The effect of after-treatment techniques on the correlations between driving behaviours and NOx emissions of passenger cars

Nitrogen oxides (NOx) emissions from passenger cars are significantly dependent on after-treatment techniques, cold start conditions, and driving behaviours. The widely-used selective catalytic reduction (SCR) technique has low effectiveness of reducing NOx emissions during the cold start period or at low speeds. As a new technique, ammonia creation and conversion technology (ACCT) system can further decrease NOx emissions to achieve cleaner travelling. The correlations between driving behaviours and NOx emissions are different for vehicles equipped with SCR and ACCT systems over cold start and warm start driving cycles. In this paper, the effect of SCR and ACCT systems on the correlations between driving behaviours and NOx emissions were explored based on worldwide harmonized light vehicle test cycles (WLTC). The results indicated that vehicle fuel consumption rates almost linearly increased with acceleration when the acceleration was higher than −0.5 m/s2. For cold start situations, vehicle speed range of 55 km/h∼ 100 km/h presented the lowest NOx emission factors over SCR and ACCT scenarios. SCR efficiency was lower than 60% for the vehicle speed range of 25 km/h∼ 40 km/h, and the efficiency of ACCT system was higher than 65% in the corresponding speed range. Regarding the warm start scenarios, average catalyst efficiency was higher than 80% for SCR system over the speed range of 25 km/h∼ 40 km/h, and ACCT efficiency was higher than 95% for all the speed and acceleration ranges. Applications of ACCT systems provided a possibility of meeting stricter emission regulations, and the relationships between driving behaviours and NOx emissions provided the guidance for eco-driving to achieve cleaner travelling.