Why Diesel Fuel Economy Drops in the Winter

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Why Diesel Fuel Economy Drops in the Winter

Diesel fuel, particularly in the northern tier states changes rather significantly from season to season. In the cold weather months generally starting in September or October refiners begin to alter the chemical composition of diesel fuels to improve cold weather operability characteristics to meet ASTM, Pipeline Operator, and Customer requirements and specifications.

Refiners talk about the components that come out of the refining process as “streams”. In a typical refinery today there can be over 180 “streams” coming from the refining of crude oil. The addition of lighter product streams are known by names such as “aromatic chemicals”, “naphtha's”,” kerosene’s” and others to #2 diesel (whether Ultra-Low Sulfur Diesel (S-15) or Low Sulfur Diesel (S-500)) will lower (improve) the Cloud Point (CP), Cold Filter Plug Point (CFPP) commonly referred to as the gel point, and Pour Point (PP) depending on how much of those components are added to the base fuel. Refiners have a lot of latitude in determining how much of and what components are used to make these improvements.

The issue from a fleet operators standpoint is that these changes lower energy (Btu) content of the fuel. It is normal for fuel economy to decrease from one to as much as five percent seasonally. This decrease can be further exacerbated by fuel racks and or distributors further cutting with kerosene to try and improve cold weather operability. The normal energy content of #2 ULSD ranges between 138,000 and 140,000 Btu’s, kerosene is much lower ranging between 130,000 and 135,000 Btu’s, whereas gasoline is about 124,000 Btu’s per gallon.

As you can see the more lighter components added to fuel, the lower the energy content. Note: ULSD has 1%-3% lower Btu content than the LSD. This is primarily due to reduction in wax content in ULSD.

So if you put all of this together in a time line, you can see that you begin using additive to improve cold weather performance at the same time the refiners are blending the fuel in a way that reduces Btu content which lowers your fuel economy, then in the spring you stop using additive at the same time the refiners are going to a “summer” blend which increases the Btu content and so your mileage goes up.

Other cold weather considerations are more idle time, slower transit speeds, more time in traffic, and even driving through snow all, of which can have a significant negative impact on fuel usage.

Diesel Doctor

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Last modified: 04/06/09
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