There were some pretty big licensing hurdles to getting the EPA to accept urea tanks as part of an emissions control system: https://iaspub.epa.gov/otaqpub/disp

There were some pretty big licensing hurdles to getting the EPA to accept urea tanks as part of an emissions control system: https://iaspub.epa.gov/otaqpub/display_file.jsp?docid=16677&flag=1 https://iaspub.epa.gov/otaqpub/display_file.jsp?docid=16677&... Also, note that the 2007 Tier 2 NOx emissions standards mentioned in that document are pretty much as strict as the 2014 Euro6 standard, at least for manufacturers which mainly produce diesel cars. The EU emissions standards were much weaker back then, giving manufacturers more time to develop tech to meet the stricter standard.