(Issue 28, pages 110 – 112)
question of aftermarket oil additives keeps coming up (Steed, Prolong, STP,
Microlon, world without end), and it always will. When a person has laid out big
money for a shiny, wonderful new Turbo Diesel, that person intends to do more
than just drive around in it. That person wants to have a relationship with that
the old days, the relationship was easy. You changed your own oil every thousand
miles, you ground your own valves, and you rotated your own tires. In fact,
there was more relationship between man and vehicle than most people wanted.
That’s why today’s cars and trucks have become such turnkey operations, with
extended oil drain intervals and no tune-ups. Just get in and drive.
way to have a relationship with the new vehicle is to buy and mount a cast
aluminum “Lone Wolf – No Club” license plate frame and some white rubber
mudflaps with jeweled reflectors. Oh, and blue dots for your taillight lenses,
to give them that distinctive purple look at night.
all that went out with the end of the 1950s. This is the 21st century here, a
time when people are concerned over things like dietary fat and bad cholesterol.
Because we are what we eat, and we want to be good, we have to eat carefully.
This applies by analogy to new trucks that have cost us $32,000. Just as we are
eating vitamin-C, DHEA, and no-flavor lean beef, so we are also tempted to pour
expensive additives into the lubricating oil of our trucks, in hopes that
performance will improve and that useful life will be extended.
read a wonderful line somewhere, which went like this; “Vitamins were
discovered in 1911. Before that time, people just ate food and died like
flies.” Something like this idea
seems to drive people today to use additives – ordinary pump Diesel fuel and
manufacturer-recommended oils can’t be enough. Aftermarket additives are,
therefore, the “vitamins” we are tempted to give our vehicles. Never mind
the fact that using “Nosmo King” anti-smoke additive adds seven cents a
gallon to the already high price of Diesel. Never mind the fact that some
highly-advertised “super” oils cost more per quart than most of us pay for a
ads are wonderfully persuasive. One I saw recently features regular guys
strolling in a junkyard. They approach a rusty clunker, start the engine, and
listen to its assortment of clatters – collapsed tappets, rod knocks, loose
wristpins. “Sounds pretty bad, Bob”, remarks one of the strollers.
“That’s right, Bill,” returns another. “We’ll try a bottle of Noo-Life,”
Bill confides to the viewer. They pour it into the oil filler and instantly the
clattering goes away (or the technician at the audio mixer cuts the treble way
down – it’s hard to tell exactly which it is). “Sounds pretty good now,
Bill,” says the pourer, turning to the viewer and holding up the now-empty Noo-Life
bottle for our inspection of the label graphics. “Why don’t you try a bottle
our minds, we know how it’s done, but in our soft hearts, we’re vulnerable,
tempted to try a bottle. Yes, we know that unscrupulous used car dealers have,
in the unregulated past, used sawdust to quiet timed-out transmissions, and we
know that thick oil or a dose of motor honey (viscosity-index improver additive)
will calm the high-frequency rattling of a worn-out engine. But, having laid out
those thirty-two thousand ones end-to-end for that beautiful new truck (that’s
more than three miles of money), it just doesn’t make sense to pass up
products that might work, right? After all, they wouldn’t let ‘em say it on
TV if it didn’t work as advertised, would they? Would they?
and why does oil work as a lubricant, anyway? I’ve touched on this topic
before in these pages (Editor’s note:
Specifically, Issue 22, page 96, and Issue 26, page 128, are great reread
material), but a deeper look always gives some fresh insight. As noted in a
previous article, there are three regimes of lubrication:
Full-film, or hydrodynamic lubrication – most of the parts in your engine are
lubricated in this regime most of the time. Viscosity and the rapid motion of
the parts drags oil between sliding parts, forming a full oil film that supports
the load. There is no contact at all between the moving parts, as revealed by
electrical conduction experiments.
Contact, or boundary lubrication – in the absence of an oil film, there is
either actual metal-to-metal contact between parts, or the parts are in some
degree protected by chemical films of oil additive used for the purpose. Such
films not only protect parts from damage, they reduce contact friction to 1/10
or less of what it would be in actual metal-to-metal friction.
Mixed lubrication – some of the load is supported by an oil film, some by
contact. This kind of friction occurs during start-up, after oil has largely
drained from engine parts. It also occurs wherever parts motion is too slow to
generate a full lubricant film – at low idle speed between cam lobes and
tappets, or near TDC between piston rings and cylinder walls, when the piston is
moving very slowly and combustion pressure is high.
what follows, I want to describe in more detail how full-film lubrication works,
and what affects it. In a later issue I’ll talk about multi-grade oils, oil
additives, and their relation to snake oils.
know that the fluids known as oils have more viscosity than, say, air or water.
Why should this be? Oils consist of molecules that are long chains, while the
molecules of low-viscosity fluids like water are small and resemble balls more
than they do chains. As one layer of fluid slides over another, long molecules
transfer kinetic energy to more potential partner molecules because they are so
long, surrounded by many other molecules. This produces a higher fluid friction,
or viscosity. The small, ball-like molecules of water, because each of them
contacts fewer other molecules, transfer kinetic energy less widely, and so
display lower fluid friction, or viscosity.
THE BOUNDARY LAYER: Near a solid surface, the situation is a
little different. Molecules – even those having the form of long chains –
are very small. At any temperature above absolute zero, they are in constant
motion. The molecules of a fluid are especially so, since in order for the fluid
state to exist, the average molecule must have enough energy of motion to
overcome any forces tending to bond it permanently to another. Therefore these
molecules vibrate, rotate, wiggle, and slither over one another constantly. At
any solid surface, these molecules collide with it steadily. Because, on the
molecular scale, even the most finely polished surface is rough, these
collisions result in rebounds at all angles, favoring no particular direction.
For this reason, therefore, the fluid near a solid surface has no net motion
along that surface. This relatively immobile layer near a solid surface is
called, reasonably, the boundary layer.
This means that in the situation discussed above, in which
one surface slides over another with a fluid between, the fluid cannot simply
slide along the surface. The relative motion has to take place in the fluid, at
some distance from the surfaces. This means that there is no escape from the
effect of the fluid’s internal friction, or viscosity. No coating we could put
on the solid surfaces would be smooth on a molecular scale, and so prevent the
formation of a boundary layer there, permitting the fluid to simply slide along
the surfaces. The relative motion always takes place in the lubricant itself, so
power must be used to overcome the slight viscous drag involved in sliding the
layers of lubricant past each other. No snake oil can change this!
PUTTING THE OIL UNDER THE LOAD: Now, why does oil remain
between the sliding surfaces, rather than being immediately squeezed out by an
applied load? Imagine a situation in which a loaded slider moves over another
surface, with a viscous fluid – oil – between. The load, by exerting
pressure on the film of oil between, tends to squeeze the oil out. If more oil
does not somehow enter the space between the surfaces, this loss of oil will
soon result in contact and possible surface damage. What can put oil into the
space between surfaces?
FORMING AN OIL WEDGE: That something is viscosity. As the
moving slider glides along, it assumes a slightly tilted position because the
oil at its rear edge has been under pressure the longest, so the most has been
squeezed out from that region. The oil film between therefore takes the form of
a wedge, thicker at the leading edge, thinner at the trailing edge. As the
slider advances, oil ahead of it does not immediately flow away because its
viscosity prevents it from doing so. Once oil enters the wedge, the only way it
can escape is to be squeezed out to the sides, or for the slider to pass
completely over it. It’s hard to squeeze the oil out because forcing viscous
oil out through such a narrow space requires very great pressure. Some does
escape, naturally, but it escapes slowly.
As the slider advances, a steady state is soon reached, in
which the rate at which oil enters the wedge at the front equals the rate of
loss through being squeezed out at the sides and at the trailing edge. Oil
enters the wedge at essentially zero pressure, but the advance of the slider,
coupled with viscosity, carries it into regions of higher pressure – high
enough to carry the load on the slider. The slider rises up on the wedge of oil
thus produced. The more viscous the oil, the thicker the wedge.
Our “slider” could be the skirt of a piston, sliding on a
lubricated cylinder wall, or it could be the lobe of a cam, rotating against a
tappet. It could even be the rotating journal of a crankshaft, turning inside a
sleeve bearing. In all cases, the load is carried by the same naturally-forming
oil wedge. Oil enters the wedge at essentially zero pressure, and once the oil
is between the surfaces, viscosity makes it easier for it to carry the applied
load than for it to be squeezed out. In the process, the frictional drag force
in the bearing is typically about one or two thousandths of the applied load.
This is why engine friction is as low as it is.
The above argument shows that viscosity is necessary if loads
are to be carried by sliding parts – it is what keeps the lubricant from
escaping out from under the load so fast that the lubricant wedge collapses. Yet
at the same time, the friction loss inherent in lubrication is produced by this
same viscosity. It is therefore obvious that a compromise is necessary here. We
must have enough viscosity to carry the loads on sliding parts, but much more
than that simply increases the friction loss in our machine.
THE VISCOSITY COMPROMISE: When engineers specify oils for
Diesel truck engines, they are obliged to make this compromise. They cannot
allow the moving parts to touch each other, because that causes accelerated wear
and parts damage. Therefore they must specify enough viscosity to keep parts
separated as much of the time as possible. On the other hand, they also know
that the more viscous the oil, the greater the force it takes to make lubricated
parts slide over each other. Too little viscosity means wear and damage. Too
much viscosity means power loss and increased fuel consumption. For example, a
change from a 30 oil to a 50 oil increases friction-loss approximately 20%.
Because well-designed engines typically lose about 15% of their power to
friction, this means 20% of 15%, or a power loss of 3%.
Shall we play with this compromise ourselves, in hopes of
either reducing friction and getting more power, or concentrate on extending
engine life? It’s obviously true that we can cut the friction of
well-lubricated parts significantly by reducing oil viscosity. This is a ploy
constantly used in auto racing. Shall we run out and get cases of watery 0W-5
oil and reap the benefits of lower friction? We don’t do this because we know
the factory chose a heavier oil to cover the full range of operating conditions
that their product will meet in use. Yes, we might be able to get away with a
lighter oil if we didn’t work our engines very hard – this is an old trick
from the Mobil economy runs of years ago. But we bought these trucks to do heavy
work, so when we’re towing that big stock trailer up the Rockies on a hot day,
we’re going to need the viscosity of the factory-specified oil.
Okay, but what about engine life? Can’t I make my engine
last longer by using thicker oil? Won’t that keep the parts separated by
thicker oil films?
Enough is enough. If factory-recommended viscosity keeps the
moving parts separated, making good oil films thicker with added viscosity gets
us nothing but added fuel consumption. Also, some of us live in Thief River
Falls, MN, where engines have to cold-start at minus forty. With that thick
stuff in the crankcase, the starter won’t turn the engine. Even if it does,
the heavy oil moves so slowly at that temperature that it will take long minutes
for flow to reach all the way to the rocker arms and other parts most distant
from the oil pump
The factory, based upon its thousands of hours of testing,
and on the years of warranty and service experience, arrives at an oil
specification for its engines. Can we get better information by watching the
motor honey ads on late-night TV?
on any link below for more TDReprints
GASOLINE AND DIESEL
(Issue 26, pages 14 – 15)
AND THE MARKETING OF AMERICA
(Issue 31, pages 126 – 129)
(Issue 28, pages 110 – 112)