Why Winter Diesel Is Adjusted Differently Than Winter Gasoline (And Why It Matters)

If you don't drive a diesel vehicle, you may not realize that diesel fuel isn't always the same formula all year round, depending on the location. In fact, winter diesel is only available during certain times of the year and it's adjusted differently than winter gasoline. The reason for that is because both fuel types actually have different cold-weather requirements.

Winter diesel is adjusted for cold weather primarily because it can develop flow problems as temperatures drop. This is due to compounds in diesel that can begin to crystallize at low temperatures, which can potentially restrict the system. To help address this problem, the EPA allows some refiners to produce No. 1 diesel as a cold weather alternative, with the option of using cold-flow improver additives as well. In contrast, gasoline is adjusted for the winter by raising its vapor pressure, which helps the fuel form a more combustible mixture when temperatures are low.

This difference matters because diesel and gasoline engines rely on two separate methods to ignite their fuel. Diesel engines use compression to generate the heat needed for ignition, while gasoline engines use a spark. This means that, on a basic level, both fuel types have different requirements when temperatures fall. So, if diesel is not adjusted, there is a chance the fuel may not flow, meaning the vehicle may not run smoothly, or even start up at all. If gasoline isn't adjusted, the vehicle may struggle to properly ignite the fuel.

Cold temperatures create problems that just aren't an issue during warmer weather, which is why diesel fuel is often adjusted each season. Because of this, winter diesel can contain more No. 1 diesel, which flows more easily in the cold. In contrast, No. 2 diesel has greater energy content and is typically preferred when freezing temperatures aren't an issue.

But how much the fuel changes actually depends on the climate, with colder areas requiring more aggressive winter blends than warmer regions. In colder states, suppliers may begin using winter blends as early as October, while southern states may wait until as late as December. Winter blends can also get stronger as time goes on and temperatures drop, with more No. 1 diesel used for extremely cold areas. As the winter season ends, the move back to No. 2 diesel gradually happens, with the changeover usually completed by late March or early April.

Seasonal changes for gasoline are due to air-quality requirements. During the summer, for example, fuel refiners have to meet lower vapor-pressure limits because gasoline evaporates more easily in hot weather. This can contribute to increased emissions, which can worsen air pollution. Since those restrictions are more relaxed during the colder months, fuel suppliers are allowed to use gasoline with more combustible properties. The transition to winter gasoline usually begins after September 15, while summer blends begin around June 1.