June 2020 - 33

which isn't too thick where you waste energy
trying to push through it and isn't too thin that
metal surfaces rubbing against one another touch
and wear. High temperature and high shear
conditions such as those found in the engine
power cylinder are particularly important for this
balance. FA-4 has a lower high temperature, high
shear than CK-4, and the benefit of that is when
you take low viscosity oil and design an engine
around that, [you] get more fuel efficiency from it."

DPF performance

Since the introduction of CJ-4 oils in 2006, engine
oil manufacturers have been placing limits on
certain chemicals in the oils to increase their
compatibility with the DPF, says Mark Betner, heavy
duty lubricant manager for Citgo. Those chemical
limits have remained in place for CK-4 and FA-4 oils
as well. By limiting harmful chemicals and working to lower oil viscosity, engine oil manufacturers
have found ways to optimize the performance of
the DPF and aftertreatment system as a whole with
improved engine oil formulations.
SAPS
Since the introduction of DPF-compatible engine
oils, engine oil manufacturers have needed to be
careful of how much sulfated ash, phosphorous,
and sulfur (SAPS) is present in the oil. While
phosphorous and sulfur are beneficial in small
amounts, aiding in wear-protection, too much of
either could cause issues with the DPF or aftertreatment system overall. As for sulfated ash, this
is the byproduct of the burning of metallic engine
oil additives. High amounts of sulfated ash will
clog the DPF, hindering DPF performance and
causing it to require increased maintenance.
"Diesel engines that utilize a diesel particulate
filter should [use] an oil that is 'low SAPS,'" says
Dan Arcy, global OEM technical manager for Shell
Lubricants. "Since 2007, all on-road heavy duty
diesel engine manufacturers recommend the use
of oils that have less than 1 percent sulfated ash."
As for phosphorus and sulfur amounts, they
are limited to 0.12 percent and 0.4 percent,
respectively.
If the SAPS rise above those levels, problems
will arise.
As mentioned above, excessive amounts of
sulfated ash will clog the DPF. A clogged DPF
means the filter cannot properly perform its job
of capturing particulate matter and ash to ensure
a less harmful exhaust is released into the environment. It also means more frequent cleanings
of the DPF or a greater chance of damage to the
DPF, resulting in increased downtime for the fleet.
Fleets should be aware, though, that all
engine oils have a certain amount of metallic
compounds that will lead to sulfated ash, says
Chevron's Booth. Any ash-forming compounds in
the engine oil will eventually hinder DPF performance. However, using engine oils with 1 percent
or less sulfated ash will decrease the frequency
of necessary DPF maintenance.
Increased amounts of sulfur and phosphorus
do not directly impact the DPF but will affect the
aftertreatment system as a whole. More sulfur or
phosphorus in the engine oil can cause poisoning

of the catalysts, says ExxonMobil's Cigala, which
will render the catalysts ineffective.
If the catalysts are not working properly, then
neither can the DPF or aftertreatment system overall.
Though lower levels of SAPS are best in the
engine oils, Exxon Mobil's Cigala warns that as
these levels are lowered, fleets must be aware that
durability and longevity of the engine and aftertreatment system could be negatively impacted if
the formulation of the engine oil is not properly
compensated to make up for the wear-protection
provided by those additives.
Engine oil consumption
In order to keep the DPF and all the other components of the aftertreatment system protected, the
engine oil must have good viscosity control.
"Viscosity dictates the separation [fluid film
thickness] between engine parts moving relative
to one another," Booth says. "If an engine oil shears
down significantly, this can result in an oil film
that is too thin to protect interacting surfaces
from wearing against one another. One such area
is the piston ring and cylinder liner. If wear occurs
between the ring and liner, then more oil can reach
the combustion chamber and be consumed."
Additionally, due to the higher temperatures
newer engines are running, oxidation of the
engine oil can occur, cutting down on the oil's
longevity, Cigala says.
Oxidation can cause deposits to develop on
the piston.
"Piston deposits can cause sticking of the
piston ring, meaning it can't seal properly, or
wear can ensue - deposits are abrasive and
make gaps between parts wider," Booth says.

40597297 | Lukas Gojda | Dreamstime

By limiting harmful
chemicals and lowering
oil viscosity, engine
oil manufacturers
have optimized
the performance
of the DPF and the
aftertreatment system.

"In either case, more engine oil can reach the
combustion chamber, and thus be consumed
or burned in the combustion process. In both
cases, oxidation and viscosity-loss, the consequence is increased oil consumption through
the combustion chamber resulting in greater
ash loading in the exhaust stream, and thus
clogging in the DPF."
CK-4 and FA-4 engine oils have the viscosity
and oxidation control fleets need, as well as the
benefit of better fuel economy.
According to Cigala, 15W-40 engine oils were
pretty much standard across the industry before
CK-4 and FA-4 oils were introduced. The benefits
of using these lower viscosity oils are a 1 to 1.5
percent fuel economy benefit when going from
15W-40 oil to a 10W-30 oil, and an additional 0.5
to 1 percent fuel economy benefit when going to
an FA-4 oil in either a 10W-30 or 5W-30. He notes
lower viscosity oils make engine starts easier in
the winter, and though some fleets believe higher viscosity oils provide better wear-protection,
the lower viscosity oils have to pass the same API
testing for wear-protection.

DPF maintenance

Maintaining a DPF involves more than just cleaning the DPF after it's clogged. Fleets also need to
use the proper engine oil to keep the DPF and
aftertreatment system as a whole running smoothly and efficiently.
"The most important thing is piston cleanliness,"
says Shell's Arcy. "This will have [a] direct effect on
DPF maintenance. There are several factors that
determine DPF maintenance intervals and overall service life. The main factor is the amount of
oil that is burned. Controlling piston cleanliness,
which in turn helps control oil consumption, will
have a big effect on DPF service life. Diesel engine
oil must provide strong piston cleanliness, otherwise oil consumption will be high, and DPF ash
buildup will be accelerated. A clean engine with
clean pistons will prevent oil from passing by the
rings and entering the combustion chamber and
being consumed [burned]."
Since the burned oil contains non-combustible materials, Arcy explains, ash accumulates
in the DPF, with the amount of ash directly
correlating to the amount of oil consumed as
well as the amount of "ash forming components
within the oil, such as detergents and antiwear components."
Engines inherently consume engine oil, notes
Chevron's Booth. There are performance characteristics of oil within the engine that can lessen or
worsen the level of consumption, but the sulfated
ash content is the greatest challenge impacting
DPF clogging. According to the research conducted
by Chevron, 90 percent of DPF clogging is caused
by engine oil-related ash, and regardless of the
engine oil used, Booth says a certain amount of
the engine oil will be consumed.
It is important for fleets to understand that
an engine oil with low-SAPS, good viscosity
control, and that follows the recommendations
of their vehicles' OEMs will have the greatest
overall impact on proper DPF and aftertreatment
system performance.

June 2020 | VehicleServicePros.com

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June 2020

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