By Gwinyai Mataruka
It’s all
about viscosity. That’s what they say. They say use thick and strong oil for
better protection. Is that so? Why does it seem like the synthetics are very
thin?
There are
certainly too many fallacies about engine oil. They never end, and will
certainly not. The engineers can do their best to formulate oil that will meet
different operating requirements, but in the end it’s all a compromise. Their
objective is to reduce friction, or lubricate, in order to extend the engine
life. The ability to lubricate means that there is always a strong film of oil
separating moving metal parts, so that they don’t rub against each other.
However,
there are so many environmental and behavioural factors that intervene. Engine
oil and the internal combustion engine are a complex interaction that cannot be
explained in a one pager. This one will therefore only look at viscosity.
Viscosity
simply refers to the thickness of the oil, or resistance to flow, or the time
that a drop of oil takes to flow down a vertical tube of a given diameter, from
one point to the other. Low viscosity means the oil is thin, whilst high
viscosity means the oil is thick. It follows that thin oil (low viscosity)
flows faster than thick oil (high viscosity).
The next
question to ask is; of what significance is this with reference to engine
lubrication? The answer is TIME and SPEED. Research shows that most engine wear
occurs during engine start up, especially during what is referred to as “cold
startingâ€, which refers to starting a cold engine. An engine becomes cold when
it has been stationary for some time, during which time the oil flows down to
the reservoir, or sump, under gravity. When such an engine is started, there is
obviously inadequate lubrication, especially for the components at the top of
the engine, such as the cam shaft. A desirable characteristic of the oil under
cold starting is that the oil takes as little TIME as possible to reach all
parts of the engine in adequate volumes and pressure to provide lubrication. A
thin oil, or low viscosity oil, will do this much better, and FASTER. It will
reach all the engine parts in seconds. This means that low viscosity oil is
more effective at reducing wear at cold starting than high viscosity oil.
This is one
half of the equation. Before we look at the other half, we need to look at one
important characteristic of oil; its behaviour with temperature. Oil has an
inverse relationship with temperature. As temperature rises, viscosity falls,
i.e. the oil becomes thinner with a rise in temperature. An engine started cold
will start rising in temperature when running. The heat is transferred to the
oil, which becomes thinner. This affects the ability of the oil to separate
surfaces moving against each other, or the ability to lubricate. Thick oil
(high viscosity) lubricates better than thin oil (low viscosity) at high
temperature. In other words, high viscosity oil tends to offer a stronger oil
film than low viscosity oil, everything else being constant. At high temperatures,
therefore, high viscosity oil is more effective than low viscosity oil.
Back to our
question.
Lubrication
engineers simply solved this by creating Multigrade Oil that behaves like low
viscosity oil at low temperatures, and high viscosity oil at high temperatures.
This is achieved through an additive called a Viscosity Index (VI) Improver.
The VI Improver allows a lubricant to have two grades in one oil. At low
temperatures, the oil behaves like thin oil, reducing low temperature wear. At
high temperature, the oil is thick, reducing high temperature wear.
Having
explained thus far, the modern engine oil is generally low in viscosity, but
with high film strength. The trend is towards low viscosity oil with high film
strength. This is important for fuel economy. Low viscosity oil offers less
viscous drag, and therefore serves on energy and fuel. This is achieved by the
use of high quality mineral base oils, or synthetics.
The Society
of Automotive Engineers (SAE) is the body responsible for automotive oil
viscosity grading. For engine oil the low temperature grade is designated by a
W, like SAE0W, 5W, 10W, 15W, 20W, and 25W. On the other hand, the high
temperature grade is designated thus SAE20, 25, 30, 40, 50, and 60. Multi-grade
engine oil will have a combination of any of the two, like SAE0W20, 5W30,
10W40, 15W40, 25W50 etc. Multi-grade SAE15W40 oil will behave like SAE15W
viscosity oil at low temperature, or on cold starting; and behave like SAE40 at
high temperature.
In general, it means that multigrade engine oil will protect at both low and high temperature, and is therefore a better choice for your engine than monograde oil. However, always check the oil specified for your engine.