The US has a diesel engine pollution regulation in particulants per mile driven for vehicles below 8600 GVW.. It doesn't take too many brains to figure out that an 8 cylinder diesel engine, on average, would put out twice as many particulants than a four cylinder diesel engine.
To get under the standard in most cases requires a high level of emission control technology ($$$), limiting sales volumes -- no matter how nice the engine would be.
And yet, if you wanted to cut the country's oil dependency, you'd want to replace the gas guzzling V8's with diesel V8's.
Just our government in action folks!
Let's hope Obama's automotive task force addresses these and other issues.