Three-way catalyst equipped stoichiometric natural gas vehicles have proven to be an effective alternative fuel strategy that has shown superior low NOx benefits in comparison to diesels equipped with SCR. However, recent studies have shown the TWC activity to contribute to high levels of tailpipe ammonia emissions. Although a non-regulated pollutant, ammonia is a potent pre-cursor to ambient secondary PM formation. Ammonia (NH3) is an inevitable catalytic byproduct of TWCduring that results also corresponds to lowest NOx emissions.
The main objective of the study is to develop a passive SCR based NH3 reduction strategy that results in an overall reduction of NH3 as well as NOx emissions from a stoichiometric spark ignited natural gas engine. The study investigated the characteristics of Fe-based and Cu-based zeolite SCR catalysts in storage, and desorption of ammonia at high exhaust temperature conditions, that are typical of stoichiometric natural gas engines. Results of the investigation showed that both, the Fe- and Cu-zeolite SCRs adsorbed above 90% of TWC generated NH3 emissions below 350-400°C SCR temperatures. Desorption or slipping of NH3 was mostly observed at exhaust gas temperatures exceeding 400°C. In terms of NOx conversions, for a same cell density substrate, Fe-zeolite showed efficiency between 60-90% above temperatures of 300-350°C while Cuzeolite performed well at lower SCR temperature from 300°C below with conversion efficiency of greater than 50%.
In order to efficiently reduce both NOx and NH3 simultaneously over longer dynamic duty cycles it was found that an active regeneration strategy for the passive SCR system must be developed. To this extent, the study extended its objectives to develop an engine control strategy that results in stoichiometric ammonia production operation followed by brief lean operation to regenerate SCR brick using high NOx slip through TWC.
