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We are living in a truly remarkable day in the history of the automobile. It is a day in which automakers are producing vehicles that are more powerful than ever before while also remaining more fuel efficient than ever.

The same factors which make vehicles more fuel efficient also help to reduce harmful exhaust emissions. These factors include onboard diagnostic systems, emission control systems such as catalytic converters and evaporative emissions systems, as well as the blending of automotive fuels to suit specific climates and geographic locations.

Changing seasons seem to always bring about fluctuations in fuel mileage for gasoline powered vehicles. While variations in ambient temperature may result in a slight difference in fuel mileage, changes in fuel composition between winter and summer blends is the key contributing factor. These different blends are also the reason that fuel prices always seem to go up in the warmer, summer months – and you thought fuel companies were just greedier during vacation season.

You might be surprised to learn that exhaust emissions are not the only concern when formulating seasonal blends of gasoline. Exhaust emissions are not even the chief concern. When fuel is stored, it has a proclivity to evaporate and release harmful fuel vapors into the atmosphere. By varying the fuel formula between seasons, fuel evaporation can be minimized and evaporative emissions significantly reduced.

Regional fuel blends also promote dramatic changes in fuel efficiency and price. In densely populated urban areas, gasoline is required to meet stricter fuel composition standards. In areas like Southern California, gasoline must contain a minimum of 5.6-percent ethanol which results in a fuel oxidation level of 2-percent by weight. This type of fuel is more expensive to produce and it burns at a less efficient rate.

Gasoline is derived from crude oil and composed of many hydrocarbons. Gasoline blending components such as alkylate gasoline (for increased octane) and straight run gasoline (lower octane) can be blended to produce a fuel that meets federally mandated specifications for Reid Vapor Pressure (RVP) and octane rating. RVP is the vapor pressure of any specific gasoline blend when it reaches 100-degrees-Fahrenheit.

When the vapor pressure of any liquid exceeds the atmospheric pressure for a particular place and climate, it will begin to boil. Think of a pot of water as it is heated and releases steam (vapor) into the atmosphere. Because of the volatility of gasoline blends, it has a higher level of vapor pressure than water and begins to boil at a much lower temperature. Certainly, gasoline does not boil as violently as a pot of water (because the temperatures aren’t nearly as hot) but the vapors released are much more harmful to the atmosphere and ozone layer.

Atmospheric pressure is created by the air over our heads. It varies between geographic locations and is affected by elevation and temperature but usually hovers around 14.7 pounds-per-square-inch (psi). Therefore, the RVP level of gasoline blends must remain at or below this approximate level in order to reduce evaporative emissions effectively. The more the ambient temperature rises, the lower the desired level of RVP.

Each compound that contributes to a gasoline blend is added with optimum fuel efficiency and minimum evaporative and exhaust emissions in mind. Of all the ingredients in any gasoline blend, butane has the highest vapor pressure. It is also the most abundant of all the ingredients. Gasoline blends with higher butane levels (winter blends) are less expensive to produce than blends with less butane (summer blends).

Winter gasoline blends have a higher molecular weight and are more volatile than summer blends because of a higher RVP and octane rating. Increased fuel volatility means that fuel can be atomized more thoroughly (increased fuel mileage) but it also evaporates more easily at higher ambient temperatures. In colder climates, this is not an issue but in locations where the temperature remains unseasonably warm, it could lead to reduced fuel mileage because of fuel evaporation. Winter blend gasoline is typically sold from mid-September through May, depending upon the severity of the climate in the region where it is sold.

In the summer months, when temperatures rise, a gasoline blend with a higher molecular weight and lower volatility is sold. The decreased volatility means a lower evaporative rate but it may also result in diminished fuel mileage. A typical summertime gasoline blend might consist of:

  • 40-percent fluid cracked catalytic (FCC) gasoline
  • 25-percent straight run gasoline
  • 15-percent alkylate gasoline
  • 18-percent reformate gasoline
  • 2-percent butane

This type of summertime mixture should yield an RVP of approximately 7.5 to 9 psi (as mandated by the federal government).

When cooler temperatures arrive, the percentage of butane will increase and one of the less volatile ingredients will decrease. The RVP can now reach approximately 15 psi because of the lower temperatures.

Gasoline blends leave the refinery as Reformulated Gasoline Blendstocks for Oxygen Blending (RBOB). When the RBOB arrives at the terminal, additional oxygenating compounds (such as ethanol) are added to produce the finished product. These oxygenating compounds can further reduce fuel mileage but they also help reduce exhaust emissions.

Excess fuel vapors may actually promote increased fuel efficiency when the vehicle is in operation. Unfortunately, when the vehicle is parked, these same vapors tend to diminish fuel mileage and pollute the atmosphere. Fuel vapors are captured by the evaporative emissions (EVAP) system and held in a reservoir with a charcoal accumulator at its core. Once the engine reaches normal operating temperature and driving conditions are ideal, these excess fuel vapors are drawn into the engine intake manifold using vacuum. These fuel vapors are then burnt as fuel by the engine, thus increasing fuel mileage and reducing evaporative emissions. It is critical to optimum fuel mileage that the EVAP system is kept in working order. If the EVAP system fails, a malfunction indicator lamp will be illuminated. EVAP system malfunctions should be repaired as soon as possible. Allowing the EVAP system to remain inoperative will certainly affect your fuel mileage in a negative manner.

Hopefully, this article has helped to unlock some of the mysterious seasonal and regional fluctuation in your fuel mileage. It is a good idea to manually calculate your fuel mileage according to the actual amount of fuel purchased and the total miles driven. Remember that variations in fuel composition can wreak havoc with electronic onboard fuel calculation devices. Make your own calculations and save yourself a world of confusion.

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