|
In simplest
terms, viscosity is a measure
that speaks to a liquid's
resistance to flow. Fluids with
a high viscosity are thick and
flow slowly. Low viscosity
fluids are thin and flow
quickly. A common example of
each are seen when comparing
molasses with water. Whereas
molasses is thick and flows
slowly (high viscosity) when
poured, water is "thin" and
flows quickly when poured (low
viscosity).
The viscosity
of lubricants is also the
measure of the lubricant's
resistance to flow. Low
viscosity lubricants are thin
and flow easily like water. High
viscosity lubricants are more
like molasses, they are thick
and have high resistance to
flow.
The viscosity
of an engine oil is significant
for three primary reasons. The
first is that the higher the
viscosity of the lubricants the
greater is its ability to
provide what is referred to as
"boundary lubrication." In
practical terms, this means
higher viscosity lubricants have
a greater ability to maintain a
boundary between moving parts.
Based on this, one might
conclude the higher the
viscosity of the engine oil the
better. But that's not
necessarily the case, and this
leads to the second primary
reason viscosity is important in
engine oils.
Whereas high
viscosity (thick) engine oils
help to maintain a barrier
between moving parts, they also
create drag in the movement of
those parts. The industry calls
this parasitic load. Rather than
burning fuel only to move a
vehicle, the higher the
viscosity of the lubricant, the
more fuel is required to overcome
the oils resistance to flow,
thus placing additional load on
the engine and driving up fuel
consumption.
The third
reason the viscosity of an
engine oil is important is a
about its cold flow properties.
As the old saying goes, "it's as
slow as molasses going up hill
on a cold day." Well engine oils
are the same, they flow more
slowly when it's cold. So
whereas a high viscosity engine
oil may be good for boundary
lubrication, they can be a real
challenge to move when the
weather is cold. What this means
is that an engine may be slow to
get lubricanted if the viscosity
is too high and the temperature
too low. For the car or truck
owner this means when they go
out to start their vehicle on a
cold day, a thinner oil will
flow more freely and lubricate
more efficiently than a thicker
oil.
The Society of
Automotive Engineers established
clear standards that define the
viscosity grade of an engine
oil. Unlike other "softer"
parameters, lubricants either
meet, or don't meet these
standards.
|
SAE
Viscosity Chart (High
Temp) |
|
100° C
(210° F) |
|
SAE
Viscosity |
Kinematic
(cSt) |
Kinematic
(cSt) |
|
|
100° C
Min |
100° C
Max |
|
8 |
4.0 |
<6.1 |
|
12 |
5.0 |
<7.1 |
|
16 |
6.1 |
<8.2 |
|
20 |
6.9 |
<9.3 |
|
30 |
9.3 |
<12.5 |
|
40 |
12.5 |
<16.3 |
|
50 |
16.3 |
<21.9 |
|
60 |
21.9 |
<26.1 |
|
Source: Engine Oil
Viscosity
Classification, J300
Jan2015, SAE. The full
publication is available
from SAE at www.sae.org. |
| |
| |
|
In The Market (5W-30) |
| |
|
VISCOSITY @100˚C
As shown
in the graph below, the
average viscosity for
the 5W-30 major brands
tested is 10.4 cSt
@100°C. Also important
to note there is a
relatively tight range
as to where the
viscosities @100°C of
the majors fall. All are
skewed to the lower end
of the acceptable API
range.
|
|
 |
| |
|
VISCOSITY @40˚C |
|
As shown
in the graph below, the
average viscosity for
the 5W-30 major brands
tested is 61.1 cSt
@40°C. As with the range
of viscosities among the
majors at 100ºC,
there is a relatively
tight range as to where
the viscosities @40°C
falls. All are skewed to
the lower end of the
acceptable API range.
 |
For details on ASTM D445 test
method click here. |
Viscosity