Secondary Emission Control towards Post China 6 Legislation

China 6 emission legislation was finalized in 2018 and the full implementation nationwide is postponed to 2021 due to COVID-19. It is foreseen that the post China 6 legislation will have more stringent primary and secondary emission requirements including further reduction of nitrous oxide (N2O) emissions and likely to include limits on ammonia (NH3) emissions. This study investigated the secondary emissions (i.e. N2O and NH3) of a variety of China 6 production gasoline vehicles operating under the worldwide harmonized light vehicles test cycle (WLTC) and random test standardized aggressive (RTS 95) cycle. It was found that N2O emissions were less than 5 mg/km on all these vehicles, far below the current China 6 limit (20 mg/km), however NH3 emissions varied from 2 to 48 mg/km among the production vehicles. The mechanisms of N2O and NH3 formation and their correlation with primary emissions were also investigated. Methods to reduce N2O and NH3 emissions were discussed from the perspective of catalyst system design, including substrate volume, platinum group metals (PGM), and washcoat loading. Finally, a gasoline ammonia slip catalyst (gASC) was proposed and tested on selected vehicles as an add-on device on targeting the reduction of NH3 emissions. NH3 emissions were reduced significantly reduced below 5 mg/km on the WLTC. © 2021 SAE International. All rights reserved.