In summer, pollution becomes a major concern for people due to an increase in smoke and ozone levels, which can damage the lungs. The summer heat promotes the formation of ozone, and the presence of the inversion layer (a stagnant layer of air) can trap pollutants in the lower atmosphere.
The Reid vapor pressure (RVP) of summer-grade fuels is different from winter-grade fuels, making it (slightly) more environmentally friendly. RVP is the vapor pressure of gasoline measured at 100 degrees Fahrenheit (38 degrees Celsius). Fuels with higher RVP evaporate more easily compared to those with lower RVP. The RVP of a specific fuel blend is based on the combination of RVP values of its constituents. Regulators are concerned about this evaporation as it contributes to ozone formation.
The RVP of gasoline must be below 14.7 psi (pounds per square inch), which is normal atmospheric pressure; if the fuel’s RVP exceeds 14.7 psi, excessive pressure can build up in the fuel tank, leading to boiling and evaporation of the fuel. Depending on the state and month, the gasoline RVP for summer fuel should not exceed 9.0 psi or 7.8 psi. Some local regulations require even stricter standards. Due to these varying RVP standards, over 14 different types of boutique blended fuels are sold across the United States during the summer.
Winter-grade fuels use more butane as an additive due to their higher RVP standard, with RVP reaching up to 52 psi (mixtures contain around 10% butane). Butane is cost-effective and abundant, leading to lower prices. Summer-grade fuels may still incorporate butane, but in smaller amounts, around 2% of the mixture.
We know that gasoline prices tend to rise in the summer, usually around Memorial Day, but when do companies start producing these different summer fuels? The Environmental Protection Agency (EPA) defines April to June as the “transition season” for fuel production. Refineries switch to producing summer blends in March and April. Gas stations must switch to selling summer-grade gasoline by June 1, while upstream facilities like stations and pumps must transition to selling summer-grade fuel by May 1. After the summer driving season, companies begin transitioning back to winter blends starting in September, with the first winter RVP allowances taking effect on September 15.
According to a 2021 EPA report, “Today, approximately 75 million Americans breathe cleaner air because of seasonal fuel programs.” However, despite the benefits, price increases, along with the use of ethanol (which is less energy-efficient than gasoline and produces more smoke) and controversial additives like methyl tert-butyl ether (MTBE), might still draw criticism towards the program.
In times of crisis or natural disasters, the EPA may waive the summer fuel mandate. On April 29, 2022, the EPA issued an emergency exemption for high-ethanol gasoline blends, allowing their sale in the summer to help lower gasoline prices, which had significantly risen due to inflation in the United States.
Two other instances that prompted the EPA to waive the summer fuel mandate were the ransomware attack in 2021 that shut down the Colonial Pipeline, which supplies natural gas to states along the East Coast, and the COVID-19 pandemic in 2020. In the latter case, reduced gasoline demand meant a longer transition from winter to summer fuels was needed due to limited natural gas storage.
Storing off-season fuels is crucial, as the reactions of winter and summer fuels differ under atmospheric pressure, especially in terms of evaporation. Storing fuel off-season may lead to evaporation and harm engine performance.