Petrol is a liquid mixture of hydrocarbons produced by fractional distillation from crude oil and used as spark-ignition engine fuel. Unleaded petrol for cars may contain a limited amount of oxygenic substances and additives such as detergents (to ensure the cleanliness of the fuel system) or antioxidants. The quality of petrol is defined by ČSN (Czech National Standard) EN 228. An important characteristic of petrol is its octane number; it indicates how resistant the petrol is to premature detonation ignition (so-called engine knocking). Pure petrol has a very low octane number and additives are therefore added to it to increase it. The octane number is included in the fuel designation and indicated on petrol station pumps. Natural 95 for cars is unleaded doped petrol which keeps the intake valves, injectors and combustion space clean. Natural 95 also has anti-corrosive and anti-oxidation effects which ensure the protection of the fuel system against corrosion.