Rethinking NOx emission factors considering on-road driving with malfunctioning emission control systems: A case study of Korean Euro 4 light-duty diesel vehicles

Abstract

This study aims to quantify the differences between the standard laboratory-based NOx emissions factor (EF) and on-road measurements, and to estimate how much NOx emissions exceed the current EF under typical Korean road traffic conditions. Four Euro 4 light-duty diesel vehicles (LDVs) manufactured in Korea were driven on a chassis dynamometer and on test routes that included urban and rural roads and a motorway. NOx emissions, average speed, and acceleration were measured and calculated for each 1 km trip. We focused in particular on the possibility of vehicle malfunctions causing increased NOx emissions, and compared NOx emissions before and after the repair of faulty exhaust gas recirculation (EGR) valves, which are a key NOx reduction system for LDVs. The current NOx EF for Euro 4 LDVs only estimates 27–31% of on their on-road NOx emissions, and low acceleration during the standard driving cycle is a part of the reason for this weak representativeness. Another is the lack of tools with which to monitor EGR valve degradation. Even with fully functioning EGR valve hardware, the capability of vehicles to control NOx emissions differ in laboratory and on-road conditions; it is weaker for the latter compared to the former because the software controls are different. The findings here provide insights for different regions where Euro 4 LDVs are still in use, depending on their age. For Korea and the EU, where the fleet is old, accurate assessments of the real risk of NOx emissions are beneficial in terms of increasing the effectiveness of control measures. In countries where the Euro 4 fleet is still young, comprehensive changes are recommended for the on-board diagnostic system and the subsequent maintenance of the hardware and software of EGR valve.