Winter Gasoline: What is the Difference?

Every year in the Fall we hear about how the gas stations are switching over to winter gasoline. The good news is that winter gasoline is a little cheaper. Have you ever wondered what the difference between winter and summer gasoline is and why one is cheaper than the other?

Gasoline is made from crude oil. Crude oil consists of a wide mix of chemicals. Refineries process crude oil and separate out the various constituents. These products are then blended into gasoline. The refinery does not convert crude oil directly into gasoline. What actually happens is the refinery uses the crude oil to make blending components which are then combined to make gasoline. Refineries have a multitude of operating units, some of which could be catalytic crackers, alkylate units and reformers. Each has a different cost to run.

When gasoline is made by blending these components, there are certain specifications that must be met. One of them is the octane rating. Another very important but less well known specification is the vapor pressure of the gas. Every fluid has a vapor pressure that depends on the temperature. Higher temperature gives more vapor pressure. When the vapor pressure is greater than the atmospheric pressure, the liquid will boil. For example, water at sea level boils at 100 degrees Celsius because water at that temperature has a vapor pressure of one atmosphere. If you climb a mountain, the water will boil at a lower temperature because the air pressure at altitude is lower. Thus the water vapor pressure reaches the air pressure at a lower temperature. The result of trying to cook in water that is boiling at a lower temperature is that you have to wait longer for your food to be done.

The vapor pressure of gasoline also rises with temperature. That means that in the baking heat of summer, the vapor pressure of gasoline in your car's fuel tank might rise up above atmospheric pressure. That would make it boil, filling your tank with gas fumes. The gas will escape into the atmosphere, causing pollution. To reduce that pollution, the gasoline must be changed so it has a lower vapor pressure at a given temperature.

The vapor pressure of gasoline depends on the components that are blended to make it. One of the cheapest gasoline blending components is butane. Butane also has a very high vapor pressure. So in the summer, gasoline has to be made with very little butane. That butane must be replaced with something more expensive. In the winter, when temperatures and thus vapor pressures are lower, cheap butane may be used in greater amounts.

So the big difference between summer and winter gasoline is the amount of butane blended in to make the gasoline. The cheap butane means winter gasoline can be a little cheaper too. Next time you hear about winter gasoline you will know what they mean. Winter gasoline means more butane!