ASK THE MXPERTS: SEASON OF THE WHICH?

Different season, different fuel.

SEASON OF THE WHICH?

Dear MXA,
I hear a lot about winter and summer gas. What is it?

During the summer, pollution is a frequent concern due to increased levels of heat, smog and ozone, which can harm the lungs. Summer heat boosts the formation of ozone, while the appearance of an inversion layer—an immobile layer of air—can trap pollutants in the lower atmosphere. Thus, gas stations carry a different fuel blend in the summer than in the winter. The difference between summer and winter-blend gasoline involves the Reid Vapor Pressure (RVP) of the fuel. RVP is a measure of how easily the fuel evaporates at a given temperature. The more volatile a gasoline (higher RVP), the easier it evaporates.

Winter-blend fuel has a higher RVP because the fuel must be able to evaporate at low temperatures for the engine to operate properly, especially when the engine is cold. If the RVP is too low on a frigid day, the vehicle will be hard to start and, once started, will run rough.

Summer-blend gasoline has a lower RVP to prevent excessive evaporation when outside temperatures rise. Reducing the volatility of summer gas decreases emissions that can contribute to unhealthy ozone and smog levels. A lower RVP also helps prevent drivability problems, such as vapor lock on hot days, especially in older vehicles. Summer-blend gasoline contains approximately 2 percent more energy than winter-blend gas, thus gas mileage is slightly better in the summer; however, the summer blend is also more expensive to produce, and that cost is passed on to the consumer.

The switch between the two fuels happens twice a year—once in the fall (to winter blend) and again in the spring (to summer blend). Gas stations typically have until June 1 to switch to summer blend and then go back to winter blend by September 15.