This FAQ article describes how the DPF and Adblue system works, discusses potential problems like filter clogging, how to manually activate an active regeneration cycle, and other FAQ. It was written for TDI but the basic description of the DPF can also apply to BMW 335d, BMW X5 xDrive35d, Mercedes Benz GL, ML 320 or 350 BlueTec, E320 Bluetec, or any diesel car that has DPF or Adblue. There were some other makes like Peugeot that used a fuel additive for NOx control but none of the cars in the US Canada use this system.
Pre clean diesels only had an oxidation catalytic converter for emissions. The V10 Touareg TDI had DPF (except for 2004) but no Adblue. Ultra low sulfur diesel ULSD was the key in the clean diesels - older fuels can clog the exhaust filters. Because the reduction in sulfur is 97%, it gives you an idea of how dramatic the change was. All new Audi TDI and VW TDI have DPF. Larger engines or heavier cars also use Adblue fluid sprayed into the exhaust to cut NOx emissions by up to 80%.
Since the article is pretty long, here's how it's organized:
There are 2 basic types of clean diesel exhaust systems on TDI. Those with adblue (2009+ VW Touareg TDI and Audi Q7 TDI) and those without (2006-2008 Touareg TDI and all other 2009+ TDI). The larger and heavier cars with V6 engines need adblue to meet emissions.
All modern clean diesels have 3 major emissions components: an oxidation catalytic converter, a DPF, and NOX catalyst (with or without Adblue). Each of these are described in detail in the FAQ article linked above. There are also numerous O2, temperature, and backpressure sensors. Because of these sensors, there is currently no way to delete the DPF or do a DPF bypass without a rewrite of the engine management computer.
Some interesting facts about Adblue fluid that I learned while writing the article: Adblue must be stored between -11oC and 55oC (12-131oF). Optimum storage conditions are in a cool, well ventilated room between -5-30oC ( 23-86oF). Below -11oC, the solution will separate into urea and ice crystals. This is why the storage tank is heated at -11oC. The more you go above 30oC, the more the ammonia will come out and while it's still considered usable, its lifespan is slightly shortened. Above 55oC, a noticeable amount of ammonia will be released and the solution can become unusable.
VW/Audi adblue part numbers
.5 gallons # g 052 910 a2 (g052910a2)
2.5 gallons # gus 052 901 a3 (gus052901a3)
10 liters (2.64 gallons) # g 052 910 a4 (g052910a4)
click the FAQ article linked above or here: http://www.myturbodiesel.com/1000q/DPF-Adblue-FAQ-VW-Audi.htm
for pictures, videos, and more detail.