Volkswagen is working on reducing NOx by 80%

NOx emissions from Volkswagen engines will be reduced by up to 80% through dual SCR technology.

The German brand is working on reducing NOx by up to 80%, and according to the carmaker, the SCR exhaust gas aftertreatment systems have been used in Volkswagen models with diesel engines. SCR technology (selective catalytic reduction) significantly reduces nitrogen oxides in the exhaust gas.

Now, Volkswagen has developed the next evolutionary stage of the SCR system, called “dual dosing”. AdBlue is selectively injected upstream of two SCR catalytic converters arranged in series. The system is used in the new Volkswagen Passat 2.0 TDI Evo with 150 CV, which already meets the technical requirements of the future Euro 6d emissions standard.

Current RDE (real driving emissions) measurements confirm Volkswagen’s type approval: in the new 2.0 TDI Evo engine with dual dosing, NOx levels are reduced by approximately 80% compared with the previous generation of the respective models.

Volkswagen bets on NOx reduction in its engines

Volkswagen will now gradually introduce the new technology across all models with 2.0 TDI Evo engines. After the 2.0 TDI Evo with 150 CV currently fitted to the Volkswagen Passat, the new Volkswagen Golf, whose world premiere is coming soon, will also feature dual dosing in all TDI variants.

The dual dosing process requires a second SCR catalytic converter located at the underside of the vehicle. Since the distance from the engine is greater, the exhaust temperature upstream of the second catalytic converter can be up to 100 °C lower. This widens the window for exhaust gas aftertreatment: even at exhaust temperatures close to the engine of +500 °C, the system can still achieve very high conversion rates. In addition, a blocking catalytic converter downstream of the SCR system prevents excess ammonia slip.

The innovative dual dosing process compensates for a system-related disadvantage of diesel engines. Modern diesel engines emit less CO2 than petrol engines because diesel fuel has a higher energy density and the combustion process is more efficient. However, diesel engines are also subject to special requirements, since fuel combustion takes place with excess air. The main component of air is nitrogen, and this reacts with oxygen during combustion, thus forming nitrogen oxides.

Ammonia is needed to reduce the nitrogen oxides produced in diesel engines. It is injected as an aqueous reducing agent (AdBlue) through a dosing module into the exhaust gas upstream of an SCR catalytic converter. Here, the solution evaporates; the reducing agent is decomposed and combined with steam to form ammonia. In the SCR catalytic converter, the ammonia reacts on a special coating with the nitrogen oxides to form water and harmless nitrogen, the main component of the air we breathe.

NOx reduction with dual SCR system for emissions

In existing exhaust gas aftertreatment systems, a closely coupled SCR catalytic converter is located between the turbocharger, the diesel oxidation catalytic converter that converts unburned hydrocarbons, and the flexible connecting piece to the silencer pipe. The SCR coating is applied to the honeycomb structure of the diesel particulate filter, allowing a single component to perform several functions. The close-coupled arrangement means that the exhaust gas temperatures required for high conversion rates can be reached quickly after a cold start: the ideal range for conversion rates above 90 percent is between +220 °C and +350 °C. These conditions are met in many operating situations.

Conversion rates do not drop above +350 °C, thanks to the dual dosing system. Temperatures at this level occur, for example, when driving at high speeds on the motorway, at high engine speeds over prolonged periods, and when driving uphill, especially if the vehicle is fully loaded or towing a trailer.