When you burn petrol you should get only CO2 and water. In practice, you don’t. There are unburnt hydrocarbons, carbon-monoxide, oxides of nitrogen (NOx), etc. We’ve all heard of them, and they all occur because air, the combustion chamber, and petrol itself, are inherently compromised.
Air is not oxygen. Air is overwhelmingly nitrogen. Only 20-odd per cent of air is oxygen. So there’s four times as much nitrogen as oxygen sucked into the combustion chamber, and a tiny fraction of it gets swept up in all that controlled chemical mayhem. That explains NOx. (However, the overwhelming bulk of nitrogen emerges unchanged from the tailpipe, albeit substantially warmer. About three-quarters of tailpipe emissions are merely inert, unchanged nitrogen.)
Carbon-monoxide, a deadly poison and suicide favourite, is what happens when there’s not enough oxygen to go around with all those carbons; it’s deformed carbon dioxide.
You have to understand that petrol beats modern metallurgy. Potentially, it produces too much heat for your engine. Perhaps you’ve heard of engines that run too ‘lean’ (ie, with too little fuel) and melt down. The reality is that engineers always tip more fuel into an engine than can be efficiently burnt by the air with which it spends its last few moments. This extra tipple of fuel is used merely to cool the chamber. If you ever get petrol on your skin, you’ll note it feels cold. That’s because it evaporates rapidly, and that sucks heat out of the environment. The fancy name for this extracted heat is the ‘latent heat of vaporisation’. Forget what you’ve heard about ‘lean burn technology’. Dictating an excess of fuel stops the combustion chamber reaching temperatures that otherwise would melt its iron, aluminium and steel underpinnings.
Petrol also still contains impurities and additives that are passed out the tailpipe, though these have been lately dramatically reduced. Benzene, tetra-ethyl lead and sulphur were major culprits in the past, but better fuel quality has largely removed them from the pollution landscape.