, oxygen and water varies the intake air composition and hence the stoichiometric adiabatic flame