regens are of two types, passive and active.

Passive is when the truck is creating enough heat in the dpf to burn the soot just due the heat.

Active is when the pcm commands the truck to send the extra fuel into the dpf to create the heat for it to burn off.

When towing you can the passive regen, but the heat means you use more def. When not towing more active regens and less def is used.

 

Exhaust comes out of the turbo, into the Selective Catalytic Reduction (SCR) system. The SCR injects Diesel Exhaust Fluid (DEF) into the exhaust to break down NOx to Nitrogen and Oxygen. The hotter the exhaust, the higher the injection rate, as hotter exhaust temps result in more NOx being produced. Therefore with higher exhaust temps, you use more DEF.

After the SCR system, the exhaust enters the Diesel Particulate Filter (DPF) which acts as a screen to catch the soot from the combustion process, the black smoke that older Diesels would make under load (rolling coal). The system uses pressure sensors to determine how clogged this filter is, and some other parameters (A grams of Soot per Liter of exhaust, and a maximum of 500 miles since last regen), to trigger a regen process. When the regen process begins, extra fuel is injected into the cylinders on the exhaust stroke, which then burns of in the exhaust system, inside the turbo, inside the SCR, and ultimately inside the DPF. The temp as EGT2 (near the DPF) can rise form a normal 550-600 degrees to more than double that (I have seen 1300+ once or twice). Think of the exhaust pipe like a chimney on a fireplace, you burn wood, soot collects inside the chimney, and eventually it will clog up, and not allow air to pass through. At this point you either call a chimney sweep, or us a product like a Chimney Sweeping Log, that dries out the soot and causes to flake and burn away. The raised exhaust temps do that same thing, the soot trapped in the DPF is burnt away to clean the filter.

The two systems are unrelated. The only reason the interact is that when the DPF regens, it raises exhaust temps, which causes the SCR system to react and use more DEF to control the NOx production. DEF does not play a part in the DPF regen, but the DPF regen can cause more DEF to be used.

I hope this helps.

 

That was the best interpretation I have seen, thank you.
I keep reading about regens and people saying that is why they are going thru more DEF. didn’t make sense to me.
Got it now!
Troy

 

Time to delete!