Just to clarify on a couple of things that I read above.
Firstly:
The DPF will only start a regen when:
a) The soot level in the DPF reaches a certain trigger point. According to VCDS, my 184 is 23.37 grams of soot.
b) Other conditions are met: coolant temp high enough, oil level not too low, diesel not less than 1/4 tank. There are others that I can't remember that will stop a regen taking place.
So - you drive 100 miles on the motorway and reach say - 23 grams of soot. Based on that, a DPF regen will not take place, even if you hammer the car - it's just not ready. You then drive those last few miles home and trigger a regen. Seems unfair doesn't it but that's how it works. Driving style has nothing to do with it at this point. If it aint ready, it aint ready.
The main thing is, if the DPF is not ready to regen (not reached the necessary soot level), then it doesn't matter that you were on the motorway etc. 5 mins ago doing 100mph, it will regen when it reaches it's trigger point. It's not an Italian tune up.
That's where VAG DPF is really good - you can see your soot levels building and basically guestimate when it will do a regen, or if it's doing one, how long you need to drive around until it's done. During a regen you can actually see the soot level dropping and it tells you when it's done.
Secondly:
Ref "how fast to drive during a regen"
It's not really about speed but exhaust temp and that is influenced by engine revs.
So - a nice hot DPF at regen is about 650 celcius. Some people think that you need to hammer the car to achieve that (fun, but not necessary). Ideally, the revs need to be about 2500+ to maintain that temp. You can drive at 50-55mph in 4th doing about 2500 revs and a passive regen can take place in 10-12 minutes.
I've done regens at 40, 50, 60 and 70mph. The one constant is the revs - keep those up above/around 2,500 and the temp stays up and the regen will finish. Just pick an appropriate gear. With VAG DPF, you can see the temp during a regen and if it starts to dip below 600, you put your put down for a few seconds and after a slight delay, the DPF temp goes back up. Maintain the revs and it will stay at temp, if the revs drop the temp drops.
An active regen is different as the car is artificially heating the DPF because you aren't driving it like you stole it or your driving type just doesn't allow it to do a passive regen. RPM's idle at 1,000 and the car effectively cooks the DPF to get it to temp. It does smell, stop/start is turned off etc. This sort of regen tends to take longer and does burn more fuel.
The technical bit that I don't fully understand is that an active regen can cause extra diesel used to heat the DPF ending up mixed with engine oil but I don't know to what extent or the exact causes (some say incomplete regens can do this).
So - a regen only takes place when the DPF reaches a trigger point. It's sods law if it happens 1 mile from home. Engine revs/temp is more relevant than speed to a successful regen.
VAG DPF will
help you indentify DPF regens and you can go for a spirited spin around the block to
help finish it off (NOTE: driving at warp speed will not make the process go any quicker).
My car on 110,000 miles and does a regen every 120 to 140 miles. It used to be 180 miles when commuting 20 miles each way. Now I do 5-8 miles most of the time, it reaches it's trigger point sooner as the car produces more soot when cold. I also think that the DPF is filling up so it regens more anyway.
Sorry about the essay. Someone might find it useful.