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
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.