White paper

Urban off-cycle NOx emissions from Euro IV/V trucks and buses

Despite meeting more stringent regulatory standards for exhaust emissions during type approval, many Euro IV and Euro V heavy-duty vehicles equipped with selective catalytic reduction (SCR) systems have significantly elevated emissions of nitrogen oxides (NOx) during in-use driving, particularly when operating in urban traffic. In some cases, actual in-use urban emission levels may be as high as or higher than those from much older vehicles with engines certified to more lenient emission standards.

These high “off-cycle” NOx emissions threaten efforts to improve ambient air quality in many European cities. And many developing countries, including Brazil, India, and China, have begun or plan to implement standards for new trucks and buses that are based on the European regulation.

This report details the technical reasons for high off-cycle emissions from Euro IV/V heavy vehicles as well as their root cause: deficiencies in the Euro IV/V type-approval process, including an unrepresentative test cycle and lack of a cold-start testing requirement, and weak in-use conformity provisions. It describes a range of technical options available to improve SCR effectiveness at low loads. And it makes specific recommendations for both Europe and developing countries, taking into account differences in how the standard is implemented in different regions. Most of the changes recommended for developing countries are patterned on changes implemented in Euro VI legislation. In addition to a cold-start testing requirement, they include the use of the World Harmonized Transient Cycle (WHTC) during engine type approval and stronger in-use conformity provisions, such as imposition of a specific not-to-exceed limit (g/kWh) for in-use NOx emissions and mandatory in-use testing to demonstrate compliance.