Well, if you insist on running them on petrol, just having a lambda sensor and closed-loop mixture control breaks the back of it.

However, if you run them on propane instead of petrol - like they do for forklifts and Zambonis, where you have to run the engines indoors without people dying - you just get carbon dioxide and water vapour out. You don't need a catalytic converter and indeed if you have one it can make the pollution slightly worse by generating nitrogen oxides.

Imagine how clean our cities could have been if we'd done this 20 years ago, instead of selling people debt to buy "Cleaner Greener Diesels".

They're still emitting CO2, but a little less of it, and from fuel that would otherwise go up in smoke at the refinery flare stack - unless you want to stop making plastics and fertilisers.