State-of-the-art diesel engines are now widely used for light-duty passenger cars due to its high efficiency and driveability, even by recognition for its environmental friendliness. The introduction of common-rail fuel system, variable geometry turbocharger, highly precise injector, diesel particulate filter and advanced fuelling logic is making diesel engines from dirty, noisy, slow into clean, powerful, pleasant ones. In Europe and South Korea, diesel cars are now very popular among various vehicle purposes.
European Union, where the share of the diesel vehicles in passenger car segment are over 50%, leads world’s pollutant regulation for diesel cars with Euro-series emission regulation standards. In case of particulate matter emissions, the regulation limit was 0.025g/km in Euro 4, the limit is now 0.045g/km in Euro 6. To meet this regulation, most of diesel vehicles are equipped with particulate filters nowadays. On the other hand, the nitrogen oxides (NOx) emission standard of Euro regulation diminished from 0.50g/km in Euro 4 to 0.18g/km in Euro 5 by 64%. Although all the vehicles on the road now satisfy the NOx limit in the vehicle emission certification test, many researchers have insisted that the vehicles emit much more NOx to the atmosphere when they are out of the certification facilities. Nitrogen oxides cause acid rain, photochemical smog and forming ozone in the air which triggers respiratory diseases and eye problems to humans so that it is needed to be regulated strictly.
Due to the limited vehicle test capability of laboratory certification tests, new vehicle certification techniques in and out of the testing facilities were developed and will be deployed in September 2017 with implementation of Euro 6c. First, in-door test New European Driving Cycle (NEDC) procedure will be replaced with Worldwide Harmonized Light-duty vehicle Test Procedure (WLTP), which represents more likely to the real-world driving conditions. Second, Real Driving Emission of Light Duty Vehicle (RDE-LDV) category will be added to the vehicle certification. Therefore, almost every diesel vehicles in future will needed to be equipped with de-NOx after-treatment systems, so the importance of the characteristics of after-treatment systems is getting critical.
Four vehicles categorized as Euro 6b were tested with WLTP in the laboratory and on-road vehicle RDE testing route in Seoul vicinity in some different conditions to investigate the engine-out NOx emission characteristics and the after-treatment system performance. With the test results, the de-NOx performance of catalysts itself and its temperature-caused effect along with raw-NOx emission characteristics will be discussed with two kinds of NOx after-treatment systems - Lean NOx Trap (LNT) and Selective Catalytic Reduction (SCR) system. The raw-NOx characteristics and a datum for minimum RDE compliance due to the temperature and engine operating conditions will also be handled.
The vehicles test were carried in and out of the laboratory. The WLTP test and some other test protocols were performed with ambient temperature of 23℃, 14℃ and -7℃ to confirm the temperature effect. Meanwhile, they were tested through the RDE-LDV test route composed of urban, rural and motorway sections. The NOx sensors were fitted to the several points of the vehicle’s exhaust systems for monitoring the NOx concentration and its reduction performance, Portable Emission Measurement System (PEMS) was used in the RDE-LDV test to measure the various gaseous emissions at the tailpipe of the vehicle.
The vehicle test results show that the engine-out NOx emissions are directly affected by the ambient temperature and the driving tendency. If the temperature of the intake air varies from the standard condition, the NOx emission will rise dramatically. And there are particular load band where the NOx formation rate is notably high. The performance of de-NOx after-treatment systems are also affected by the temperature. The NOx reduction ratio of LNT on regeneration phase were critically related to the catalyst and exhaust gas temperature. The overall NOx reduction ratio of LNT is decided by the capacity of the catalyst and exhaust gas temperature. On the other hand, the SCR system has relatively low-sensitivity to the temperature compared to the LNT system, which shows better NOx reduction performance even in cold-start conditions.