Experimental and simulation investigation of the gasoline engine aftertreatment system

Nowadays, lean burn strategy for gasoline direct injection (GDI) engine is gaining increasing popularity due to its superior fuel economy and lower greenhouse gas emissions. However, it is technically challenging to meet the stringent emissions regulations on NOx with a conventional three-way catalyst (TWC). Moreover, low-temperature emissions control, especially during engine cold start, has been another focal point to researchers. To this end, it is urgently demanding to introduce strategies for improving the low-temperature and lean-NO\(_x\) removal performance of gasoline engine aftertreatment catalysts. Applying chemical promotors (i.e., hydrogen) was shown to be beneficial for enhancing catalysts activity but has been barely studied for lean burn GDI engine exhaust. The work presented in this thesis investigates the potential of adding hydrogen to improve the gasoline engine aftertreatment performance in reducing vehicle emissions. Corresponding experimental works were performed using a gasoline research engine testing bench with modified aftertreatment system consisting of catalysts, heating devices and hydrogen introduction system. Precious hydrogen injection set-up was designed and built to allow both continuous and periodical hydrogen injection strategy. Simulation studies were carried out using a one-dimensional model developed via AVL Boost to model the catalyst light-off performance, providing auxiliary information to reveal the underlying causes of hydrogen effect. The hydrogen effect was studied using a TWC and a lean NO\(_x\) trap (LNT) catalyst. Firstly, the impact of adding small amount of hydrogen upstream TWC was examined. It was found that hydrogen addition (0.5%) upstream of the TWC greatly reduced its light-off temperatures. The poor lean NO\(_x\) removal efficiency over TWC was also enhanced, but the overall efficiency was still not very satisfactory. Therefore, the implementation of a LNT downstream the TWC was studied. The TWC-LNT system performance under various conditions was examined to help understanding the real exhaust chemistry over the catalysts. Periodical engine rich operation is required for the LNT regeneration, thereby causing associated fuel penalty (3.51%) compared to constant lean operation. Therefore, the concept of periodically injecting hydrogen to regenerate LNT instead of the cyclic engine rich operation was assessed. When engine was operating under lean/stoichiometric cycling operation, it was observed that injecting hydrogen during the stoichiometric phase has potential to improve the LNT NO\(_x\) removal performance and the engine fuel economy simultaneously. Subsequently, the effect of hydrogen addition on LNT performance at various temperatures (especially low temperature) was investigated. The influence of hydrogen injection amount and injection strategy was also assessed. Overall, the experimental results indicate that hydrogen addition upstream can improve the low temperature performance of lean gasoline engine aftertreatment catalysts