Page 48 - Energize August 2021

TECHNICAL

Local manufacturer Sasol introduced 10 ppm diesel to the The soot particles themselves are sub-micron in size when
market in late 2013 as part of the initial roll-out strategy, but to formed, but with progressive fuel usage, large quantities of these
date 10 ppm is still not widely available nationwide. Sadly, falling particles are continually deposited in the oil and will eventually
behind on fuel quality limits the availability of new engine technology agglomerate. Dispersants now have to fight a battle on two fronts
as low sulphur fuels are essential to enabling advanced control – with the use of EGR systems and with the requirement to increase
technologies and fuel‐efficient designs. oil drain intervals.
As already discussed, the EGR system recirculates a
Durability small amount of cooled exhaust gas back into the combustion
If all the requirements already covered were not difficult enough for chamber which lowers the combustion temperature, which in
lubricant blenders to contend with, there are also additional trends turn reduces the NOx gases. However, recirculating exhaust gas
affecting future diesel engine oil formulations such as increased oil also creates a multi-pass opportunity for soot to accumulate in
drain intervals, smaller sump levels, higher running temperatures the engine oil.
and shear forces – all of which put increased stress on the lube oil. Extending oil drains also increases soot levels in the oil. If
The oil inside your engine has become a warzone. soot is not adequately dispersed by the engine oil, it can cause
To support greater engine durability under these punishing sludge to form on rocker and front engine covers, bearings to fail,
conditions, lubricants are expected to resist increased oxidation and valve bridges and fuel injection links to wear, and filters to plug.
thermal degradation, keep deposits and sludge to a minimum and The durability of the lubricant and additive system in relation to
keep engine components cleaner for longer as well as neutralise the the ability to disperse soot and maintain a regime of reduced
cocktail of acidic species between drain intervals. wear has led to significant changes in additive formulation.
Let’s review the additives that have been tasked with turning a Increasing concentration of soot contamination causes a
can of oil into a bulletproof lubricant. variety of problems, including sludge formation, higher operating
temperatures, loss of anti-wear performance, increased viscosity
Detergents and filter plugging. Added to this the additional soot loading
Detergents are cleaning agents that contain metals. They work at increases the strain on the dispersant’s ability to function
high temperatures in the combat zone of the engine (pistons, rings, optimally.
liners and valves) to reduce or remove deposits on surfaces and In response to this issue, lubricant blenders have had to
also in the bulk of the oil. They also neutralise acidic compounds increase the treat rate of this additive and these days, dispersants
formed during the combustion of diesel or due to base oil are typically one of the major components making up between 30 to
oxidation. The all-important Total Base Number (TBN) of the oil is 60 per cent of the additive package.
an expression of this neutralisation ability. The amount of TBN an However, as lubricants move to lower viscosity for enhanced
oil contains is based on the intended application and fuel used, fuel economy, the thickness of the polymeric-based dispersant
specifically the amount of sulphur in the diesel. The majority of becomes problematic and use of higher treat rates of thicker
metallic detergents is based on either calcium or magnesium dispersants complicates formulation, resulting in the use of
attached to an oil-soluble organic soap – typically sulphonates, lighter base stocks, which in turn can result in higher volatility
phenates or salicylates. lubricants.
However, because magnesium-based detergents provide a higher
TBN per unit of sulphated ash produced, they are now favoured in Antioxidants
formulations. A reduction in TBN is expected with many low SAPS Oxidation is a form of irreversible chemical deterioration of the
oils. While the TBN of new oil is important, the ability of oil to retain lubricant. It is caused by the base oil combining with oxygen,
TBN over extended drain intervals is arguably more critical than the sulphur and nitrogen to form harmful compounds. It can also be
absolute value in the new oil. As already discussed, the introduction caused by additive depletion, contaminants in the oil such as
of low SAPS oils goes hand in hand with the use of low sulphur fuels, metal particles, soot, fuel and glycol as well as increased operating
which should produce less sulphur-derived acids in the oil. With this temperatures.
in mind, it could be argued that the base number of low SAPS oils Oxidation creates oil-insoluble high-molecular-weight molecules
does not need to be as high. However, the lower amounts of sulphur- that increase the viscosity of the lubricant, accelerate wear and
derived acids that the oil is exposed to could be partially offset by eventually lead to varnish-formation, typically on pistons and
higher acid dewpoints from higher EGR rates, by higher amounts of valves in engines. These varnish deposits can cause valve and ring
nitric acid and from oxidation acids resulting from higher engine oil sticking, eventually leading to accelerated wear.
temperatures. TBN will therefore still need to be maintained at a level The use of EGR systems can increase the rate at which the
to provide adequate protection against these acids. oil oxidises as many of these systems rely on the engine’s coolant
system to reduce exhaust gas temperatures, which increases the
Dispersants engine running temperature. This elevates the engine oil sump
Dispersants are non-metallic, ashless cleaning agents that inhibit temperature, which in turn puts more demand on the lubricating
sludge-formation by keeping insoluble contaminants like soot properties of the oil. Exacerbating the situation is the added
dispersed in the lubricant and preventing them from coating requirement of increased oil drain intervals, which means the oil
metal surfaces. needs to be in use for longer.

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