And, in theory, it was an easy sell - diesel engines are more efficient than petrol engines, so running costs are cheaper. Using less fuel should mean lower emissions.
In practice, however, laboratory measurements of CO2 emissions from diesel and petrol engines are the same, according to Martin Adams at the European Environment Agency (EEA). And as diesel cars tend to be bigger and heavier, any advantages in efficiency are wiped out.
As a result, average CO2 emissions from diesel cars are only fractionally lower than those from petrol cars, figures from the UK's Society of Motor Manufacturers show. The industry counters that of course emissions would be greater from larger cars, and maintains that when comparing like-for-like models, diesels do emit noticeably less.
Just how ineffective they are is staggering. Tests conducted by the ICCT show that modern diesel cars emit on average seven times the EU limit for NOx.
Ouch...A separate test showed that some individual cars emit even more - an Audi A8 emitted 22 times the limit. Only three cars - an Audi A5, a VW Golf and a BMW 3-series - complied with EU regulations.