Clean Diesel Exhaust Systems and Dieselgate

How the exhaust systems on 2.0L TDI “Clean Diesels” work, and why they matter if you want to understand “Dieselgate” and how VW cheated emissions testing.

In 2009, VW released its new “Clean Diesel” TDIs, which were designed to meet the changing  BIN5/LEV2 emissions requirements implemented by the EPA. VW TDIs were unable to meet these changing requirements in 2007 and 2008, so no TDIs were released in those years, and time was spent developing a new exhaust system instead. It was supposed to (had it actually been used correctly) solve the nasty emissions problems commonly associated with diesel engines--large amounts of soot, nitrous oxide gas, and excess carbon monoxide. Not only would make diesel emissions clean, the exhaust system could do it and retain the TDI’s famous power and economy.

Clean Diesel Exhaust System


The exhaust system contains the following components:

  • Oxidation Catalytic Converter
  • Particulate Filter
  • Nitrogen Oxide Catalytic Converter
  • H2S Catalytic Converter

During normal operation, the exhaust gas passes through the components in order. Each part has a different function in purifying exhaust gases.

The most important part to understand in relation to Dieselgate is how the DPF (Diesel Particulate Filter) functions. The DPF requires heat in order to filter out soot and change the chemical makeup of the exhaust gases. It has three different phases of function, and these in turn dictate how much fuel is needed and when it needs to be injected. The phases are:

  • Passive Regeneration
  • Warm-up Phase
  • Active Regeneration

During Passive Regeneration, the fuel is injected normally. However, during the Warm-Up Phase, a change in fuel injection occurs:

“To heat up the cold oxidation catalyst and particulate filter as quickly as possible and thus bring them to operating temperature, the engine management system introduces a post-injection after the main injection. This fuel combusts in the cylinder and increases the combustion temperature. Through the air flow in the exhaust gas tract, the resulting heat reaches the oxidation catalyst.”

--VW Self Study Program 826803

During the third phase, Active Regeneration, an additional post injection occurs, and this fuel actually vaporizes in the the combustion chamber, so even more heat is available to the DPF filter.

 Cutaway of DPF


These changes in fueling and determinations about when to activate each phase of regeneration are all controlled by the ECM. Which is where VW’s emissions cheating occurred--when the car determined it was being tested, it increased fueling for the warm-up cycle and for the active regeneration. The amount of NOx emissions, and how well the NOx filter is able to do its job, is a direct result of the gas conversion that takes place in the DPF. To conserve fuel, VW decreased fueling for the warm up cycle and active regen, resulting in higher NOx emissions but better fuel economy. The DPF system was still able to trap particulate matter and regenerate, but overall it didn’t convert as much exhaust gas (to prep the exhaust gas for the NOx filter later on) because it wasn’t up to temperature.

In Europe, affected diesel vehicles are only receiving a software update, which, as you could guess, will negatively affect fuel economy. How much fuel economy they will lose is yet to be seen. In the US there’s been a lot of loose talk about retrofitting cars with AdBlue tanks, which would be one potential fix. Adblue injection, or urea injection, breaks down the NOx gases and allows the NOx filter to work more efficiently. 2012+ Passats are equipped with Adblue. However, adding Adblue onto a vehicle would cost VW a huge sum of money in parts and labor. If the fix in Europe is any indication, and based on what we already know about how these exhaust systems work, the 2009+ 2.0L TDIs should only require a software fix at a cost to fuel economy.