| Summary: | In China, where in-use heavy-duty diesel vehicles producing NOx and particulate emissions for air pollution are required to undergo emission spot inspection at check stations, it is significant to adopt a simple method to evaluate emissions due to traffic jams, especially in big cities. To realize convenient vehicle emission spot inspection, this paper investigates the effects of exhaust temperature on the Selective Catalytic Reduction (SCR) de-NOx conversion efficiency and presents a method for evaluating the SCR operation state based on ammonia storage characteristics. The paper proposes a real-road driving test procedure and verifies it by measuring NOx from a heavy-duty diesel vehicle in an on-road driving test. The results show that the SCR de-NOx efficiency experiences three operation states. In state I, SCR works and injects urea, resulting in high de-NOx efficiency (>90%); state III occurs during the cold, starting with the lowest de-NOx efficiency (<50%) due to a lack of NH<sub>3</sub>; and state II is a transition stage caused by the ammonia storage, with a certain conversion efficiency (50–90%) when SCR de-NOx efficiency linearly relates to exhaust temperature. Whether the SCR works normally can be judged based on the SCRs operation states. This method is simple and easy to implement in different SCR operation states, and it is effective and repeatable.
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