Computational Fluid Dynamics Study of Alternative Nitric-Oxide Emission Mechanisms in a Spark-Ignition Engine Fueled with Hydrogen and Operating in a Wide Range of Exhaust Gas Recirculation Rates for Load Control

Nitric oxide (NO) emissions are practically the only ones emitted from spark-ignition (SI), hydrogen-fueled engines, and their reliable prediction is important in engine simulation codes. In this work, the reaction mechanisms of nitric oxide are investigated in such engines during load variation by using a very wide range of exhaust gas recirculation (EGR) rates, up to 47%. For that purpose, a three-dimensional computational fluid dynamics code is applied, which has been developed by the authors and validated for its main sub-models, such as the heat transfer and combustion. The latter one includes the thermal NO mechanism, widely known as “Zeldovich mechanism,” whereas two alternative production paths have been included, viz. through the NNH and N2O species formation in order to improve the numerical predictions. The NNH path has been shown to be favored under lean and low-temperature combustion conditions, especially for hydrogen flames, whereas the N2O path becomes important for lean flames irrespectively of the fuel used, whereas such flames have many similarities with highly-diluted mixtures by recirculated exhaust gases. The calculations are compared with available measurements concerning the NO exhaust emissions, in order to quantify the applicability of the alternative production paths at such conditions and applications. For the high load cases (low EGR rates) the NO predictions have good accuracy because the thermal NO is the main production path. However, at mid and low engine loads, when the combustion temperature is lower and higher EGR rates are used, a significant discrepancy exists between the calculations and the measured data, which is improved when the NNH route is taken into consideration.