RDE Challenges Existing Aftertreatment Technology
According to a survey we have recently conducted, the optimization of aftertreatment systems is a key challenge in regard to upcoming Real Driving Emissions (RDE) regulations. Why is RDE such a challenge for the existing aftertreatment technology?
Before the introduction of RDE, the emission compliance in Europe is focusing on the NEDC. This cycle is characterized by mainly low engine load and speed area, resulting in low exhaust temperature level and moderate engine out NOx emissions. RDE will cover a much wider operating range of the engine. Depending on the vehicle power to weight ratio, even full load conditions can be reached at least during accelerations. The engine out NOx emissions increase at these conditions, even when taking into account improved engine technologies like e.g. low pressure EGR systems. Therefore the performance of the aftertreatment system has to increase and at the same time, this increased performance has to be realized in a very wide temperature range, which is a challenge particularly for NOx storage catalysts, which lose their storage capacity at high temperature.
What are the specific strengths of LNT based systems in regard of RDE?
Especially when the LNT is positioned directly downstream of the turbine, it has a very good light off behavior and provides high NOx conversion rates at low loads and exhaust temperatures, which will still be relevant with RDE. Furthermore the LNT technology is very attractive regarding costs and packaging when compared to active SCR systems. Another advantage is that LNT does not need an additive, whereas active SCR systems use AdBlue®. With RDE, the AdBlue®-consumption of active SCR systems will increase and the customer probably has to take care for refill from time to time, which might cause acceptance issues.
How far is the optimization of control models an important step for ideal system performance under real driving conditions?
In order to limit the fuel consumption penalty of LNT based systems, the timing of the rich engine phases, which are required for LNT regeneration, has to be calculated very precisely. Also with regard to secondary emission like HC and CO breakthrough, the regeneration process has to be modeled highly exactly. Therefore, advanced models of the LNT have to build up the NOx loading and unloading behavior as accurately as possible, taking into account various boundary conditions.
What are the advantages of combined LNT and passive SDPF configurations under real driving conditions?
The LNT is producing NH3 during the regeneration at rich conditions. This NH3 can be stored in the SDPF and an additional NOx reduction takes place. Partial shifting NOx conversion from the LNT to the passive SCR is an advantageous measure for decreasing fuel consumption penalty and increasing NOx performance. Especially in transient operation with higher temperatures, the additional NOx reduction can compensate the decrease of the LNT´s NOx storage capacity and the frequency of LNT regenerations can be reduced with beneficial impact on the fuel consumption penalty compared to systems with LNT only.
Taking a look into the future, how much room for performance improvement do you still see for the existing aftertreatment technologies? Given that the planned RDE regulation will probably not be the end of the story, which aftertreatment strategy has the most promising potential beyond Euro 6?
From current point of view, the combination of both active LNT and active SCR systems provides the highest emission performance. The catalysts are still being improved with regard to their temperature dependency. For example, state-of-art Cu-zeolites typically tend to show a lower NOx-conversion at high temperatures, which might be improved in the future. Furthermore the actuators and sensors which are used for the control of the aftertreatment system have to be improved regarding accuracy, robustness and response time. And the layout of the aftertreatment system has to be optimized regarding thermal losses and uniformity of exhaust gas and NH3 entering the catalysts. A very important topic is also the exhaust back pressure increase caused by the exhaust system, which shall be minimized e.g. by using advanced substrates with higher porosity and thinner walls.
Thank you very much.
Dr. Bastian Holderbaum is the Manager for Passenger Car Diesel Engines at FEV GmbH, Germany. He has held positions as a technical specialist for diesel exhaust aftertreatment and passenger car diesel engines.