Burning propane in car engines produces carbon dioxide and water, no sulphur oxides (because there's no sulphur in the fuel, or such a tiny amount that it is ne

Burning propane in car engines produces carbon dioxide and water, no sulphur oxides (because there's no sulphur in the fuel, or such a tiny amount that it is negligible) and no nitrogen oxides. You can actually measure this. It burns leaner than petrol resulting in complete combustion so there's no CO or HC in the exhaust, either. Indeed, if there's enough CO and HC in the environment from other vehicles you'll actually be burning that too giving it (in a sense) negative emissions figures.