Combining the NOX storage catalyst and an SCR system in diesel passenger cars to fulfill the most stringent EU emission limits

1. October 2015 | Engineering Service

The upcoming EU emission legislation will challenge engineers to keep the NOX emissions of diesel-powered passenger cars at a very low level under various boundary conditions. A key element to achieving this is the availability of highly efficient  NOX aftertreatment technologies. Hyundai Motor Company (HMC) and FEV investigated the performance of a combined  NOX aftertreatment system.

After-treatment System and Demonstrator Vehicle

The after-treatment system that was investigated within this study consisted of a NOX Storage Catalyst (NSC) that was directly connected to the turbocharger, followed by an SCR-coated Diesel Particulate Filter (SDPF) catalyst. A water-cooled AdBlue® injector with a peak dosing pressure of 50 bar was mounted between the NSC and the SDPF and a mixer unit was located at the front cone of the SDPF converter. An additional SCR catalyst was installed in an underfloor position to prevent NH3 slip. The after-treatment system was installed in a Hyundai Santa Fe SUV with a 2.2-liter diesel engine and a 6-speed automatic transmission. The engine is equipped with cooled, high-pressure Exhaust Gas Recirculation (EGR) including an EGR cooler bypass, an intake throttle, and a single stage variable-geometry turbocharger (VGT).

Control Logic

In order to control the interaction between the NSC and SCR components, new after-treatment software functions were developed. The main modifications in the control logic consist of a slice model for the NSC, an ammonia (NH3) model to predict the NH3 mass flow downstream of the NSC and the provision of NSC control logic for the regeneration events including an integrated state coordinator for the combined NSC and SCR aft er-treatment system.

RDE Measurements

In a first step, the system performance was validated on the chassis dynamometer over various cycles such as WLTC, RTS95, FTP-75, and HWFET. The NOX tailpipe emissions were kept safely below the EU6-limit in the WLTC and even in the highly dynamic RTS95. The results in the FTP-75 and HWFET showed very low tailpipe emissions as well, such that even SULEV30 emission targets would be fulfilled with some further refinement of the DeNOX strategy. Additional Real Driving Emissions (RDE) investigations were carried out on a street route with an installed Portable Emission Measurement System (PEMS). Under smooth driving conditions very low NOX conformity factors between 0.38 and 0.69 were obtained. Additional tests with dynamic accelerations showed, however, how sensitive the NOX emissions are to the traffic and driving conditions, clearly indicating that a thorough definition of the boundary conditions under which a certain emission compliance should be fulfilled is a requirement. “The results of this study are very promising and show that modern high-efficiency diesel engines and highly sophisticated combined after-treatment systems have the potential to reach very low NOX tailpipe emissions even under real driving conditions,” concludes Joschka Schaub, Technical Specialist for diesel powered passenger cars at FEV. As a major advantage of the combined after-treatment with NSC and SCR, a comparatively low AdBlue® consumption between 0.2 and 0.5 l/1000 km was observed. These values would avoid the requirement for a refill by the customer.

Comparison of four different RDE tests

Comparison of four different RDE tests