I've been making some further observations of the DPF via VAG-COM since the OP, and have learned a few things:
1. Somewhat counterintuitively, the value for "Particle filter carbon mass (act.)" never changes; it stays at zero, day-in and day-out. I'm guessing this would be the value determined by an actual carbon sensor if the DPF had one, which it apparently does not.
2. On the other hand, the value for "Particle filter carbon mass (spec.)" does change, on the order of once every 5 minutes or so, while you drive. It can go up, or it can go down. As I mentioned in my OP, when I first discovered this parameter its value was 14.4. This morning, when the missus and I went out for brunch, its value was 12.0 prior to starting the engine (still only 3 regens shown by VAG-COM, so its decrease was not caused by a regen), and over the course of a 20 minute mostly-city drive it increased, in three steps, to 12.9. I recall reading sommers that ECU determines when a regen is needed based on two things: (i) the pressure differential across the DPF, and (ii) the results of a
model of carbon accumulation based on your driving profile. So I'm guessing that "Particle filter carbon mass (spec.)" stores the predicted carbon accumulation calculated by this model.
3. OK, this one's really interesting. When you log the pre-DPF and post-DPF temperatures while driving, when the engine is under load (accelerating or climbing a hill) you'll observe that the pre-DPF temperature is higher than the post-DPF temperature...no big surprise here. BUT...when the engine is
not under load (while coasting, driving on the flat with no acceleration, or idling) the pre-DPF temperature drops more than the post-DPF temperature does, so that the post- temperature actually ends up (and stays)
higher than the pre- temperature. The graph below documents this (data is from a fully warmed-up engine that had been idling for about 3 minutes before data collection started):
(Between 140 and 180 seconds on this graph I brought the RPMs up to 2000 (while still in Park), because I want to use the pressure differential at 2K RPM in Park as my standard measure of DPF obstruction in future).
Notice how the post-DPF temperature stays 12-15 *C higher than the pre- temperature (except while I'm revving the engine)? There are only a few things that could account for this:
(a) The DPF has an electric heater. Not a chance. Fuggedabadit!
(b) Condensation of vapor to liquid within the DPF is heating it. This is formally possible, but I don't believe it. How much condensation would you be seeing at around 200 *C? And if you were seeing enough to heat the DPF by 12-15 degrees, you'd have liquid pouring out of your tailpipe.
(c) By far the most plausible explanation is that the DPF experiences some measure of
passive regeneration during normal operation...i.e., some soot is oxidizing continuously inside the DPF, which would indeed heat it somewhat. In my recent readings regarding DPFs I've seen it mentioned several times that passive regeneration can indeed occur, but it is more or less efficient in different exhaust system designs. It is
most efficient when the NOx trap or cat is placed
after the DPF, because NOx is a strong oxidant and so will promote some oxidation of soot in the DPF at normal operating temperature. If I'm reading correctly the picture of the exhaust system
recently posted by Drivbiwire, that appears to be the case in the '09: the NOx storage catalyst is downstream of the DPF(?).
So I'm thinkin' this is a good thing! The more
passive regeneration the DPF does, the less
active regeneration it needs...and, thus, the less of a hit it imposes on fuel economy, since active regen requires extra fuel supplied via post-combustion injection, whereas passive regen is essentially free. Yay, VW!
Something else which this observation suggests is that the more you drive in such a way as to stay out of
the smoke zone, the less soot you deposit in the DPF, and thus the better its chances of keeping itself unobstructed via passive regeneration rather than requiring an active regen...and, thus, the better your MPGs. So there's yet another way in which smoke kills your fuel economy, folks.
