Octane Number, Day After Pump

Gasoline is a mixture of many components, with different volatilities and octane numbers. One could mix high and low octane number liquid hydrocarbons to make a commercial grade gasoline.

However, since some of the components are more volatile than others they will basically start a gravity separation when car is not running causing an evaporation at the surface and a distillation process in your fuel tank, with the more volatile components separating as vapors and being captured by the charcoal cannister and introduced in your intake manifold by the evap system when the car is running, like in a carbureted engine.

Gasolines that come with high octane number after distillation of crude and being applied other chemical processes are naturally high octane number gasolines and do not require as many octane boosting additives.

The biggest problem of having highly volatile additives in your gasoline is they will be distilled in  your fuel tank, separate first and then sent in the intake which would lower your octane number only after a few evap cycles or a few trips.

Decreasing of octane number would mean automatically decreasing timing advance that is adjusted by the computer continually using the knock sensor, thus decreasing efficiency, power and mileage and making you use more carbon per mile and pay more money for less.

One cheap way to mitigate with problems due to volatility of low quality gasoline and its additives is to make fuel tanks with a smaller evaporation surface, like installing then vertically maybe, behind the rear seat, which would probably decrease the safety rating of your vehicle. Another would be to seal the fuel tanks, and that would mean you would have some pressure inside, but maybe not as much as in the case of GPL tanks.

There could be other solutions, like capturing the vapor, making it liquid under pressure and putting it back in the tank but it would be an engineering nightmare to make a compressor to liquify gasoline on a car, mainly due to lack of lubrication properties of gasoline.

One more feasible variant of this solution would be thermally insulating the tank and cooling of your gasoline using your AC already in place, especially in hot summer days. Just one more branch of the AC cold line passing through the fuel tank.

Using a small mixer beater rotating in the tank would prevent gravity separation of the more volatile components at the surface in hot summer days when the car is not running. A plastic sheet floating at the surface of the gasoline maybe one inner pouch inside the tank that would wrinkle and collapse when empty, with no air inside and allowing no vapors out?