Summertime, When the Living’s Easy

Memorial Day marks the start of summer – sending millions of drivers to their nearest gas station to fill up for that first summer trip.  But have you ever stopped to wonder why gas prices seem to go up around Memorial Day and then fall again around September? Today’s edition of SBFF explores the relationship between a little known aspect of fuel regulation – Reid Vapor Pressure (RVP) – and its impacts on consumers, and the environment during the summer driving season.

Summer Gasoline Blends

In the United States, gasoline is a blend of various petroleum hydrocarbons as well as renewable fuels, such as ethanol.  Gasoline has two major properties that fuel refiners control, octane and Reid Vapor Pressure (RVP). To learn more about octane, check out our recent Fact Sheet: A Brief History of History of Octane in Gasoline. 

When blending fuels for different regions and regulations, refiners have a number of petroleum hydrocarbons to choose from, each providing different properties to the final blended fuel.  While refiners keep the octane values constant throughout the year, the RVP rating changes according to the seasons, due to air quality standards set forward in the 1990 Clean Air Act Amendments.

There are three primary blend types used in the United States: summer blends, winter blends, and Reformulated Gasoline (RFG). Summer blend-fuels are so named because they are required by EPA during the hotter summer months to control the formation of smog.  

Summer Blends Lower Smog Levels

When the air temperature reaches over 100 degrees Fahrenheit, gasoline vaporizes quickly, meaning it can boil in open containers and cause vapor lock in engines, a phenomenon whereby the fuel is unable to combust.  While most modern engines are designed to control for vapor lock, these so-called evaporative emissions are a precursor to ozone.

Commonly referred to as smog, ozone is a complex mixture formed when air pollutants react with each other in the presence of sunlight. Ground-level ozone impairs lung functioning and contributes to increased incidences of asthma and other lung diseases, especially among children and the elderly. A hazy summer day, often referred to as a “bad air day” is the result of ozone formation. Over time, ozone exposure is linked to serious health impacts.

To reduce the formation of ozone in summer months, the EPA regulates the volatility of gasoline in the summer months.  Refiners accomplish this by changing the fuel blend in anticipation of the warmer season. EPA requires retail gasoline stations to offer summer blends between June 1 and September 15.

Gasoline Blends Vary Nationwide

Reid Vapor Pressure (RVP) is a measure of gasoline volatility, or its ability to vaporize. Each component of gasoline adds to the overall RVP of a gasoline blend, and certain blending components have a higher RVP rating than others. To cut smog levels, EPA requires that summer blends have an RVP cap of no more than 9 pounds per square inch (psi). In some National Ambient Air Quality non-attainment areas, the cap is as low as 7.8 psi. Additionally, EPA allows states to require stricter standards (as low as 7.0 psi) in regions with poorer air quality. Refiners accomplish the various summertime RVP targets by using different proportions of the various gasoline components than they do in the wintertime.  

Fig 1: U.S. Summer gasoline requirements, courtesy of EIA

Another fuel, Reformulated Gasoline (RFG), is required year-round in about a third of the country, due to even stricter ozone caps. Since RFG has a lower RVP rating than the required summer blends, RFG areas are not subject to the use of summer blends. Additionally, Alaska, Hawaii and U.S. territories do not have to comply with federal fuel volatility regulations. The net result is quite a few different gasoline blends, depending on the region and the time of year.

Summer Blends Are More Expensive

According to the National Association for Convenience & Fuel Retailing, gasoline prices in the summer months are on average, 52 cents higher per gallon.  It’s a common misconception that the seasonal price hike of gasoline is due to increased demand.  While demand does peak at about 10 percent more in August compared with January, demand is only one piece of the price increase. 

The summer blend is more expensive for a number of reasons beyond demand: 1) use of more expensive blending components that comply with lower RVP ratings, 2) a more lengthy refining process to produce summer blends, and, 3) a resulting lower yield per-barrel which translates to higher priced fuels. The cheapest and most abundant gasoline blending component – butane – also has the highest RVP rating.  Therefore, to comply with summertime RVP, refiners typically lower the amount of butane they use in finished gasoline.  This both decreases available gasoline supplies as butane is reduced from gasoline, and increases blending costs – causing the price spike. 

Ethanol’s Effect on RVP

Figure 2: Ethanol’s effect on RVP, courtesy of NREL

Currently, about 95 percent of U.S. gasoline is sold as E10 (10 percent ethanol, 90 percent gasoline). Adding 10 percent ethanol to gasoline raises gasoline’s RVP rating by one psi, to approximately 10 psi, according to the National Renewable Energy Lab. Because of this phenomenon, the EPA granted E10 a one psi waiver (commonly referred to as the one pound waiver) in 1992. Currently, higher blends of ethanol, such as E15, do not have a waiver. 

While neat ethanol actually has a very low RVP rating (2 psi), the chemical interaction between small volumes of ethanol and gasoline causes RVP to increase.  As the volume of ethanol in the fuel is increased beyond 10 percent, this effect is erased, and the RVP of the gasoline begins to drop. At higher blends, such as E30, RVP is just below 9 psi.  Because the RVP of gasoline drops as ethanol content increases, blends higher than E50 have a lower RVP than that of the base gasoline, and no RVP waiver is required.