Europe had better diesels than the US years sooner because our fuel had too much sulfur in it which wreaked havoc on the magic pumps, injectors and emissions systems. My 2004 TDI has a "PD" engine with each injector having its own built-in pump. Common Rail systems seem to be better in terms of simplicity and lower emissions, but they required low-sulfur fuel. We have only had that since December 2010.

(I think VW possibly also didn't want to license the necessary common-rail patents for a while, or something. They have a common rail system now.)

AFAIK, diesels sold in the US are as good as European ones now (but they still have an advantage in the array of sizes available, number of car models which offer them, etc.).

(At least as I understand it,) Due to the higher compression ratios of diesels, they naturally have more NOx than gas engines. Emissions control systems have to work harder to counteract that.

But the higher compression ratios also give you higher efficiency.

So, there's a tradeoff.

Since smog and ground-level ozone are directly created by NOx, it's a big deal if a diesel car is turning off its emissions controls.

VW really, really messed up in doing this.

Mercedes and BMW (and Chrysler and ...) diesels aren't sold as magic super MPG machines the way VW pushes theirs, so maybe they didn't feel the pressure to do similar emissions cheating. Or maybe they were smart enough to realize they'd eventually be caught and the risk wasn't worth it. But I hope CARB and EPA are checking them (and everyone else) to make sure they're not fudging their numbers all the same.

Cheers,
Scott.