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Summer blend gasoline vs winter blend gasoline

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Have you ever wondered why there is a Summer Blend Gasoline and a Winter Blend Gasoline?  And have you ever wondered why the price of gasoline is generally less in the winter time verses the summer time?  Many believe it is because of the lower demand for fuel in the winter time because people are traveling less.  While this could contribute to the lower price it is generally the Clean Air Act that causes the price decrease and for there to be a winter blend of gasoline.  More precisely the EPA controls the Reid Vapor Pressure (RVP) of gasoline.  Reid Vapor Pressure (RVP) is the measure of the vapor pressure, or volatility, of gasoline at 100 degrees fahrenheit.

Air pressure has a RVP of 14.8 psi.  Summer blend gasoline has a RVP generally between 7.8 and 9 psi.  As the RVP of gasoline rises more of the liquid gasoline becomes vapor.  We want the gasoline vapor in the air we breathe at a minimum.  By controlling the RVP of gasoline the EPA is helping to keep gasoline vapors in the air within an acceptable range.

The atmospheric pressure will change with variations in altitude, the amount of moisture in the air and with changes in temperature.  With gasolines higher volatility it is the large temperature change from summer to winter that lowers the RVP or volatility of gasoline.  The refineries, pipelines and fuel storage facilities will add butane to gasoline to keep the RVP high enough in the winter months so gasoline has sufficient volatility.  In the winter time approximately 10% to 12% of the makeup of gasoline is butane.  Butane is also very inexpensive and plentiful.  By having 10% of the volume of gasoline butane the benifits are twofold.  The butane concentration increases the gasoline supply and the production cost of gasoline is less.  Plus the 10% butane mixture contributes approximately 5.2 psi to ther RVP of the gasoline.  The higher vapor pressure of the gasoline leads to a fuel which is more volatile at cold temperatures.  Gasoline engines need the higher volatility to help start when it is cold.  Who knew?  Butane in our gasoline reduces our costs per gallon and helps enable the fuel to ignite in our engines through the cold fall and winter months.

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