A new global NOx emissions formation model, formulated by a single analytically derived algebraic equation, is developed with relevance to post-flame gases. The model originates from subsets of detailed kinetic schemes for thermal and N2O pathway NO formation, needs no calibration and is quick to implement and run. Due to its simplicity, the model can be readily used in both 1D and 3D-CFD simulation codes, as well as for direct post-processing of engine test data.
Characteristic timescales that describe the kinetic nature of the involved NO formation routes, when they evolve in the post-flame gases independently the one from another, are introduced incorporating kinetic information from all relevant elementary reactions. These timescales are then included in the formulation of the single algebraic equation that describes the global, kinetically controlled, temporal NOx emissions evolution in the burned gas mixture, as determined by the simultaneous contributions of the two considered NO formation pathways.
After being validated against engine-out NOx emissions data of two lean burn large gas engines, the model is then used to assess the behavior and significance of the thermal and N2O pathway mechanisms under operating conditions typical of large gas engines. It is also used to highlight the errors in the calculation of NOx emissions commonly made in cycle simulations of lean burn gas engines by neglecting the contribution of the N2O pathway mechanism.