NOx emissions from Euro IV busses with SCR systems associated with urban, suburban and freeway driving patterns

• NOx emission of two Euro IV busses with SCR was measured using PEMS.
• NOx emission factors in terms of g/(kW·h) and g/km under urban, suburban and freeway driving patterns were acquired.
• Results showed that NOx emission factors cannot meet Euro IV standard limits on three driving patterns.
• Measured NOx emission factors were higher than NOx predicted by IVE model.
• Adjustment of NOx emission factors is needed to improve the estimation of Euro IV vehicle emission in China.

NOx and particulate matter (PM) emissions from heavy-duty diesel vehicles (HDVs) have become the most important sources of pollutants affecting urban air quality in China. In recent years, a series of emission control strategies and diesel engine polices have been introduced that require advanced emission control technology. China and Europe mostly have used Selective Catalytic Reduction (SCR) with urea to meet the Euro IV diesel engine emission standard. In this study, two Euro IV busses with SCR were tested by using potable emission measurement system (PEMS) to assess NOx emissions associated with urban, suburban and freeway driving patterns. The results indicated that with the SCR system, the urea injection time for the entire driving period increased with higher vehicle speed. For freeway driving, the urea injection time covered 71%–83% of the driving period; the NOx emission factors from freeway driving were lower than those associated with urban and suburban driving. Unfortunately, the NOx emission factors were 2.6–2.8-, 2.3–2.7- and 2.2–2.3-fold higher than the Euro IV standard limits for urban, suburban and freeway driving, respectively; NOx emission factors (in g/km and g/(kW·h)) from the original vehicles (without SCR) were higher than their corresponding vehicles with SCR for suburban and freeway driving. Compared with the IVE model results, the measured NOx emission factors were 1.60–1.16-, 1.77–1.27-, 2.49–2.44-fold higher than the NOx predicted by the IVE model for urban and suburban driving, respectively. Thus, an adjustment of emission factors is needed to improve the estimation of Euro IV vehicle emissions in China.