Page 46 - Energize August 2021

TECHNICAL

engines with roller follower valve train systems have relatively little
boundary lubrication and friction modifiers may not demonstrate
significant fuel economy benefit, while engines without roller
followers may show more benefit.

Viscosity index improvers
Another additive group gaining in importance as the technical
demands placed on engine lubricants continues to grow is
viscosity index improvers (VIIs). VIIs are large polymer additives
which partially prevent the oil from thinning out (losing viscosity)
as operating temperatures increase. They are also responsible
for better oil flow at low temperatures, resulting in reduced wear.
VIIs contribute to improved fuel economy primarily through shear
thinning and viscosity temperature properties.
Properly designed VIIs support higher lubricant viscosities in the
Blocked DPF from DACK
hotter engine operating environments for robust wear protection,
while maintaining lower viscosities in moderate engine temperature
environments, which provides fuel economy benefits. As the use There are three major mechanisms for possible interference
of lower viscosity engine oils increases, the role of VIIs in providing between the lube oil’s components and aftertreatment devices:
wear protection will follow suit. Selection of the correct VII also poisoning, deactivation and accumulation of ash deposits derived
allows lubricant formulators to meet the minimum HTHS viscosity from the lubricant’s additive package. We will take a closer look at
while lowering kinematic viscosity at the same time. Finally, these mechanisms and the additives that cause them.
combining FMs with the correct VIIs can create a synergistic effect
which improves overall engine operating efficiency. Sulphated ash
The term sulphated ash relates to the amount of incombustible
Emission system compatibility metallic ash that remains as a result of engine oil combustion. This
The most important emission aspect of modern-day diesel engine ash is mostly derived from the engine oil’s detergent and anti-wear
oil formulation is its compatibility with exhaust aftertreatment additive chemistry. Calcium and magnesium-based detergents and
technologies. The chemical composition of a diesel engine oil zinc-based anti-wear and antioxidant additives, most commonly
contains, among other things, sulphated ash, phosphorous and Zinc dialkyl dithiophosphate (ZDDP), are the most common sources
sulphur. These substances form part of the additive chemistry of lubricant-derived ash.
that provides the oil with the following attributes: detergency, All engines, even the most modern, consume a small amount
neutralisation of acids, anti-wear and antioxidants. of oil as part of their normal operation. The oil is burned in the
Unfortunately, these chemicals are proving problematic with combustion chamber along with the fuel, and the resulting small
the current emission technology on offer and, as a result of these amount of residual ash from the oil is later trapped in the diesel
compatibility issues, lubricant manufacturers are facing chemical particulate filter (DPF). During regeneration, the particulate matter
restrictions which are aimed at protecting emission control systems, (PM) is oxidised and removed from the filter, but the ash from the
by limiting the levels of these chemicals used to formulate a lubricant cannot be oxidised and remains in the filter, causing the
lubricant. DPF to become irreversibly blocked.
This has given rise to a new class of engine oils – low SAPS. As the ash particles accumulate, they result in irreversible
The abbreviation SAPS stands for “sulphated ash, phosphorus filter blockage, which also increases back pressure to the engine,
and sulphur”. A low-SAPS engine oil is therefore an oil with a
low proportion of sulphate ash, phosphorus and sulphur. These
oils are also designated “low-ash” due to their low tendency to
ash formation. The requirement to use low-ash additives in the
formulation of modern diesel engine oils may sound simple but
developing this sort of oil is a true challenge for every lubricant
producer.
While some additives have organic alternatives containing little
or no sulphur and phosphorous and which do not contribute to
sulphated ash, some important anti-wear and detergent additives
do not have organic alternatives. Until effective replacements are
found for those additives without alternatives available, a careful
balancing and reduction in the concentrations of SAPS-contributing
additives is required to ensure that the engine oil meets all the
performance requirements that engines demand. Badly oxidised oil

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