For the past couple years I have been following the development of the diesel Mazda6. Most of you likely have never heard of such a thing because while the car has been available in other markets, Mazda has not introduced it into the US.
Mazda wanted to have a diesel sedan that could achieve the required emissions standard without the use of an emission treatment liquid. Mazda's engineers did it. However, Mazda wasn't happy with the performance of the engine. Mazda seems to know their customer base pretty well and decided that Mazda drivers would be unhappy with the lack of "Zoom Zoom" from the diesel engine. They have kept delaying the US release until they could solve the problem.
Now my guess, (and this is only a guess) is that Mazda felt that if VW could achieve a peppy diesel engine that didn't require emission additives, the they should be able to do it as well. They just couldn't figure out how. Instead of introducing a car that would disappoint and instead of cheating, they delayed. My guess is that with the VW scandal, the US market for diesel cars will take such a hit that Mazda will give up on a diesel Mazda6 for the US market. Too bad.