Diesel particulate filters are almost mandatory on European diesels (hard to pass new emissions standards without one), but I'm not sure how eco-friendly they are on a global scale. Yes, they reduce particulate matter almost to zero, but they need to be burned out every now and then. When initially testing emissions and fuel economy for a new model, the car is new and the DPF is not clogged. I wonder how those tests would go during a burnout?
For PSA diesel engines (Peugeot/Citroën, but also certain Fords, Volvos, Subarus) there is a fuel additive called Eolys, which is about 5% cerium and 80%-90% hydrocarbons. When the ECU initiates burnout, fuel is added to raise exhaust temperatures, and the Eolys fluid is injected into the fuel going to the engine, lowering the temperature at which PM starts to burn (from 550°C to 450°C). The cerium does not burn, but stays in the filter (I believe it binds whatever is left over after burning the PM). After a while, this cerium residue starts to clog the DPF and it should be replaced after 120k km.
VW also has a DPF technology with a urea-based fluid.
So, at its best, a PF diesel is very clean. But if it is driven efficiently (=low exhaust temperatures), then it will be burned out quite often (=more fuel+additive) and will need a replacement filter sooner (=extra resources, not to mention costs). EO(ff)C-ing a diesel with a PF may lower the already low temperatures even further, but then new, hi-tech
turbodiesels shouldn't be cycled on and off too often. If you go racing on the Autobahn every now and then, then you're OK, but for everyday driving your ECU is going to start adding fuel (and fuel additives) every so often.
Another question: If we are concerned about catalyst cooling, then should we be DFCOing? When engine braking, air is pumped through the engine, then through the cat, cooling it from the inside. Yes, compressing it inside the cylinders raises its temperature somewhat, but upon exiting it is still much cooler than during normal combustion, and so also cooler than the cat.