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Originally Posted by Rocklobster
Interesting insight. Thanks.
It was certainly my assumption that at least until recently it was the US emissions that drove the bus at least on the DEF subsystems. Further the 6 cyl engines i assumed were quite different and while a good test bed for theory were unlikely to have been simply copied as-is over to the 4-cyl engine. The 4cyl diesel offering was new for the 2014 model year in the US and i thought i had read a few places that prior to that,where offered over seas, did not have the DEF system. As with all emissions control systems they are a single system of associated sub systems all designed to meet tailpipe emissions laws. So, adding a DEF sub-system to an emissions system which already includes the DPF would at the very least incur some different tuning, testing, calibration, control, etc...
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It seems that the DEF/SCR system is almost entirely separate from the rest of the systems; we're not even sure that the EGR system is integrated with it (both affect NOx) as EGR seems much more affected by throttle position and boost and such. It appears to be set up to minimize NOx by itself... with the DEF/SCR designed to reduce NOx below the limits.
The DEF/SCR system was an option in the EU for as many years as it's been on the USA/CA cars - but it doesn't appear that many were sold. In looking around at things, I noticed that the DPF canisters (and DOC) are different on US and EU cars: curious, but perhaps not unexpected.