I wonder if cetane has any affect on oil ash build up. Anyone know if higher concentrations of cetane increases the burn temperature of diesel fuel? Would a better burn produce less ash? I've been using a good additive from day one, but I don't see my DPF accumulation data being any different than the rest of the posts.
I believe cetane is very important.
VW in Australia recommends diesel with minimum cetane number of CN51 (CZ51 in german) for DPF equipped vehicles and CN49(CZ49) for non-DPF vehicles. Standard diesel here is minimum cetane 46, typically higher, but borderline on the CN51 VW recommendation.
I presume the 120k mile DPF inspection time is based on tests using fuel that meets the VW CN51 standard. EN590 diesel standard also specifies the same CN51 since 2004.
Higher cetane fuel results in less smoke. On our non-DPF PD TDI, the smoke is readily visible on heavy throttle and standard fuel. A cetane improving additive (apparently usually 2EHN) or a "high cetane fuel" reduces visible smoke using the "smoke in the following vehicle's headlights as seen in the rear vision mirror at night" test.
BP launched in 2008 in Australia an "Ultimate Diesel" product which contained a nitro-type (probably 2EHN) cetane improver. Two years later, they removed it due to safety risk assessment - presumably explosion risk of undiluted 2EHN at fuel storage sites. So there are no out of the pump diesel products in Australia that meet the CN51 specification.
2EHN safety information:
http://www.atc-europe.org/showdoc.asp?doc_id=294
2EHN info from a large manufacturer:
http://www.eurenco.com/en/2ehn/index.html
I read a research paper that found 2EHN alone as an additive increased injector fouling, so I believe 2EHN must be used in an additive which also contains an injector cleaner. A local diesel mechanic uses both a smoke-reducing additive and a periodic injector cleaner (BG 244K) every 7500km (<5000miles).
So, I believe that fuelling a DPF vehicle with fuel which does not meet the VW CN51 specification will result in more smoke. More soot gets DPF-regenerated to more ash, which builds up in the DPF, until it fails. Where fuels have a CN<51, which I understand is most of USA, which is worse than most of Australia, DPF life must be significantly shorter than engineered for CN>=51. At least in the USA, you have an extended DPF warranty, presumably part of the compromise to permit sale of DPF-equipped vehicles in a country where the fuel does not meet VW/VAG specifications.
Unless evidence appears to the contrary, I continue to run a cetane improving (smoke reducing) additive in both our diesel cars.
A poster on an Australian VW forum recently had a DPF failure at 75000km on Mk5 Golf GTD and still under warranty, but VW refused to repair under warranty claiming the DPF is a "consumable item". DPFs cost about AUD2500, compared to a few hundred euros via ebay - something odd going on there - ?price gouging or inferior second quality products being sold on ebay.
At this point, I believe the early DPF failure problem is caused by low cetane fuel not meeting VW specification CN51. In markets where such fuel is not readily available (non-EU), VW is dodging liability for releasing cars with DPFs which are bound to fail early and at great expense to unsuspecting owners.