FOR the past six years, Volkswagen has been advertising a lie: “top-notch clean diesel” cars — fuel efficient, powerful and compliant with emissions standards for pollutants. It turns out the cars weren’t so clean. They were cheating. The vehicles used software that cleverly put a lid on emissions during testing, but only then. The rest of the time, the cars spewed up to 40 times the legal limit of nitrogen oxide emissions. The federal government even paid up to $51 million in tax subsidies to some car owners on the false assumption of environmental friendliness.