Yes, the refineries are blending summer gasoline. Summer gasoline is more expensive for refineries to make since it does not have butane in it. This is not the only reason why gasoline is more expensive in the summer though. Butane is cheaper than the other gasoline components. In the winter, refineries add butane to gasoline to increase the vapor pressure of the gasoline, which will help start your car in cold weather. The butane does not have as much energy as the other components that make up gasoline and therefore summer gasoline will give you better gas mileage. In many areas the summer gasoline is required to have a low vapor pressure (no butanes or other light gasoline components) to help with air pollution (smog).
Gasoline is blended from components produced from several different processes. One component comes from a FCC (Fluidized Catalytic Cracking) unit, which cracks heavier oil like heating oil into high-octane gasoline. Basically a FCC unit takes a larger molecule and breaks into smaller ones. One of the smaller ones is gasoline. Another gasoline component comes from reforming straight run gasoline into reformate in a CCR (Continuous Catalytic Reforming) unit. Straight run gasoline comes straight from the crude tower. A crude tower is the first unit in a refinery that separates crude oil into light, medium, and heavy fractions. Once of these fractions is called straight run gasoline. It is gasoline but it has a very low octane rating like 50-60. Only low compression engines can use straight run gasoline. Our Contour engines would probably be destroyed if we used straight run gasoline. A CCR unit takes the straight run gasoline and catalytically changes some of the straight run material into aromatic material. Refomate has a lot of aromatics (including benzene) in it. Aromatics have high octane ratings and make good gasoline blending components but gasoline can only have so much aromatics in it. This is especially true for benzene. Another blending component in gasoline comes from an Alkylation unit, which makes alkylate. Alkylate is made in an Alky unit from butene (not butane) and isobutane. An Alky unit basicly takes to two small molecules and makes a larger molecule (alkylate). Alkylate also has a high octane rating.
FCC gasoline (from a FCC), reformate (from a CCR), and alkylate (from an Alkylation unit) are your basic gasoline components. There are other processes that make gasoline but this post is already getting long! A gasoline blender will mix all of these components to meet vapor pressure, octane, drivability index, and many other specifications to make the gasoline that we have in our tanks. They are possibly a hundred different gasoline blends. This explanation how gasoline is made is very basic and was intended for people who know very little about how gasoline is made.
