The Volkswagen Group’s V6 TDI engines for the new Evo 3 generation were mostly developed by Audi. Engineers are forced to increase the effectiveness of exhaust gas purification as a result of ever-stricter emission rules.
This necessitates, among other things, greater design volumes of the catalytic converters in the case of exhaust after-treatment. Audi has managed to combine all technology in a small package with the latest new development for the V6 TDI.
The exhaust gas turbocharger is situated behind the engine, in front of the firewall, where the flow of the two exhaust gas tracts on the exterior of the two cylinder-bank sides converges.
An NSC oxidation catalyst is located immediately downstream in the exhaust system. NOx Storage Catalyst is the name of the substance. A diesel particulate filter with SCR is immediately after it (SDPF).
Selective Catalytic Reduction is known by the acronym SCR. In the exhaust system, underneath the underbody of the car, is where the second SCR catalyst is situated.
Up to the regeneration step, nitrogen oxides can be momentarily stored in the oxidation catalyst (NSC) close to the engine. After a cold start, for example, this catalyst is still effective at low engine operating temperatures.
Through a brief mixture enrichment that is started by the engine management unit, regeneration takes place.
As a result, the catalyst not only briefly stores the nitrogen oxides and then neutralizes them, but it also converts unburned hydrocarbons and carbon monoxide into carbon dioxide and water vapor utilizing the oxygen molecules from the momentarily stored NOx.
The infusion of the AdBlue ingredient initiates a further stage to minimize nitrogen oxides. The entire system is known as twin dosing because this aqueous urea solution is delivered into the exhaust system at two sites where temperatures differ, utilizing one dosing module at each position.
The AdBlue ingredient is then transformed into ammonia in the exhaust system through the chemical reaction of urea thermolysis.
The second SCR catalyst is placed farther downstream in the exhaust system, and the unconverted nitrogen oxides that are present in the ammonia react with each other and the SCR-coated diesel particulate filter (SDPF) that is located near the engine. Since elemental nitrogen makes up around four-fifths of our planet’s atmosphere, this causes the production of water and nitrogen.
The aqueous AdBlue urea solution should be dosed twice for best results. It makes use of the varying conditions in various exhaust system components to boost the system’s overall efficacy while being modified for various operating circumstances.
Audi can transform more than 90% of nitrogen oxides in this way across a broad operating and temperature range. Therefore, double dosing significantly helps to achieve NOx emission limitations.