When the VW/Audi group were caught cheating their exhaust emission tests I added a discussion about the waste ALL car manufacturers seem to encourage. Recent experience with certain cars, specifically Peugeot & Renault diesels, makes the VAG debacle pale into insignificance in my view!
The later diesel engines have a filter in the exhaust system which cuts the output of diesel soot into the atmosphere known in France as a FAP & in the UK a DPF (diesel particle filter). Peugeot's web site describes the system thus:-
"If you buy an HDi Peugeot, or one of our diesel hybrid vehicles, the chances are it will have a Diesel Particulate Filter. And while you’re driving around, the DPF (or FAP in French), as it’s known, will be cutting the soot coming out of your engine by 99.9%.
For the geeks among us, it works like this: the FAP catches diesel particulates in a silicon carbide filter. It then eliminates them by burning them at a high temperature in a ‘regeneration’ process controlled by the engine’s computer.
Peugeot developed the world’s first FAP engine back in 2000 for the 607 – it was hailed as a technological breakthrough in environmental protection.
And, just to reassure you, a FAP has no effect whatsoever on driving performance and only needs servicing every 110,000 miles."
Sounds wonderful, doesn't it? What is not mentioned is that the filter needs to clean itself & this is automatically carried out by the car. A special fluid is injected into the fuel line causing the exhaust to burn extremely hot thus burning all the soot in the filter. It relies on the exhaust temperature being high enough to automatically burn off, (motorway driving) and or using the ECU to alter the vehicle timing to control the process. In city driving or short trips cleaning does not take place fully, leading to blocking of the filter.
Recently I have had a couple of Peugeots in with "anti pollution fault" lit. One of the causes can be a blocked FAP filter. These cars rarely travel more than a few kilometers a day so will never get hot enough to automatically clean themselves. Peugeot computers can make a car clean its own filter if it is not too blocked.
So the filter just saves up all the soot in the filter & chucks it out when no one is looking (at speed) but if the car is doing a forced clean at the dealers, this is what it looks & sounds like.
So thats where all the soot went!