Comparative investigation of NOx emission characteristics from a Euro 6-compliant diesel passenger car over the NEDC and WLTC at various ambient temperatures

Euro 6-compliant diesel passenger cars have tended to feature lean NOx traps (LNTs) to satisfy the increasingly stringent NOx regulations. This paper focused on NOx emission characteristics by the NOx sensors installed at both before and after LNT from a Euro 6 diesel vehicle on a chassis dynamometer. The vehicle was repeatedly driven according to the new European driving cycle (NEDC) and the world-harmonized light-duty vehicle test cycle (WLTC) at various ambient temperatures (23, 14 and −5 °C). LNT regeneration was detected twice in the NEDC but 5 times in the WLTC due to the longer period, increased mileage and more frequent acceleration in the WLTC. Accordingly, the NOx conversion rate was higher, and the NOx emission factor was lower for the NEDC than for the WLTC. Additionally, as the ambient temperature decreased, the NOx concentration increased considerably. Because of the poor mixing of fuel and air and the reduced combustion efficiency and stability, the exhaust gas recirculation (EGR) rates decreased. Because the LNT did not reach the light-off temperature (LOT) in the cold start phase, the chemical reactions in the LNT did not occur, and only NO was detected. For the LNT regeneration process, the EGR rates decreased, and the fuel rates increased to release the stored NOx in the LNT. The emissions characteristics of the products, such as NO, NO2, N2O and NH3, during regeneration varied depending on the stored NOx, the after-treatment temperature, and the vehicle driving condition. The results from this comparative study can contribute to monitoring the NOx emission characteristics and optimizing the engine management system to meet future emission regulations including WLTC and low ambient temperature conditions.