[mikestew]

Diesel has a problematic history in the US, though, and US emissions standards are tough on diesel by-products like NOx. I would like to know how much of that is anti-competitive and how much is genuine concern for the environment.

Genuine concern. US cars were virtually crippled in the 70s due to NOx standards (power-robbing EGR valves being the main culprit, IIRC). For example, the high-end version of the Chevrolet Camaro in 1976 was getting 165bhp out of a 350ci V8 small block (for comparison, I think the plain Jane 2015 Corvette my parents just bought puts out 450bhp from the modern version of that engine). The straight six cylinder version put out less power than my 1.4L four cylinder Scion.

All that smog in the LA valley back in the 70s? That was NOx, so at least in the case of California, those standards were there for a reason. Besides, small diesels were just a gleam in VW's eye when those standards were put in place, so there was nothing to anti-competitive about.