Essentially:
- diesel cars produce soot and nitrous oxides, which kill people around them
- in Europe we focus on reducing CO2, which favours diesel, but doesn't factor in the deaths due to emissions
- The Americans OTOH, focus on dangerous emissions, and don't care so much about CO2
Volkswagen have spent a lot of money developing their diesel technology, whereas Japanese and American manufacturers tend to stick with petrol and hybrid cars.
It turns out that that Volkswagen cars in America far exceed the legal limits for dangerous emissions, but the car was programmed to detect when it was running on a rolling road (for emissions testing), and basically drastically reduce power and fuel efficiency, in order to cut emissions.
Now around half-a-million US Volkswagen diesel car owners are going to have their cars reprogrammed, so that power and fuel efficiency are reduced, and VW is going to have to pay billions in compensation, both to owners and the US government.
A further 10.5 million cars might be affected around the globe.
www.bbc.co.uk/news/business-34325005