The emissions system at the heart of the global Volkswagen diesel mess comprises many complex components, all of which have to work perfectly and consistently all the time, or the vehicle will fail to meet pollution standards.
Here are the components VW used on its 2009-15 diesel-powered vehicles and the functions they are designed to perform:
- Lean NOx trap, or LNT: Placed in the exhaust system downstream of the engine’s exhaust manifold, this device stores nitrogen oxide, or NOx, when the engine is running lean. When the trap begins to fill, the engine switches to a richer running mode -- that is, the fuel-injection system sprays more fuel into each cylinder than is needed to propel the car. The richer fuel mixture causes the stored NOx in the LNT to be converted into nitrogen. It is then released from the LNT into the exhaust system.
- Diesel particulate filter, or DPF: Also, located downstream of the exhaust manifold and typically after the LNT, the DPF traps soot or fine diesel particulates. That usually is burned off when the engine is running at high speeds, such as on the highway.
- Oxidation catalytic converter: Located near the exhaust manifold, it reduces hydrocarbons and carbon monoxide emissions.
- Selective catalytic reduction, or SCR: A tank, usually in or near the trunk, that contains urea, or diesel exhaust fluid, that periodically is injected into the exhaust system just after the oxidation catalytic converter. According to the Diesel Technology Forum, urea sets off a chemical reaction that converts nitrogen oxide into nitrogen, water and traces of carbon dioxide, or CO2.
- Exhaust gas recirculation, or EGR: Located on the cylinder head, exhaust manifold or intake system, a valve plumbed into the exhaust system routes a small amount of exhaust gas back into the engine to cool the combustion chamber. Because diesel combustion occurs at very high temperatures, it produces higher NOx than gasoline engines. EGR helps reduce NOx emissions.
- Engine control module, or ECM: Located inside the car or under the hood, this is the computer that manages the fuel-injection, ignition and emissions system as well as the automatic transmission. VW cheated by programming the ECM to have two modes. The illegal “dyno” mode detected when the car was being tested on a dyno, such as during an EPA or yearly state emissions test, and operated the emissions system to ensure the NOx emissions were at or below the legal limit. The “defeat” mode became active when the vehicle was driven on the street, switching off key parts of the emissions system.