SageBrush wrote:IssacZachary wrote:I read somewhere recently that the main VW defeat was to not send unburned fuel to the NOx trap for reducing the NOx. Is this true?? It sounds plausible. I wonder why they can't just use DEF instead.
There are multiple reasons of varying importance
You have heard most of them, but I think one under reported (and the most important) is that the pollution control devices have a short lifetime.
SCR doesn't deteriorate AFAIK. DPF does.
For NOx treatment, either LNT or DEF. Both will last more than DPF, which lasts around 160 000km before it needs maintenance (ash removal).
SCR uses DEF fluid to clean exhaust. Fluid is very cheap but system costs significantly, especially noticeable on cheaper vehicles.
LNT uses diesel to clean exhaust. System is cheap but additional fuel burned is nonsense (very expensive). This is what VW did - reduced fuel consumption. Which, on the other hand, reduces all other pollutants, excluding NOx.