Page 45 - Energize August 2021

TECHNICAL

The fuel consumption of an engine is affected by, among other Groups I, II and III are mineral oils derived from crude oil. Group
factors, the friction that must be overcome in the engine. While IV is a fully synthetic oil and Group V is for all base oils which
engine friction is affected largely by design considerations, the are not included in one of the other groups. Group III base oils,
engine lubricant can also play an important role. although crude-derived, are sometimes described as synthesised
In principle, every engine runs more smoothly and economically hydrocarbons (synthetic).
with a less viscous (thinner) oil. Yet the thinner the oil, the harder it Differences in base stock composition affect the performance
is to build a hydrodynamic stable oil film which prevents mechanical of finished lubricants; consequently, base stocks are considered
contact between moving metal surfaces and therefore wear, but non-fungible in many lubricant formulations. This is particularly true
this can be offset by the use of friction modifier and viscosity index in high-performance diesel engine oils.
improver additives. Group I base stocks are high in aromatics, sulphur and nitrogen,
The trend toward improved fuel economy has led to the all of which have a negative impact on lubricant performance and,
introduction of lower oil viscosity grades such as 5W30 and 10W30 as performance standards tighten, the impurities in this group
which are now commonplace in heavy-duty diesel engines. will make them unacceptable in many engine oil formulations –
When one speaks of engine lubricants, the tendency is to especially those formulations designed for modern diesel engines.
refer to the SAE grading system, which defines both low and high These inherent issues with group I base oils have led many
temperature viscosity requirements – typically kinematic viscosity – formulators to focus on Groups II/II+ and III base oils due their lower
which is a measure of an oil’s resistance to flow under the force of volatility, aromatic and sulphur contents, better oxidation stability
gravity at specific temperatures. and higher viscosity index.
However, there is another important type of viscosity that is
associated with increased fuel efficiency: high temperature high Friction modifiers (FMs)
shear (HTHS) viscosity, which is a fluid’s resistance to flow under Friction modifiers are a group of additives that are gaining in
conditions resembling highly loaded journal bearings in fired internal popularity. They are typically used in engine oils to lower metal-to-
combustion engines. metal friction between interacting component surfaces and in doing
In an operating engine, the lubricant is exposed to more than so, reduce wear and improve fuel economy.
just gravity. The lubricant is required to lubricate and protect However, their effectiveness is dependent on the lubrication
engine components under hotter and more severe operating regime within the engine, which is also affected by engine design.
conditions, and HTHS dynamic viscosity is the current industry An engine is a very complex system that at any one time can have
standard that best predicts the oil’s behaviour in these operating multiple frictional regimes occurring simultaneously. The typical
conditions. It should be noted that engine lubricants can have regimes encountered in this environment are hydrodynamic or
the same SAE viscosity grade but different HTHS viscosity, full-film lubrication – where two metal surfaces are completely
so understanding the relationship between these readings separated by an unbroken lubricant film; boundary lubrication –
is becoming more important – especially as more engine where occasional metal-to-metal contact takes place between
manufacturers move towards lower HTHS engine oils in the drive surfaces and, finally, mixed lubrication which is a combination of the
for improved fuel economy. The requirement for lower viscosity other two.
oils has also led to changes in base oil selection. For engine components lubricated hydrodynamically, the friction
The American Petroleum Institute (API) base oil classification varies with the viscosity of the oil. The thinner the oil, the less the
system groups base oils according to their purity and viscosity friction, relatively speaking. For engine components experiencing
Index (VI). The system uses physical and chemical parameters to boundary or mixed lubrication where there is some form of surface
divide all base stocks (oils) into five groups – Groups I, II, III, IV contact, FMs are used to effectively reduce friction. For example,
and V. However, base stocks, even in the same “group”, may differ
widely in their molecular composition, and physical and chemical
properties depending on the feedstock and processing parameters
used by the refiner.

API service symbol donut Viscosity index improver

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