NO Formation and Decomposition Models for DI Diesel Engines

An algebraic engineering model for emissions of nitric oxides (NO_x) from direct injection (DI) Diesels, first suggested by Mellor et al. [1], is evaluated with results from engine tests involving the injection of pure nitric oxide (NO) into the intake air of a 2.4L high speed direct injection (HSDI) Diesel engine [2]. The model is based on a two-zone representation of the DI Diesel spray plume flame. As originally suggested by Mellor et al. [1], NO forms in zone 1, which is characterized by the adiabatic stoichiometric flame temperature at start of combustion (SOC), and decomposes in zone 2, which is characterized by the end of combustion (EOC) temperature. Engine-out NO_x emissions are correlated using ratios of characteristic fluid mechanic mixing times to characteristic chemical kinetic times (Damköhler numbers). The kinetic times for NO formation and decomposition take into account both the extended Zeldovich and nitrous oxide mechanisms [1].

The model is first rederived accounting for NO in the intake air and then used to evaluate the mixing times for each zone. The bulk EOC temperature estimated using the dual cycle, which was suggested by Mellor et al. [1] to be the appropriate temperature for zone 2, is found to be too low because it results in an unrealistic range of mixing times. Due to the temperature dependence of the NO decomposition reactions, NO decomposition is suggested to be characterized by a temperature near the stoichiometric flame temperature.