For a petrol engine to work efficiently, petrol must burn smoothly without premature detonation, or knocking. Severe knocking can dissipate power output and even cause damage to the engine.


When petrol engines became more powerful in the 1920s, it was discovered that the most extreme knocking effect was produced by a fuel composed of pure normal heptane, while the least knocking effect was produced by pure isooctane (the prefix ‘iso’ is used in chemistry to show that a substance has the same chemical formula but a different structure). This discovery led to the development of the octane scale for defining petrol quality.