Investigation of nitric oxide emission mechanisms in a SI engine fueled with methane/hydrogen blends using a research CFD code

Abstract

The reaction mechanisms of nitric oxide (NO) emissions are investigated in a spark-ignition (SI) engine fueled with hydrogen/methane blends, for variable equivalence ratio (load variation) and variable hydrogen content. For that purpose, a research three-dimensional computational fluid dynamics code (3D-CFD) is applied, being capable of simulating in high detail the in–cylinder processes. The combustion model of the CFD code includes not only the thermal NO mechanism, widely known as ’Zeldovich mechanism’, but also other alternative NO production mechanisms. Therefore, the NO emission modeling has been enhanced here with the addition of other production paths, such as via the NNH and N2O species formation and the prompt NO mechanism. The NNH path has been shown to be favored at lean and low-temperature combustion conditions especially when hydrogen is present, whereas the N2O path becomes important for lean flames irrespectively of the fuel used. Prompt NO becomes important in stoichiometric and fuel-rich conditions and is also examined here to evaluate its contribution to the NO emissions. Focus is also directed on the local/spatial NO production pattern at each location in the cylinder during the combustion and expansion periods. The calculations are compared with available measurements, in order to quantify the use of these alternative production paths at such conditions and applications. In all cases, the thermal NO mechanism is the dominant production path. On the other hand, with lean flames it has been observed that the N2O path produces significant amount of NO, while the contribution of the NNH mechanism is small, but it is recommended to be also considered in the calculations for increasing the accuracy of the results. The prompt NO is not favored at such conditions, even for the stoichiometric case, and could be even omitted from the combustion model.