Q--When do the refineries stop supplying the summer blend of gasoline and how much less does it cost them to produce the non-summer blend?
A--I really couldn’t get a fix on the costs for blending gasolines, but here is what Dan Larson, a spokesman for BP told us about blending:
Refiners seasonally adjust the volatility of gasoline for two reasons: to maintain performance and to manage evaporative emissions. The former helps prevent problems like vapor lock in the summer and hard starting in the winter, and the latter minimizes fuel evaporation and the emission of volatile organic compounds (VOCs) into the atmosphere, which can help reduce ground-level ozone formation. The warmer months (June 1 through Sept. 15 for most areas) are considered ozone season for refining purposes and this generally means gasoline should have a lower Reid vapor pressure (Rvp.) to prevent vapor lock and to reduce VOC emissions.
Ozone season comes to an end on Sept. 16 when refiners increase the vapor pressure in stages through November. Winter volatility regulations vary, but in many states with the coldest weather, winter blends can run up to 15 lb of Rvp. The process is reversed beginning March 1 as refiners begin stepping back down the Rvp of their gasoline to the summer level. Most refiners begin shipping summer gasoline in early-May so their product meets summer specs by June 1. Generally, Rvp is managed by adjusting butane levels in the gasoline blend. The rule of thumb says that for every two-percent increase in butane, a one-pound increase in Rvp is the result. Since butane is also an octane enhancer, when it is removed refiners achieve pump octane levels by blending in other compounds.