NO_x Emissions Reduction of a Natural Gas SI Engine Under Lean Conditions: Comparison of the EGR and RGR Concepts

In natural gas SI engines under lean conditions, NOx emissions reduction can be realized by injecting an additional mass flow rate to inlet gases. It can be easily done in situ using two techniques: EGR (Exhaust Gas Recirculation) or RGR (Reformed Gas Recirculation) which is an improvement of the usual EGR configuration. Exhaust gases are catalyzed before being reintroduced at the engine inlet. Reformed gases contain carbon monoxide and hydrogen in addition to carbon dioxide, steam and nitrogen dioxide that compose usual recirculated gases. In order to compare EGR and RGR concepts, the study is divided in three stages. Firstly, a “two-zone” thermodynamic model has been developed and validated on a large open chamber SI engine (18L CHP plant engine, fuelled by natural gas and equipped with data acquisition). Both in-cylinder pressure and NO_x emissions have been compared between numerical results and experimental data. A good agreement is obtained, the error is less than 3%. Secondly, a widespread model of steam reforming on a Ni/MgOAl₂O₃ catalyst has been used to compute in particular CO and H₂ concentrations. Numerical results lead to a good concordance with experimental data from literature. Finally, SI engine and reformer models have been linked. RGR and EGR configurations have been numerically compared considering the same recirculation mass flow rate. According to the results, RGR is the best way to decrease significantly nitrous oxide emissions, while keeping good engine performance.