Transient emission characteristics of a heavy-duty natural gas engine at stoichiometric operation with EGR and TWC

The recent issued emission regulations have more stringent standards for the transient emissions of natural gas engines. When equipped with cooled exhaust gas recirculation (EGR) and three way catalyst (TWC), natural gas engines operating at stoichiometric conditions are capable of achieving extremely low transient emissions. In this paper, the transient emission characteristics of a heavy-duty natural gas engine at stoichiometric operation with EGR and TWC are experimentally studied based on the world harmonized transient cycle (WHTC). The results show that both the raw carbon monoxide (CO) and total hydrocarbon (THC) emissions are higher in the hot start test while the raw NOx emissions are higher in the cold start test. When measurements are conducted after the TWC, all these emissions are higher in the cold start test. Among all the sub-cycles of WHTC, the urban sub-cycle plays the most important role for all three emissions in both cold start and hot start conditions except the after three way catalyst (ATWC) CO emissions in the cold start test, where the motorway sub-cycle takes up the highest percentage. In summary, CO, nitrogen oxide (NOx) as well as non-methane hydrocarbon (NMHC) emissions can meet the requirements of Euro VI emission standards, whereas CH4 emissions are higher than the corresponding standard, suggesting that improvements in the performance of the catalyst are essential.