Until about the early 2000s you could get a grant to cover the cost of converting your petrol car to run on propane. There's masses of it that has to be burnt because it's a waste product from cracking heavier fractions to make plastics feedstocks, and it burns producing only carbon dioxide and water - no HC, no CO, no particulates.
They run warehouse forklifts on it, because unlike diesel or petrol you don't die if you breathe the exhaust fumes.
But, there was more profit to be had in buying everyone's cars for a couple of hundred quid and "scrapping" them - in reality, even 30 years later there are still millions lying in fields that haven't been touched - and then selling people very expensive finance packages so they can buy a diesel car instead.
So now we have stinky diesels everywhere and everyone is in debt. Working as intended.