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

•We performed an experiment operating an IC engine on methane-oxycombustion.•We used two models to predict burned-gas temperatures and NOx formation.•NOx emissions in oxycombustion vary linearly with N2 volume concentration.•CO2 working fluid produces higher thermal efficiency and lower NOx than CO2 and H2O.•NOx mechanisms tuned for air accurately predict emissions in low N2 environments.

This paper investigates the formation of the pollutant nitric oxides (NOx) 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 NOx regulation levels without after-treatment systems.NOx emissions in g/kW h 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, NOx 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 NOx 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 NOx 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 NOx formation, and with results indicating that air-calibrated NOx mechanisms maintain reasonable accuracy in low-N2 environments.