Im this picture you can see that a DPF is kind of like looking at a ‘chessboard’. The picture actually shows you inlets visible from one side only, if you were to see the DPF from the other side as well then there would be inlets on the other side as well but each inlet has a dead end, like a long tunnel that is bricked off at the end and it is these tunnels which get full of soot from the DPF’s natural process.
The walls of the DPF are semi-porous which means the soot cannot get through the side walls but the vapours can and because the soot cannot get out, this is what the re-generation is required for.
The re-generation can burn the ash into a semi-liquid which exits the exhaust system but the soot has nowhere to go and it is for this reason why a DPF eventually needs cleaning, so that the soot can get out of the DPF.