Spark-ignited engine NO_x emissions in a low-nitrogen oxycombustion environment

This paper investigates the formation of the pollutant nitric oxides (NO_x) in the low-nitrogen (N₂) environment of methane oxycombustion in a spark-ignited (SI) internal combustion engine. Working fluid composition, N₂ concentration, O₂ concentration, compression ratio (CR) and spark-timing have been investigated to evaluate the feasibility of operating such a system below NO_x regulation levels without after-treatment systems.NO_x emissions in g/kWh are shown under equivalent CR, intake temperature, and indicated mean effective pressure (IMEP) at maximum brake torque spark-timing, to have an approximately linear dependence on N₂ concentration from no N₂ to normal air combustion. At a given N₂ concentration, NO_x emissions were found to be adversely correlated with power, thermal efficiency, and the coefficient of variation of IMEP. It was found that with 2-3% N₂ by volume in the working fluid, it was possible to reduce NO_x emissions to satisfy regulation levels, but this corresponds to non-ideal engine performance in other metrics. Satisfying regulations while operating at the maximum thermal efficiency required the N₂ concentration be reduced to 1-2% by volume.The system was simulated using an AVL Boost model, with results indicating that the increasing NO_x concentrations at higher O₂ cases and earlier spark-timings can largely be attributed to higher burned-gas temperatures. An additional simulation utilizing CHEMKIN and the GRI 3.0 mechanism was used to estimate NO_x formation, and with results indicating that air-calibrated NO_x mechanisms maintain reasonable accuracy in low-N₂ environments.