In diesel engines, you have to balance price, performance and pollution. Most designs can only deliver in two categories, but Volkswagen claimed to win in all three with its turbocharged direct injection diesels. The claim was too good to be true.

Scientists say diesel, with its inherently good fuel economy, could be tweaked to be efficient, quick, clean and cheap, but getting combustion right is a challenge.

Volkswagen's U.S. chief Michael Horn (center left, gray hair), faces cameras before a House hearing on the company's emissions cheating