Exemption of petrol from additional excise duty limits levy above the prescribed fixed per litre rate under delegated finance powers. Exemption of petrol from that part of the additional duty of excise which exceeds the amount calculated at the prescribed per litre rate, effected by the Central Government under delegated powers from the Finance Act and the Central Excise Act, caps additional excise liability on petrol at the prescribed per litre amount while leaving tariff classification and ordinary excise liabilities intact.