Question

in some countries a particular fuel is used as an additive to increase the efficiency of petrol such a fuel is called as......

Answer 1

Fuels that are added to petrol or other fuels, to increase fuel efficiency, are known as Fuel Additives.  

Common Fuel Additives are alcohols such as Methanol (MeOH), Ethanol (EtOH), Isopropyl Alcohol etc. They increase the Octane Rating of petrol, by inhibiting corrosion and providing a lubricating effect. It thus allows the use of higher compression ratios, for greater power and efficiency, in internal combustion engines.