Gasoline blends fall into two categories as it pertains to Environmental Protection Agency (EPA) standards: summer-blend and winter-blend. The difference between the two, somewhat literally, boils down to differences in Reid Vapor Pressure (RVP). Reid Vapor Pressure is a measure of a fuel’s volatility, that is, its tendency to evaporate at higher temperatures.
The most important RVP benchmark for gasoline is 14.7 pounds per square inch (psi), which is normal atmospheric pressure. Beyond that, surplus pressure would accumulate in the gas tank, eventually building to the point that the fuel would boil away and evaporate. Typically, the EPA mandates that gasoline RVP not exceed 7.8 psi, although it allows for certain exceptions up to 9.0 psi. This relatively lower RVP gasoline is known as summer-blend, mandated for use when possible because the lower RVP results in fewer volatile organic compounds and nitrogen oxides being released upon combustion.
When temperatures cool, gasoline is less prone to vaporization. Readers of this blog may recall a personal experience turning their key in cold weather to the screeching of their ignition instead of the rumble of their engine starting. To combat this issue, the EPA allows for higher RVP fuel to be used during periods of colder temperatures. Butane, with a RVP of 52, is a commonly used additive. While summer-blend gasoline may contain only 2% butane, winter-blend weighs in at about 10%. This increased volatility allows for more vapor to form, which more readily catches the spark from the spark plug, which makes it easier for you to start your engine and begin blasting the inside of your windshield with hot air so you can get where you’re going.
So, what does this amateur chemistry lesson have to do with your bottom line? It depends on the month. Butane is inexpensive and easy to source, which is a major reason winter-blend gasoline tends to be cheaper. Generally speaking, demand for gasoline in the United States is lowest in February, so many refineries (up to about 25% in any given year) use this time to go offline for maintenance and repairs. They also take this offline time to retool for the EPA’s summer regulations. Refineries switch over to summer-blend production in March and April. Gas stations have by June 1st to switch to selling summer-grade gas, while terminals and other facilities “upstream” from pumping stations must switch by May 1st.
While the varieties of winter-blend gasoline are limited, various state and regional regulations have resulted in 14 different fuel specifications during the summer months, depending on the geography. Refineries must produce enough of these specialized gas varieties to prevent any supply shortages, complicating production and distribution schedules and further contributing to the elevated seasonal price tag. This higher-cost, lower-volatility fuel is prevalent until about the middle of September, when the EPA allows for the first increase in RVP on the 15th. These allowances remain in place until the following spring, and the cycle repeats.
Gasoline inventories and production have been higher than expected in recent months, contributing to a decrease in retail prices. It will be interesting to see how the annual shift to a more expensive formulation will impact prices at the pump this spring.