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Fuel Lubricity
What is it? What are the
important effects?
Lubricity:
The property of a lubricant that
causes a difference in friction under conditions of
boundary lubrication when all the known factors
except the lubricant itself are the same. The lower
the friction the higher the lubricity. [Kajdas, C.,
S.S.K. Harvey, and E. Wilusz, Encyclopedia of
Tribology, Elsevier, New York, 1990.]
Lubricity is actually a very
difficult property to characterize. In spite of the
intent of the definition's attempt to separate the
lubricity as a fluid property, it is also strongly
dependent on the method used to measure it and on
the characteristics of the solid surfaces being
lubricated. For example, as friction occurs small
particles of the solid material may be removed and
entrained in the lubricant. In some applications,
these particles will be swept away by a flow of
lubricant while in others, the particles stay in the
vicinity of the surface contact. Particles that are
present in the area of surface contact may act very
differently. In some cases, they may act as an
abrasive to increase wear while in other cases, the
particles may shield the surface from further wear.
When trying to characterize lubricity, it is
important to use a measurement technique that
correlates well with the actual lubrication
situation. In the case of diesel fuel, the fuel acts
as a lubricant for the finely fitting parts in the
diesel fuel injection system. While all diesel fuel
injection systems depend on the fuel to act as a
lubricant, rotary pump style injection systems seem
to be the most sensitive to fuel lubricity.
Why and where is it
important?
The need for diesel fuel lubricity
has been recognized for many years. Most early
concern focused on the use of #1 diesel fuel in
place of #2 diesel fuel under cold weather
conditions. Higher wear rates with #1 diesel fuel
would be aggravated by #1 diesel fuel's lower
viscosity. However, when the U.S. Environmental
Protection Agency mandated that the sulfur content
of on-highway diesel fuel be lowered from 5000 ppm
to 500 ppm in 1993, fuel lubricity captured national
attention. There is still disagreement about what
specific fuel changes are caused by the sulfur
reduction that result in lubricity reduction. Some
have suggested that sulfur compounds themselves
provide lubricity, others have suggested that
nitrogen compounds or naphthenic hydrocarbons are
responsible. In any case, there is general agreement
that the hydrotreating process used by petroleum
refineries to remove sulfur results in lower fuel
lubricity. Recent regulations by the EPA to further
lower the sulfur content of diesel fuel to 15 ppm
are expected to make the fuel lubricity even worse.
The addition of small amounts of
biodiesel (0.25% to 2%) to diesel fuel has a
dramatic effect on the lubricity of that fuel. Pure
biodiesel and high level blends have excellent
lubricity.
Measurements of Lubricity .
There are two methods that are
commonly used to measure lubricity, the Scuffing
Load Ball On Cylinder
Lubricity Evaluator (SLBOCLE
- ASTM D 6078-99)
and the High Frequency Reciprocating Rig (HFRR
- ASTM D 6079-99).
The
apparatus used for the SLBOCLE test involves
placing a steel ball
bearing
against a rotating steel ring whose lower edge
is immersed in the test fluid. Weight is
gradually applied to the ball until a “scuff”
mark is seen on the
rotating ring
. The tangential force is also measured
and the point of scuffing is indicated by a
large increase in the friction coefficient. The
EMA has indicated that a weight of 3150 grams is
representative of an acceptable lubricity
level. The higher this number, the better the
fuel lubricity.
The
HFRR test also uses a steel ball but in this
case the ball is held against a stationary disk
and the ball is reciprocated back and forth
across the disk with a frequency of 50 hertz.
The applied load is 200g and the test duration
is 75 minutes. The wear scar produced on the
disk is measured and a scar diameter of less
than 450 micron is considered to be acceptable.
Schumacher and Adams
[10th Biennial Bioenergy Conference –
Bioenergy 2002, Boise, Idaho, Sept. 22-26, 2002]
have measured the effect of low-level blends of
soybean-based biodiesel on biodiesel that has
been produced to meet 15 ppm sulfur levels.
Figure 1 shows SLBOCLE results for No. 2 diesel
fuel with small amounts of biodiesel. As little
as 1% biodiesel changed the diesel fuel from an
unacceptable level to an acceptable level.
Figure 1.
SLBOCLE for #2 Diesel
2004 Tier 2 Fuel, Biodiesel, and Biodiesel
Blends
Figure 2 shows the same
effect for No. 1 diesel fuel that has also been
treated to lower the sulfur contain to less than
15 ppm. In this case, the lubricity of the
original No. 1 diesel fuel was so low that even
2% biodiesel was not able to bring the lubricity
back up to the acceptable level of 3150 grams.
However, the lubricity was greatly improved and
it is unlikely that the engine would suffer
damage from short term use at a lubricity level
of 2880 grams.
Figure 2.
SLBOCLE for #1 Diesel
Fuel, Biodiesel, and Biodiesel Blends
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