They do different things, DPF is for soot, DEF is for NOx.
They have a relationship in that for an engine to have met NOx emissions up to 2010 without DEF it needed more EGR to run cooler, because heat and pressure is what creates NOx from air. However when the engine runs cooler it creates more soot causing the DPF to need more often regeneration. So DEF helps reduce regeneration indirectly only because the EGR can now be reduced increasing temps, which also helps mileage and power. It doesn't necessarily change how regeneration is done, only how often it's needed.
For 2010 emission engine makers realized they couldn't meet the tougher standards without DEF, they tried everything, Navistar really tried and failed. EPA doesn't care how you get emissions down, just so happens DEF is the only viable way to get NOx down right now. Volkswagen got caught tweaking EGR which reduced power and mileage but only while testing instead of going with DEF, then on the road it ran hotter but no DEF, so it put out 40 times the NOx.
Here is a diagram, notice how the DEF system is after the DPF:
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You know, I've never measured mine. The DEF is so cheap and I use so little of it, I just don't see the need for measuring. If I'm doing a lot of traveling, I'll keep it about 3/4 full....if I'm setting for a long period of time, only about 1/4 full.
In any case, I guess i can say....who cares?
Ron, Sandie and Lilly 2013 Tiffin Phaeton 42LH 400 ISL | 2011 GMC Terrain SLT-2
Roadmaster All Terrain | US Gear Unified Brake System | Pressure Pro
I asked the original question since my consumption seems high compaired to what the dealer and Cummins says 2% and wanted to know if I should have it looked at next time I take it to Freightliner for service.
Thanks to all the replies.