How are there different grades of automotive gasoline made?

Question

How are there different grades of automotive gasoline made?

Answer 1

I have seen a diagram which shows it nicely, maybe  worth seaching for?

Basicly, they pump the crude oil into a giant tank which is seperated into hoizontal sections with passageways between. Heat is applied at the bottom and the heavier stuff (tar) moves towards the top and the lighter stuff (petrol and diesel) moves towards the bottom. I don't know why it seperates out upside down like that, but it does. All the different grades are tapped off from thier own level in a continuous process.

Answer 2

There's a base gasoline.  Chemicals that make it harder to ignite accidentally (anti-knock chemicals) are added to make higher grades.  More here:

http://www.atsdr.cdc.gov/toxprofiles/tp7...

if you can't read that, try:

http://64.233.167.104/search?q=cache:CqT...