Automotive Fluids - Lubricating Oils & Greases, Fuels, Coolants & Brake Fluids
KEW Engineering Ltd
in its simplest form is a Base Stock. It
can be either a mineral base stock derived from crude petroleum or a man-made
synthetic base stock.
are then added to the oil to improve, suppress or add new properties to the oil.
Grease is an oil and additive package held in suspension in a soap or thickening agent.
we need to understand how an oil lubricates the moving surfaces before we can
start to look at the oil itself.
control - to minimise surface damage to components and thus ensure long
service life before the parts wear out.
control - to minimise corrosion and rusting of surfaces from acids and
moisture in the oil and the environment.
control - to remove heat from the working surfaces and components and
transfer the heat away to a point such as a cooler or casing.
transmission - to support loads or transfer power from one point to
another such as on the brakes.
In the case of engines and transmissions there are a number of components
moving relative to each other; some rolling and some sliding against each other.
At some point the components will be stationary, then starting to move,
and at some stage maybe under light loading or possibly under severe loading.
Added to which, the temperature can vary partly owing to running
conditions (start-up, for example) or partly due to ambient and seasonal
There are two main lubrication regimes that will protect the components,
namely; Boundary lubrication and Thick Film lubrication.
Boundary, as the name implies, is where the two component's surfaces are in direct contact but lubrication is provided by the Friction Modifiers in the additive package (adpac) acting on the component surfaces to control friction and wear.
Thick Film, as the name implies, is where the two component surfaces are separated by a film of oil. This separation is dependent on various factors,
Thick Film lubrication can be further sub-divided into either separation
of components sliding or rolling against each other, known as Hydrodynamic and
In the diagram below (Fig. 1) the typical scenario of an engine starting
shows what might happen where sliding contact occurs such as at the plain
bearings of the crankshaft and camshaft.
Figure 1 – Lubrication
between two sliding surfaces
the starting phase the static friction is followed by boundary friction (high
friction/high wear). The friction
and wear is controlled by additives. Under extreme load it may remain boundary
increasing speed the sliding surfaces are partially separated by the lubricant,
this phase is called mixed film lubrication (medium friction /medium wear).
In this phase the emergency lubrication film formed by the additives
protects the sliding surfaces. In
some instances this may remain as a mixed film.
even higher speeds the sliding surfaces are separated from each other by a
hydrodynamic fluid film, in this phase the lowest value of wear is achieved. In
this phase the viscosity acts to separate the components. Additives may not be
required depending on the nature of the machine.
- With rolling contact such as between the elements of an anti-friction
bearing, or cam rollers, or at the pitch line of a gear tooth, immense pressure
is generated at the point of contact thanks to the small contact area.
This pressure is significant to cause the components to deform
elastically and trap enough of an oil film.
The pressure acting on this oil film is immense, sufficient to turn it
into a solid layer momentarily. This
solid layer is only 1 or 2µm thick but this is sufficient to ensure there is
separation of the two surfaces.
Both forms of Thick Film lubrication are subject to:
Particle Contamination – results in Abrasive
and Surface Fatigue damage in sliding and rolling applications respectively,
and is most susceptible where the particle size is equal to or slightly
greater than the film size or clearance.
Moisture – results in damage to the bearing
surfaces as a result of loss of film strength, and possible hydrogen
embrittlement damage in rolling contacts.
Viscosity selection – crucial to ensure
sufficient film strength, but too high a viscosity selection will also
result in increased fluid friction and thus drag and overheating, leading to
wasted power. This is one
of the reasons for modern engines using lower viscosity oils.
The same is true when the viscosity is too low, mixed film and boundary
contact can occur resulting in increased friction and wear, as well as
overheating which will exacerbate the problem. See
The same is true when the viscosity is too low, mixed film and boundary contact can occur resulting in increased friction and wear, as well as overheating which will exacerbate the problem. See Viscosity explained
Correct Additive Package – the correct range of friction-modifiers are necessary to cope with all load types at all temperature ranges.
Start-up/Shut-down – more than 70% of wear
takes place during start-up owing to insufficient speed to achieve complete
separation, hence the correct additive package is essential to protect
during this critical period of starting and warming up.