Living near Chicago has its advantages, people say. I’m still trying to think of one. The one good thing that happens in winter here is the high-priced summer blend is over
Refineries brew their summer blends by removing hydrocarbons that are more inclined to evaporate in hot weather. These chemicals, called volatile organic compounds (VOC’s), react with airborne pollutants in the summer sun to form ozone, one of the main components of smog. From June 1 to September 15, the Environmental Protection Agency (EPA) mandates that pumps in more than 30 large cities must meet special low-evaporation criterion.
Summer blend gases started in 1995, as required by the Clean Air Act’s 1990 amendments and even cleaner, mixture was phased in the summer of 2000. Since enacted, there have been sharp spikes in fuel prices every spring as summer blends roll out. This is not so much because it’s expensive to make the gas (the added cost per gallon is only 1 or 2 cents) but because refineries generally try to sell every last bit of winter fuel before mixing in the slightly more expensive summer batch. Sometimes they allow the stock too deplete too far which creates shortages before the first deliveries of summer blend entering the supply chain. (Nice, huh?) The return to normal blends in the fall causes a far less pronounced spike because the industry, free from summer standards, doesn’t bother selling off the summer gas before mixing in the less pricey stuff.
The difference between conventional summer and winter blend gasoline has to do with the Reid Vapor Pressure (RVP) of the fuel as it relates to the volatility of a gasoline. The more volatile a gasoline, the more likely it will evaporate as the temperatures rises; evaporated gasoline donates to unhealthy ozone and smog levels. Summer gasoline has a low RVP and is less likely to evaporate when equated to the high RVP winter grade. The EPA states conventional summer blend gasoline contains 1.7 percent more energy than winter blend gas, which is the reason the summer blend gets a bit better gas mileage.
So why don’t we use the summer blend year-round? The main reason is that summer blend gas doesn’t work as well in the winter. Summer blend’s low evaporation rate makes engines less likely to stall in hot weather, however can make them difficult to start in the cold.
The RVP is the vapor pressure of the gasoline blend when the temperature is 100F/38C. Normal atmospheric pressure varies, but is usually around 14.7 pounds per square inch (psi). Atmospheric pressure is caused by the weight of the air over our heads. If a liquid has a vapor pressure of greater than local atmospheric pressure, that liquid boils. For example, when you heat a pot of water, the vapor pressure increases until it reaches atmospheric pressure, at that point, the water begins to boil. (Thank you meteorology class, I actually understood this!)
In the summer, when temperatures can exceed 100F/38C in many locations, it is important that the RVP of gasoline is well below 14.7 psi. Otherwise, it can cause pressure in gas tanks and gas cans, and it can boil in open containers. BOOM! Gas that is evaporated ends up in the atmosphere and contributes to air pollution. Consequently, the EPA has declared that summer gasoline blends may not exceed 7.8 psi in some locations, and 9.0 psi in others.
The RVP of the gasoline blend rests on on how much of each element is in the blend and what the RVP is of each component. Butane is a relatively inexpensive ingredient in gasoline, however, it has the highest vapor pressure at around 52 psi.
In the gasoline blend, each component adds a portion to the total RVP. Simply, in the case of butane, if there is 10% butane in the winter blend, it will contribute around 5.2 psi (10% of 52 psi) to the overall blend. This means that in the summer, the butane fraction must be very low in the gasoline or the overall RVP of the blend will be too high.
And that’s why gasoline prices generally fall back in the fall, and spring forward in the spring, just like the clock!!