Why octane and not icosane uses as gasoline?

Answer 1

To be an efficient fuel for cars, a hydrocarbon must be able to vaporise at the pressure in the pistons of the engine. The minimum temperature before a fuel will vaporise is called the flash point. Relatively low flash points are good for fuel because once it is vapourised, the fuel effectively has the highest possible surface area possible, making it very easy to ignite (high rate of reaction).

Good fuels must have a relatively low boiling point (but not too low, otherwise you get something called knocking, when the fuel vaporises too easily and actually ignites before the spark plug sparks it. The fuel ignites twice, making the cycle not as efficient as before because it is now out of sync).

Octane has a chain of 8 carbons, and icosane has a chain of 20 carbons. More carbons in the chain means a higher relative molecular mass and usually a larger contact area for intermolecular forces (van der Waal's forces in hydrocarbons) to act. So generally, boiling points increase as the carbon chain lengthens, so icosane will have a much higher boiling point (hence flash point) than octane. In fact, at room temperature and pressure, octane is a liquid, but icosane is a solid. Imagine trying to fill up your car at the petrol (gas) station with solid lumps of fuel!

The boiling points make all the difference. Hope this helped :)