> But for diesels, it seems that there is trade-off between fuel efficiency and NOx emissions Correction: there is a trade-off between fuel efficiency, NOx emi

> But for diesels, it seems that there is trade-off between fuel efficiency and NOx emissions Correction: there is a trade-off between fuel efficiency, NOx emissions, particulate emissions, and cost. Urea injection + diesel oxidation catalyst + diesel particulate filters pretty much solves all emissions problems with diesel engines, and that's what GM and Ford diesels do, both in Europe and in the US. It isn't cheap for sure, but the 2014 Chevrolet Cruze diesel is worth every dollar in premium over the gasoline version.