For a car's engine to work well, a mixture of gasoline and air must be sprayed inside.
Inside the car, the mixture receives an electric spark and explodes, forming gases.
These gases increase the pressure inside the combustion cylinder, causing a piston to move, which makes the engine run.
It is the intensity of the energy released during the explosion that causes the car to move.
Depending on the mixture, gasoline, when compressed, can explode ahead of time, reducing the intensity of the explosion.
Good gasoline if it has high compressive strength without premature explosion.
Scientists use two hydrocarbons as standard to measure gasoline quality. Heptane, which has very low resistance to compression, is given the value 0 (zero) and 2,2,4-trimethyl-pentane isoctane, which has high compressive strength, value 100 (one hundred).
So a 90 octane gasoline is one that behaves as if it were a mixture of 10% heptane and 90% isoctane.