Impact of NO₂ and NH₃ in exhaust on NOx sensors in vehicles with different fuel types

NOx sensors are widely used in the emission control systems of heavy-duty diesel vehicles to reduce real-world NOx emissions. The forthcoming Euro 7 regulation will, for the first time, mandate reliable and accurate on-board NOx monitoring (OBM) for all vehicles powered by internal combustion engines, including diesel, gasoline, and natural gas (NG) engines. However, concerns have been raised regarding the cross-sensitivity of commercially available NOx sensors to ammonia (NH₃), which may compromise measurement accuracy. Although various correction models have been proposed to account for this effect, few studies have quantitatively evaluated the interference or elucidated its underlying mechanism. In this study, Pt/Rh catalysts were used to mimic the reactions occurring within NOx sensors, and the responses to and interferences among NO, NO₂, N₂O, and NH₃ were quantified using a flow reactor coupled with a quantum cascade laser (QCL) detector. The results showed that, at 800 °C, the nominal operating temperature of NOx sensors, the influence of N₂O was negligible, while the response factors to NO₂ ranged from 75 % to 85 %. More importantly, the response factors to NH₃ exceeded 75 %, indicating a strong potential for cross-sensitivity. Vehicles fueled by gasoline or NG with aged three-way catalysts (TWCs) may exhibit NH₃ emissions exceeding 200 ppm, which can lead to substantial errors in NOx sensor readings. Therefore, the application of NOx sensors to vehicles equipped with TWCs should be approached with caution.