Real-world NO_x emission characteristics of China-6 heavy-duty special-purpose vehicles

Special-purpose vehicles (e.g., concrete pump trucks, garbage trucks, water sprinklers) are currently certified under the same emission standards as conventional road transport vehicles, even though certification test cycles are primarily designed for the latter. In practice, the engine loads and exhaust temperatures of special-purpose vehicles differ markedly from those of road transport vehicles, resulting in distinct emission behaviors. Using remote-monitoring data, this study analyzed the driving speed, engine load, exhaust temperature, and NOx emission characteristics of seven types of special-purpose vehicles under real-world conditions. The results show that several vehicles exhibited brake-specific NOx emissions exceeding 0.69 g/kWh, the portable emission measurement system (PEMS) limit. The operating conditions of special-purpose vehicles can be divided into driving states and working states. During driving states, exhaust temperatures generally satisfied the operational requirements of SCR catalysts, keeping NOx emissions below 1.0 g/kWh. In contrast, during working states—when vehicles were often stationary or moving at very low speeds—engine output was primarily used to power auxiliary equipment. This led to sustained low-load operation and exhaust temperatures frequently below the threshold for effective SCR functionality, causing NOx emissions to rise sharply to 5.0–6.0 g/kWh. Since working states typically accounted for over 75% of total operating time, their contribution dominated overall emissions. These findings indicate that the in-use NOx emissions of special-purpose vehicles substantially exceed current regulatory limits. Therefore, emission control requirements tailored to the unique operating conditions of special-purpose vehicles—particularly their working states—are urgently needed.