The Formation of Ammonia in Three-Way Catalysts Fitted to Spark Ignition Engines - Mechanisms and Magnitudes
[ 1 ] Instytut Silników Spalinowych i Napędów, Wydział Inżynierii Lądowej i Transportu, Politechnika Poznańska | [ P ] employee
2022
Journal year: 2022 | Journal number: 2022-01-1026
scientific article
english
EN Exhaust gas aftertreatment systems can, under certain conditions, create undesired chemical species as a result of their elimination reactions. A prime example of this is ammonia (NH3), which is not formed in the combustion reaction, but which can be formed within a three-way catalyst (TWC) when physicochemical conditions permit. The elimination of NOx in the TWC thus sometimes comes at the cost of significant emissions of NH3. Ammonia is a pollutant and a reactive nitrogen compound (RNC) and NH3 emissions should be analyzed in this context, alongside other RNC species. Examination of the literature on the subject published over the past two decades shows that ammonia, a species which is currently not subject to systematic emissions equirements for road vehicles in any market, is often identified as forming the majority of the RNC emissions under a range of operating conditions. It is expected that upcoming automotive emissions legislation (e.g. the European Union’s Euro 7/VII standards; perhaps also China 7) will set an NH3 emissions limit, mandating the measurement of NH3 under laboratory conditions. A pool of European passenger cars was tested on a chassis dynamometer under laboratory conditions, using the WLTP test procedure; ammonia emissions were measured using a dedicated analyzer. Other test procedures were used to generate further data NH3 formation trends and gain further insight, particularly into the relative proportions of NH3 and NOx in the exhaust gas and the impact of cold start and powertrain warmup. Much of the literature focuses on periods of high load as causing significant NH3 emissions; here it was found that cold start is a very significant contributor. Separation of the effects of powertrain temperature and load confirm that TWC behavior determining NH3 emissions is non-trivial. As a result, NH3 emissions can be highly sensitive to test parameters.
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