Freightliner
A friend of mine who until recently a financial guy for Daimler at Freightliner headquarters told me that Freightliner is offering diesel engines in which the air coming out is cleaner than the air going in. I'll email him for details, he's no longer in the area.
He did explain to me why there are no modern 4-door convertibles. There isn't enough torsional strength in the body if there is no top and the body is that split up. Hadn't thought of that.
Arf