Year 12s: what will be the first way you research your future options? >>

additives in fuel (oxygen)

why does adding additives containing oxygen improve car performance?

I think it's because in petrol engines you tend to run at roughly stoichiometric mixtures of petrol and air (i.e. just enough to completely combust the petrol without forming CO and generate the maximum power). However, if you have too much air you can sometimes get spontaneous combustion of petrol in the wrong place (ahead of the flame front), leading to knocking. If you mix oxygen-containing additives in, then you add more oxygen to improve the power generation but you may have less risk of knocking because the additives are mixed in with the fuel.

I'm not sure about this though, I don't specialise in this and only learnt odd bits about combustion engines a while back.