Nitrogen Enriched Air (NEA) has shown great potential in NOx reduction without the drawbacks of exhaust gas recirculation (EGR). Use of NEA in stationary natural gas engines has shown up to 70% NOx reduction with a modest 2% nitrogen enrichment. However, nitrogen enrichment beyond a point leads to degradation in engine performance in terms of power density, brake thermal efficiency and unburned
hydrocarbons. Optimizing the nitrogen enrichment levels to reduce NOx without performance degradation of the engine would greatly benefit the advancement of the air separation membrane technology. Development of fast and robust modeling tools to compute the temporal variation of the in-cylinder engine pressure, temperature and NOx formation can aid experimental efforts in determining the optimum enrichment levels for a given engine operating condition. This work presents a methodology to compute engine-out NOx for engines with and without nitrogen enrichment. Temporal variation of in-cylinder engine pressure and temperature can be obtained by a solution of the energy equation. Using these temperature and pressure values, along with the instantaneous composition of the working fluid, one can evaluate the equilibrium concentration of the combustion products. Since the NOx formation freezes a few crank angle degrees after the completion of combustion, it is instructive to examine whether the equilibrium computation can provide a reasonable estimate of engine-out NOx. To this end, engine-out NOx computed by using the above-mentioned procedure was obtained as a function of equivalence ratio for cases with nitrogen enrichment of 2% and no nitrogen enrichment. The results showed that the equilibrium NOx concentrations a few crank angle degrees after end of combustion were close to those reported experimentally in stationary natural gas engines. These results suggest that it would be possible to use equilibrium chemistry computations to evaluate various NOx mitigation strategies.