I think better idea is to prohibit all hydrocarbon-powered cars (gasoline, diesel, LPG, CNG) from large cities after some date (e.g. 2030). Automotive companies will know they must develop good and cheap electric cars or go bankrupt after this date. Urging them to be "green" is foolish, they will always find the ways how to cheat (look at recent Volkswagen trick).
They may have cheated, but they are paying for it.
Heads are rolling, plus Volksagen has had to stop selling some variants in a few countries.
They don't get away scot free.