So if I’m understanding this correctly, the SCR ‘cleaning’ process uses DEF and the DPF Regen uses diesel fuel. Both processes occur independently of each other.
This is my 3rd diesel but never had all this emissions stuff to deal with.
The 09 TDI required no DEF and the 06 Cummins 5.9 (2nd owner) was modified before I bought it so it never had any emissions stuff on it, no Cat, just a Magnaflow muffler and 4” exhaust (no emissions testing in my part of the state).
Never had a CEL in either vehicle with over 400,000 miles combined.
Your 09 Jetta had the LNT system.. that was an NOx Absorber, and it would absorb NOx (until tested!, it was not doing many regens, due to the cheat) then it would regenerate using fuel in the exhaust stream to react with the NOx to produce N2 and water... making the NOx "trap" ready for more action.
The SCR is totally different.. it is a catalyst bed, where the DEF can react with NOx in the exhaust stream. It never absorbs NOx, so it is never needing a "clean". The reason why the EPA mandates the warnings and reduced power if the DEF runs out, is because without the DEF, there is no absorption of the NOx, and hence no controls of NOx emissions, or very little, the EGR is another way to limit NOx production in the combustion chamber, and the new Diesels are running lower compression ratios which also helps limit NOx in the combustion process. EGR has been very problematic in other ways, and they are looking to not use EGR in the future, but I thing SCR will be around for the long haul.
Your 06 Cummins would not have been made with any of this stuff, the requirements came in force in 07.5, and the first trucks had many, many issues.
The DPF Regen uses fuel primarily to attain the very high temperatures needed to complete the burn on the accumulated soot, the fuel use in that case is to get the temperatures up to burn the soot.. where with the LNT trap temperature was only part of the action, the other was the need for the un-burned hydrocarbons to react with the absorber to chemically combine to produce N2, CO2, and water.. and with that clear out the accumulated NOx in the trap.
The LNT on my Cummins ('09) requires more than 30 minutes to complete a regen, on the Gen 1 Cruze its at most 20 minutes, and that difference is largely due to not having the LNT, the SCR is much better technology, but more complex, more expensive, and hence the VW cheat which literally saved $330 per car.. had they done SCR back in 2009, and not cheated they would have been much better off.. but they also saved $600 per car to use a very crappy HPFP that had epic failures... so I'm glad I got rid of that car fast once the data started adding up.