For at least 20 years, diesel cars have been much more popular in Europe than in the United States.
Because diesel combustion is more efficient than gasoline combustion, the engines help drivers save money on fuel–and produce lower emissions of greenhouse gases.
However, European countries have only recently adopted a similar strict level of emissions standards to the U.S.
That’s led to public concerns over air quality, due to the high levels of nitrous oxides (NOx) and other harmful substances from diesel exhaust produced by engines without modern emission controls.
Up through last year, the diesels sold throughout Europe that would not have been legal to sell in the U.S. for most of a decade.