DPF fuel consumtion

[JimT]

Why the DEF usage to DPF fuel usage calculation is inherently flawed:

DEF usage is based on SCR conversion efficiency. This is determined by data provided by the outlet NOx sesnor. There are specific levels of outlet NOx that the truck must try to achieve and DEF is consumed in the SCR canister as a means to achieve this goal. If everything is working perfectly then on paper the DEF used would be proportional to the fuel burned, but that's never going to happen in the real world. In reality there are so many variables that go into the amount of NOx produced (and therefor the amount of DEF needed to reduce that NOx) that equating them is never going to be accurate. Driving patterns alone will cause differences, not to mention the operation of the EGR valve, EGR cooler, turbo, multiple engine sensors and even exhaust leaks.

So if the DEF usage under normal engine operation fluctuates then there's no way to accurately determine the % of DEF used over-all to the amount of fuel burned exclusively by the DPF. This is probably why you can't get any good answer from Cummins as they most certainly understand the situation. While they might talk about averages and %'s for sales purposes (based on static testing?) they absolutely wouldn't want to provide those numbers as definitive proof for calculations based on real world operation.

Edit:
Also, the DEF doser valves, at least in some of the older trucks, are known to leak sometimes. And, the DPF doser can also leak fuel into the exhaust too. These factors can further skew the numbers you're trying to use as the basis for calculation.