Unfortunate news today, as Mazda has delayed the release of its Skyactiv-D clean diesel engine destined for its Mazda6 sedan...

 This is the second time Mazda has issued a delay for the somewhat revolutionary, 14:1 compression, all-aluminum 2.2L inline-four. It's rumored that the engine cannot meet the NOx requirement in Tier 2 Bin 5 (U.S.) emission standards. If you recall, Mazda designed the Skyactiv-D around the idea of not needing selective catalytic reduction (SCR, or urea injection) to keep NOx levels in check (the engine does have low-and-high temperature EGR loops though). However, it appears that's exactly what this engine will need if Mazda wants to sell them in North America.

 When the 2.2L debuted in Japan in 2012 it met the country's 2009 emission standards, in which a NOx limit of 0.08 g/km is imposed. But by comparison, the U.S. Tier 2 Bin 5 NOx limit is 0.044 g/km (or 0.07 g/mi). From this data you can deduce that the Skyactiv-D falls somewhere between these two limits—reason enough to have to re-engineer the emissions system. Mazda stated that "further development is required to deliver the right balance between fuel economy and Mazda-appropriate driving performance."

 At the present time, it appears that no one is going to get around using SCR going forward—at least if they want to meet Tier 2 Bin 5 emission regulations in the U.S.

Sources:

DieselNet

Forbes