The selective catalytic reduction system (SCR) is an essential method to reduce NOx emissions from heavy-duty diesel engine-powered vehicles, which include conventional diesel buses and diesel-electric hybrid buses. Using wireless remote communication technology, the SCR system status and NOx emissions were reviewed for ten fully-operational hybrid buses from Hangzhou China in this research. Under the internal combustion engine mode, the main factors studied were vehicle speed, engine operation conditions and environment temperature, impact on the SCR catalyst outlet temperature and NOx concentration and dosing rate of the urea injector of the SCR system. The research result shows that (32.4±4)% of the operational time of the hybrid buses with SCR system was spent in internal combustion engine mode, and under (26.9±11)% of the operation time of this mode, the SCR system did not dose urea. The average NOx emission reduction rate of the SCR system, when operating normally with hybrid buses, is about 59%. The main reasons for the high NOx emission of the diesel-electric hybrid buses in operation condition are poor satisfaction of the requirements of the SCR system control strategy and the SCR catalyst's low temperature conversion efficiency. Whenever the speed of the hybrid buses was above 40 km·h-1, the SCR catalyst outlet temperatures were higher than the 230℃, and the NOx emission concentrations were significantly reduced, as the urea injector working proportion and urea quantity increased. In the winter, the SCR catalyst outlet temperature and urea injection quantity dropped with the reduced environment temperature, which led to increased NOx emissions.
Keywords: NOx emission; ambient temperature; bus; hybrid; remote communication technology; selective catalytic reduction system (SCR system).