A research on the NO_X reduction effects of SCR systems in city buses

NO_X emissions from three buses with SCR systems are measured by using portable emission measurement system in real-world conditions, the NO_X emission factors in both conditions (with and without urea injection) are compared and their NO_X reduction rates of SCR systems are calculated. The results show that the State-IV diesel bus has the lowest NOx reduction rate of SCR system due to its low exhaust temperature and less volume of catalyst, whose effects gradually become apparent only when vehicle speed is higher than 30km/h, with an average NO_X reduction rate of only 10%; the State-V CNG bus has the highest NO_X reduction rate due to its high exhaust temperature even at low speed and higher proportion of NO₂ in NO_X emission, and its NO_X reduction rate basically does not change with vehicle speed, resulting in an average NO_X reduction rate of 85%; while the NO_X reduction rate of State-V diesel bus is in-between with an average NO_X reduction rate of around 70%. As for the emission of NO_X + HC, the lowest is the State-V diesel bus, the highest is the State-IV diesel bus, while the NO_X + HC emission of State-V CNG bus, due to its high emission of methane, is close to but slightly lower than that of State-IV diesel bus.