Paulcotes message regarding DPFs got me thinking about something;
I've observed a manual regeneration process. Initially, the engine speed is raised to approximately 3500rpm. The exhaust temperature sensor shows the temperature gradually rising.
As soon as the temperature reaches 200 deg C, the additional post-injection process is started and the temperature rapidly rises to approximately 600 deg C (downstream of the pre-catalyst).
Now comes the bit I'm not sure about;
The main injection timing is retarded to approximatey 15-30 deg ATDC. However this alone can't cause the temperature to rise as high as 600 deg C when stationary (no load). Plus the vapourisation of the post-injected fuel would lower the temperature.
I assume therefore that the post-injected fuel is oxidised in the pre-cat, and it is this that actually gives the huge rise in exhaust gas temperature.
If my theory is correct, then a faulty pre-cat would not oxidise the post-injected fuel and hence the temperature would never get high enough to oxidise the soot particles in the downstream DPF. This would result in a strong smell of unburnt diesel fuel to be emitted from the exhaust.
So, given that the ECU won't start regen until EGT is > 200degC, we can assume that this is the required EGT for proper function of the pre-cat.
Therefore, if there is a smell of unburnt fuel from the exhaust, then one of the following must be true:
1. The EGT sensor is giving an inaccurate reading (higher than reality) and so the ECU is allowing regen to start before the required EGT is condition is met in reality.
2. The pre-cat is faulty and not oxidising the post-injected fuel despite the required EGT condition being met.
I'm hoping someone with a better understanding of DPF theory can confirm or debunk my theory as I can't find such detailed info elsewhere on the net.