The EPA has decided that diesels put out too much soot. So now, we have what is called a DPF (Diesel Particulate Filter). This mounts in the exhaust and traps soot. It is monitored by a pressure sensor as well as temp sensors. When the PCM decides that the DPF has trapped enough soot that it has become restrictive, it enters "regen" mode. In regen mode, the PCM is going to inject raw diesel fuel into the exhaust by injecting it into the cylinders on exhaust stroke.

When that raw diesel fuel hits the DOC (Diesel Oxidation Catalyst), it makes the inside of the DPF (which is directly down stream from the DOC) extremely hot. Like a self cleaning oven, the inside of the DPF will get so hot that it will reduce the soot in the DPF to ash. That ash will fall to the bottom of the DPF. The ash never leaves the DPF.

This extreme heat is why you will see some funny looking, ugly, exhaust tips on a modern diesel (2008 up). These tips must never be removed or capped over with chrome tips that may seal them off. They are made so that they will draw cool air into the exhaust before it leaves the pipe. That way it will cool the exhaust gases enough that hopefully it will not blister the side of the truck, or burn someone's legs to the bone who happens to be standing next to the truck if it goes into regeneration mode.

Here is a pic of the tips. There are some other ports further up the pipe too.

Also, some early '08 production Ford F-Super duties had a problem with literally blowing flames from the pipe. I suggest showing caution when near the tail pipe of any '08 and up diesel. Those are supposed to have been corrected by a recall. The recall was a reflashing of the PCM.

YouTube - 6.4 Ford diesel flame thrower option

Any questions?