Someone started a similar thread and there was a good bit of information regarding DPF's. Short of it is they're great in theory but the application for most cars is not ideal, with some causing major issues.
Basically it came down to the DPF being too far away from the turbo, meaning it is much colder, and uses a lot of excess fuel to burn the particles up. There are 2 ways of doing this: 1) install an extra fuel injector just before the dpf or 2) pump extra fuel into the engine with the intention of it passing unburnt into the exhaust. (The second is the cause of fuel in the oil in some of the ingenium diesels.)
The most successful (read least troublesome) designs have the dpf as close to the turbo as possible, with a separate injector where necessary. As the exhaust after the turbo is frequently above 4-500°C the additional injector doesn't have to be used very often, as the dpf will be cleaned during hard acceleration, when exhaust temperatures jump towards that 750° magic number.
Apparently the dpf in the D5 is a fair way away from the turbo, as it has a catalytic converter in there too.
At least, that's how I've understood it
FINN - '72 88" S3 - 2.286 petrol - yet to go on it's first adventure
SOLD - '08 D3 4.0 V6 - 265/65/R17 on X5 rims
GONE '96 D1 300Tdi - 2" lift, 32" tyres, HD rear axles, lockers :(
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