Research objective: This research investigates the formation of NH3 and N2O in the Three-Way Catalyst (TWC) of a Euro VI MD-SI engine fueled with Liquefied Petroleum Gas (LPG). Through evaluation of the exhaust gas flow rate, composition, and temperature during a homologation cycle, as well as measuring the emissions of NH3 and N2O upstream and downstream of the TWC, the main operating conditions of the TWC are identified. The data obtained from this study is then used to conduct a synthetic gas test bench (SGB) study, controlling temperature, humidity, and H2 presence to determine the critical conditions for NH3 and N2O formation. Methodology: This study employs a research methodology integrating work on an engine test bench with an SGB. The engine test bench had two gas analyzers to measure regulated (CO, UHC, and NOx) and unregulated (NH3 and N2O) species. The engine test bench was utilized to conduct WHTC homologation cycles in cold-start and hot-start conditions to obtain the TWC boundary conditions in real driving operation and to assess the exhaust gas composition upstream and downstream of the TWC. These test results will define the test conditions in an SGB equipped with an FTIR and a micro-GC for measuring H2 concentration. The tests in SGB were performed under three conditions (dry, water presence, and water with hydrogen presence) using a temperature ramp from 100°C to 600°C, with increments of 10°C/min, for different air-to-fuel ratios representative of rich conditions (as required by the engine calibration to meet legislative limits). Results: Results achieved during the WHTC analysis showed how levels of NH3 and N2O before TWC were lower for both species except for NH3, around 50ºC during the cold-start test. However, after TWC emissions of both species increased, NH3 was generally higher than N2O emissions, whose top levels were hit in the range of 150ºC to 320ºC. Space velocity analysis of the WHTC found that the most frequent values were around 15,000 h-1. Therefore, this value was selected to emulate conditions in the SGB along with the exhaust gas composition. The results of the SGB tests further confirmed the dependence of NH3 and N2O formation on exhaust gas composition and temperature. The inclusion of water in the exhaust gas increased the formation of both species, while the presence of H2 resulted in the highest formation of NH3 and a decrease in N2O formation temperature without changes in the maximum concentrations. Limitations: The study used an engine calibrated to operate under rich to fulfill Euro VI emission limits, and this calibration was kept for all tests performed; this prevents analyzing the operation of the TWC in lean operating conditions. However, there were no possibilities in the engine test bench to measure the H2 concentration; hence, the concentrations imposed in the SGB were estimated from chemical balance. In addition, the water level in exhaust emulation was around 5% because of vaporizer limitations, which is lower than the typical value in the exhaust gas from LPG stoichiometric combustion. Novel part: Authors have conducted studies focused on pollutant emissions measure and control, whose tests have been carried out in engine facilities similar to those employed in the first part of this study, and on the operation of other aftertreatment systems. However, this is the first time a catalyst is analyzed using a synthetic gas bench and applying data from the engine test to understand the TWC physicochemical processes better. Furthermore, the formation of secondary pollutants, specifically N2O and NH3 in the aftertreatment, is relevant, and the information about it in LPG engines available in the literature is limited. Conclusion: During WHTC homologation cycles, levels of NH3 and N2O emitted by the engine were lower. However, these levels increase after TWC, reaching concentrations around 90 ppm in the worst case for both species. Additionally, the formation of N2O increases by temperatures between 150ºC and 320ºC, while NH3 formation had a broader temperature range. Results in the SGB confirm that the exhaust composition and temperature have great importance on NH3 and N2O formation, and levels of NH3 were higher than N2O. Furthermore, water and H2 increase the formation of NH3 and reduce the formation temperature of the N2O. Research has been supported by PID2020-114289RB-I00 funded by MCIN/AEI/10.13039/501100011033.
Dr.-Ing. Enrique José Sanchis, Lecturer, CMT - Universitat Politècnica de València